Class 9 Exam > Class 9 Notes > Chapter 3 Atoms and Molecules
Chapter 3 Atoms and Molecules - Class 9 PDF Download
| Download, print and study this document offline |
Please wait while the PDF view is loading
Page 1
Chapter 3 Atoms and Molecules
Elements – The purest form of matter that are made up of a single type of atoms. E.g. Carbon (C), Oxygen (O).
Compounds – Compounds are made up of two or more type of elements combined chemically in fixed ratio by their
masses. E.g. Carbon dioxide (CO 2).
Atoms – The smallest constituent particles of matter that cannot be broken down further. E.g. Atom of oxygen is O,
nitrogen N
Molecules – The form of matter made up of two or more atoms that can exist independently. E.g. Oxygen and nitrogen
can exist as single atom. They combine with another atom of their own kind to form a molecule of Oxygen i.e. O 2 and
nitrogen i.e. N 2.
Laws of Chemical Combination
1. Law of Conservation of mass
Mass can neither be created nor be destroyed. It can only be converted from one form to another.
2. Law of Constant Proportion
Two or more elements combine in fixed proportions (ratio) by their mass to form a compound.
Example
? Carbon (C) + Oxygen (O 2) Carbon dioxide (CO 2)
3g of carbon combine with 8g of oxygen to produce 11g of carbon dioxide.
? Similarly, 9g of water decomposes to produce 8g of oxygen and 1g of hydrogen.
Dalton’s Atomic Theory
1. Matter is made of tiny particles called atoms.
2. Atoms can neither be created or be destroyed.
3. All the atoms of an element have same mass and chemical properties.
4. Atoms of different element have different atomic mass and chemical properties.
5. Atoms combine in the ratio of whole numbers to form compounds.
6. The relative number and kinds of atoms are constant in a given compound.
Atoms
? The smallest constituent particles of matter that cannot be broken down further. E.g. Atom of oxygen is O,
nitrogen N.
? However, an atom is made up of protons, neutrons and electrons.
? Atoms are very small.
? Radius of hydrogen atom = 10
-10
m
? Radius of a water molecule = 10
-9
m
Symbols of elements
? Various elements have been assigned a symbol, which is specific to that element and is used refer to it in various
reactions.
? Dalton was the first scientist to use symbols
IUPAC or International Union For Pure and Applied Chemistry approves the symbols for elements.
Atomic Number – Number of protons or electrons in an uncharged atom is known as its atomic number.
Atomic mass – Number of protons and neutrons in an atom is called its atomic mass. Its unit is atomic mass unit (amu or
u).
Page 2
Chapter 3 Atoms and Molecules
Elements – The purest form of matter that are made up of a single type of atoms. E.g. Carbon (C), Oxygen (O).
Compounds – Compounds are made up of two or more type of elements combined chemically in fixed ratio by their
masses. E.g. Carbon dioxide (CO 2).
Atoms – The smallest constituent particles of matter that cannot be broken down further. E.g. Atom of oxygen is O,
nitrogen N
Molecules – The form of matter made up of two or more atoms that can exist independently. E.g. Oxygen and nitrogen
can exist as single atom. They combine with another atom of their own kind to form a molecule of Oxygen i.e. O 2 and
nitrogen i.e. N 2.
Laws of Chemical Combination
1. Law of Conservation of mass
Mass can neither be created nor be destroyed. It can only be converted from one form to another.
2. Law of Constant Proportion
Two or more elements combine in fixed proportions (ratio) by their mass to form a compound.
Example
? Carbon (C) + Oxygen (O 2) Carbon dioxide (CO 2)
3g of carbon combine with 8g of oxygen to produce 11g of carbon dioxide.
? Similarly, 9g of water decomposes to produce 8g of oxygen and 1g of hydrogen.
Dalton’s Atomic Theory
1. Matter is made of tiny particles called atoms.
2. Atoms can neither be created or be destroyed.
3. All the atoms of an element have same mass and chemical properties.
4. Atoms of different element have different atomic mass and chemical properties.
5. Atoms combine in the ratio of whole numbers to form compounds.
6. The relative number and kinds of atoms are constant in a given compound.
Atoms
? The smallest constituent particles of matter that cannot be broken down further. E.g. Atom of oxygen is O,
nitrogen N.
? However, an atom is made up of protons, neutrons and electrons.
? Atoms are very small.
? Radius of hydrogen atom = 10
-10
m
? Radius of a water molecule = 10
-9
m
Symbols of elements
? Various elements have been assigned a symbol, which is specific to that element and is used refer to it in various
reactions.
? Dalton was the first scientist to use symbols
IUPAC or International Union For Pure and Applied Chemistry approves the symbols for elements.
Atomic Number – Number of protons or electrons in an uncharged atom is known as its atomic number.
Atomic mass – Number of protons and neutrons in an atom is called its atomic mass. Its unit is atomic mass unit (amu or
u).
One atomic mass unit is defined as the one twelfth (1/12
th
) the mass of one carbon-12 atom.
For e.g. The atomic mass of oxygen atom is 16. It means that it is 16 times heavier than one twelfth (1/12
th
) the mass of
one carbon-12 atom.
Some common elements and their atomic number and masses:
Molecule
The form of matter made up of two or more atoms that can exist independently. E.g. Oxygen and nitrogen can exist as
single atom. They combine with another atom of their own kind to form a molecule of Oxygen i.e. O 2 and nitrogen i.e. N 2.
Molecules of elements
1. Molecules of element – made of one or more atoms of the same element combined chemically to form a stable
substance.
Some elements usually non-metals are very reactive at normal conditions and hence react with more atom/s of their own
kind to form a molecule.
Atomicity – number of atoms in a molecule.
Monoatomic – A molecule that contain only one atom
Diatomic – A molecule that contain two atoms.
Triatomic – A molecule that contain 3 atoms.
Tetra-atomic - molecule that contain four atoms.
Element Symbol Atomic number Atomic mass Way to
remember 1 to
20 elements
Hydrogen H 1 1 High
Helium He 2 4 Heels
Lithium Li 3 7 Liye
Beryllium Be 4 9 Bena
Boron B 5 11 Bhi
Carbon C 6 12 Cal
Nitrogen N 7 14 Nani
Oxygen O 8 16 Or
Fluorine F 9 18 Fufa
Neon Ne 10 20 Nachenge
Sodium Na 11 23 Na
Magnesium Mg 12 24 Mang
Aluminium Al 13 27 Allah
Silicon Si 14 28 Se
Phosphorus P 15 31 Pepsi
Sulphur S 16 32 Soda
Chlorine Cl 17 35 Cola
Argon Ar 18 40 Aur
Potassium K 19 39 Kaju
Calcium Ca 20 40 Cake
Page 3
Chapter 3 Atoms and Molecules
Elements – The purest form of matter that are made up of a single type of atoms. E.g. Carbon (C), Oxygen (O).
Compounds – Compounds are made up of two or more type of elements combined chemically in fixed ratio by their
masses. E.g. Carbon dioxide (CO 2).
Atoms – The smallest constituent particles of matter that cannot be broken down further. E.g. Atom of oxygen is O,
nitrogen N
Molecules – The form of matter made up of two or more atoms that can exist independently. E.g. Oxygen and nitrogen
can exist as single atom. They combine with another atom of their own kind to form a molecule of Oxygen i.e. O 2 and
nitrogen i.e. N 2.
Laws of Chemical Combination
1. Law of Conservation of mass
Mass can neither be created nor be destroyed. It can only be converted from one form to another.
2. Law of Constant Proportion
Two or more elements combine in fixed proportions (ratio) by their mass to form a compound.
Example
? Carbon (C) + Oxygen (O 2) Carbon dioxide (CO 2)
3g of carbon combine with 8g of oxygen to produce 11g of carbon dioxide.
? Similarly, 9g of water decomposes to produce 8g of oxygen and 1g of hydrogen.
Dalton’s Atomic Theory
1. Matter is made of tiny particles called atoms.
2. Atoms can neither be created or be destroyed.
3. All the atoms of an element have same mass and chemical properties.
4. Atoms of different element have different atomic mass and chemical properties.
5. Atoms combine in the ratio of whole numbers to form compounds.
6. The relative number and kinds of atoms are constant in a given compound.
Atoms
? The smallest constituent particles of matter that cannot be broken down further. E.g. Atom of oxygen is O,
nitrogen N.
? However, an atom is made up of protons, neutrons and electrons.
? Atoms are very small.
? Radius of hydrogen atom = 10
-10
m
? Radius of a water molecule = 10
-9
m
Symbols of elements
? Various elements have been assigned a symbol, which is specific to that element and is used refer to it in various
reactions.
? Dalton was the first scientist to use symbols
IUPAC or International Union For Pure and Applied Chemistry approves the symbols for elements.
Atomic Number – Number of protons or electrons in an uncharged atom is known as its atomic number.
Atomic mass – Number of protons and neutrons in an atom is called its atomic mass. Its unit is atomic mass unit (amu or
u).
One atomic mass unit is defined as the one twelfth (1/12
th
) the mass of one carbon-12 atom.
For e.g. The atomic mass of oxygen atom is 16. It means that it is 16 times heavier than one twelfth (1/12
th
) the mass of
one carbon-12 atom.
Some common elements and their atomic number and masses:
Molecule
The form of matter made up of two or more atoms that can exist independently. E.g. Oxygen and nitrogen can exist as
single atom. They combine with another atom of their own kind to form a molecule of Oxygen i.e. O 2 and nitrogen i.e. N 2.
Molecules of elements
1. Molecules of element – made of one or more atoms of the same element combined chemically to form a stable
substance.
Some elements usually non-metals are very reactive at normal conditions and hence react with more atom/s of their own
kind to form a molecule.
Atomicity – number of atoms in a molecule.
Monoatomic – A molecule that contain only one atom
Diatomic – A molecule that contain two atoms.
Triatomic – A molecule that contain 3 atoms.
Tetra-atomic - molecule that contain four atoms.
Element Symbol Atomic number Atomic mass Way to
remember 1 to
20 elements
Hydrogen H 1 1 High
Helium He 2 4 Heels
Lithium Li 3 7 Liye
Beryllium Be 4 9 Bena
Boron B 5 11 Bhi
Carbon C 6 12 Cal
Nitrogen N 7 14 Nani
Oxygen O 8 16 Or
Fluorine F 9 18 Fufa
Neon Ne 10 20 Nachenge
Sodium Na 11 23 Na
Magnesium Mg 12 24 Mang
Aluminium Al 13 27 Allah
Silicon Si 14 28 Se
Phosphorus P 15 31 Pepsi
Sulphur S 16 32 Soda
Chlorine Cl 17 35 Cola
Argon Ar 18 40 Aur
Potassium K 19 39 Kaju
Calcium Ca 20 40 Cake
Name Symbol of molecule Atomicity
Argon Ar Monoatomic
Helium He Monoatomic
Oxygen O 2 Diatomic
Chlorine Cl Monoatomic
Nitrogen N 2 Diatomic
Sodium Na Monoatomic
Aluminium Al Monoatomic
Copper Cu Monoatomic
Phosphorus P 4 Tetra-atomic
Ozone O 3 Tri-atomic
2. Molecules of Compounds
Compounds are made up of two or more type of elements combined chemically in fixed ratio by their masses.
E.g. Carbon dioxide (CO 2).
? Ratio mass of Carbon and Oxygen in CO 2 = 12/16*2 = 3:8
(16*2 because atomic mass of oxygen is 16 and there are two atoms of oxygen in CO 2)
? Water – H 2O; Ratio of hydrogen and oxygen in H 2O = 1*2/16 = 1:8
Ion
Charged species are called ions.
? Atoms are made up of electrons, protons and neutrons.
? Electrons are negatively charged; protons are positively charged while neutrons do not carry any charge.
? Protons and neutrons are present in the center, in a nucleus and electron revolve around the nucleus.
Fig. An atom of carbon
The number of electron and protons is same in an uncharged atom. For example, Sodium’s atomic number is 11 and an
uncharged atom of sodium contain 11 electrons as well as 11 protons.
However, an atom can gain electrons to become negatively charged and a loose electron to become positively charged.
Explanation – there are 11 electrons and 11 protons in a neutral sodium atom. If sodium loses one electron, the number of
proton remain 11 and the number of electrons reduces to 10. Since, there is one more positive charge than negative
sodium becomes positively charged.
That can be written as
Na Na
+
+ e
-
Sodium produces sodium ion on losing one electron.
Types of ions
1. Cations – A positively charged ion is called cation. Cations are formed when an atom/molecule loses
electron. Metals usually form cations.
Page 4
Chapter 3 Atoms and Molecules
Elements – The purest form of matter that are made up of a single type of atoms. E.g. Carbon (C), Oxygen (O).
Compounds – Compounds are made up of two or more type of elements combined chemically in fixed ratio by their
masses. E.g. Carbon dioxide (CO 2).
Atoms – The smallest constituent particles of matter that cannot be broken down further. E.g. Atom of oxygen is O,
nitrogen N
Molecules – The form of matter made up of two or more atoms that can exist independently. E.g. Oxygen and nitrogen
can exist as single atom. They combine with another atom of their own kind to form a molecule of Oxygen i.e. O 2 and
nitrogen i.e. N 2.
Laws of Chemical Combination
1. Law of Conservation of mass
Mass can neither be created nor be destroyed. It can only be converted from one form to another.
2. Law of Constant Proportion
Two or more elements combine in fixed proportions (ratio) by their mass to form a compound.
Example
? Carbon (C) + Oxygen (O 2) Carbon dioxide (CO 2)
3g of carbon combine with 8g of oxygen to produce 11g of carbon dioxide.
? Similarly, 9g of water decomposes to produce 8g of oxygen and 1g of hydrogen.
Dalton’s Atomic Theory
1. Matter is made of tiny particles called atoms.
2. Atoms can neither be created or be destroyed.
3. All the atoms of an element have same mass and chemical properties.
4. Atoms of different element have different atomic mass and chemical properties.
5. Atoms combine in the ratio of whole numbers to form compounds.
6. The relative number and kinds of atoms are constant in a given compound.
Atoms
? The smallest constituent particles of matter that cannot be broken down further. E.g. Atom of oxygen is O,
nitrogen N.
? However, an atom is made up of protons, neutrons and electrons.
? Atoms are very small.
? Radius of hydrogen atom = 10
-10
m
? Radius of a water molecule = 10
-9
m
Symbols of elements
? Various elements have been assigned a symbol, which is specific to that element and is used refer to it in various
reactions.
? Dalton was the first scientist to use symbols
IUPAC or International Union For Pure and Applied Chemistry approves the symbols for elements.
Atomic Number – Number of protons or electrons in an uncharged atom is known as its atomic number.
Atomic mass – Number of protons and neutrons in an atom is called its atomic mass. Its unit is atomic mass unit (amu or
u).
One atomic mass unit is defined as the one twelfth (1/12
th
) the mass of one carbon-12 atom.
For e.g. The atomic mass of oxygen atom is 16. It means that it is 16 times heavier than one twelfth (1/12
th
) the mass of
one carbon-12 atom.
Some common elements and their atomic number and masses:
Molecule
The form of matter made up of two or more atoms that can exist independently. E.g. Oxygen and nitrogen can exist as
single atom. They combine with another atom of their own kind to form a molecule of Oxygen i.e. O 2 and nitrogen i.e. N 2.
Molecules of elements
1. Molecules of element – made of one or more atoms of the same element combined chemically to form a stable
substance.
Some elements usually non-metals are very reactive at normal conditions and hence react with more atom/s of their own
kind to form a molecule.
Atomicity – number of atoms in a molecule.
Monoatomic – A molecule that contain only one atom
Diatomic – A molecule that contain two atoms.
Triatomic – A molecule that contain 3 atoms.
Tetra-atomic - molecule that contain four atoms.
Element Symbol Atomic number Atomic mass Way to
remember 1 to
20 elements
Hydrogen H 1 1 High
Helium He 2 4 Heels
Lithium Li 3 7 Liye
Beryllium Be 4 9 Bena
Boron B 5 11 Bhi
Carbon C 6 12 Cal
Nitrogen N 7 14 Nani
Oxygen O 8 16 Or
Fluorine F 9 18 Fufa
Neon Ne 10 20 Nachenge
Sodium Na 11 23 Na
Magnesium Mg 12 24 Mang
Aluminium Al 13 27 Allah
Silicon Si 14 28 Se
Phosphorus P 15 31 Pepsi
Sulphur S 16 32 Soda
Chlorine Cl 17 35 Cola
Argon Ar 18 40 Aur
Potassium K 19 39 Kaju
Calcium Ca 20 40 Cake
Name Symbol of molecule Atomicity
Argon Ar Monoatomic
Helium He Monoatomic
Oxygen O 2 Diatomic
Chlorine Cl Monoatomic
Nitrogen N 2 Diatomic
Sodium Na Monoatomic
Aluminium Al Monoatomic
Copper Cu Monoatomic
Phosphorus P 4 Tetra-atomic
Ozone O 3 Tri-atomic
2. Molecules of Compounds
Compounds are made up of two or more type of elements combined chemically in fixed ratio by their masses.
E.g. Carbon dioxide (CO 2).
? Ratio mass of Carbon and Oxygen in CO 2 = 12/16*2 = 3:8
(16*2 because atomic mass of oxygen is 16 and there are two atoms of oxygen in CO 2)
? Water – H 2O; Ratio of hydrogen and oxygen in H 2O = 1*2/16 = 1:8
Ion
Charged species are called ions.
? Atoms are made up of electrons, protons and neutrons.
? Electrons are negatively charged; protons are positively charged while neutrons do not carry any charge.
? Protons and neutrons are present in the center, in a nucleus and electron revolve around the nucleus.
Fig. An atom of carbon
The number of electron and protons is same in an uncharged atom. For example, Sodium’s atomic number is 11 and an
uncharged atom of sodium contain 11 electrons as well as 11 protons.
However, an atom can gain electrons to become negatively charged and a loose electron to become positively charged.
Explanation – there are 11 electrons and 11 protons in a neutral sodium atom. If sodium loses one electron, the number of
proton remain 11 and the number of electrons reduces to 10. Since, there is one more positive charge than negative
sodium becomes positively charged.
That can be written as
Na Na
+
+ e
-
Sodium produces sodium ion on losing one electron.
Types of ions
1. Cations – A positively charged ion is called cation. Cations are formed when an atom/molecule loses
electron. Metals usually form cations.
2. Anions – A negatively charged ion is called anion. Anions are formed when an atom/molecule gains
electrons.
Chemical Formula
The symbolic representation of the composition of elements in a compound is called its chemical formula.
*Valency – The combining capacity of an element of molecule is called its valency.
*Details are given in the next chapter.
Valency of some Ions
Rules to write a chemical formula:
? Valencies of the ion should be balanced (the compound should not have any charge as a whole).
? If a compound consists of a metal and a non-metal, name of symbol of the metal should be written
first followed by the name or symbol of the non-metal.
? If the compound is formed from polyatomic ion, the ions should be enclosed in a bracket before
writing the ratio.
Examples
1. Formula of sodium chloride
Name Sodium Chloride
Symbol Na Cl
Valency 1 1
Formula NaCl
Valency Metallic
ions
Symbol Non-metallic
ions
Symbol Polyatomic
ions
Symbol
1
Sodium Na
+
Hydrogen H
+
Ammonium NH4
+
Potassium K
+
Hydride H
-
Hydroxide OH
-
Silver Ag
+
Chloride Cl
-
Nitrate NO3
-
Copper Cu
+
Bromide Br
-
bicarbonate HCO3
-
Iodide I
-
2
Magnesium Mg
2+
Oxide O
2-
Carbonate CO3
2-
Calcium Ca
2+
Sulphide S
2-
Sulphite SO3
2-
Zinc Zn
2+
Sulphate SO4
2-
Iron (II) Fe
2+
Copper (II) Cu
2+
3
Aluminium Al
3+
Nitride N
3-
Phosphate PO4
3-
Iron (III) Fe
3+
Page 5
Chapter 3 Atoms and Molecules
Elements – The purest form of matter that are made up of a single type of atoms. E.g. Carbon (C), Oxygen (O).
Compounds – Compounds are made up of two or more type of elements combined chemically in fixed ratio by their
masses. E.g. Carbon dioxide (CO 2).
Atoms – The smallest constituent particles of matter that cannot be broken down further. E.g. Atom of oxygen is O,
nitrogen N
Molecules – The form of matter made up of two or more atoms that can exist independently. E.g. Oxygen and nitrogen
can exist as single atom. They combine with another atom of their own kind to form a molecule of Oxygen i.e. O 2 and
nitrogen i.e. N 2.
Laws of Chemical Combination
1. Law of Conservation of mass
Mass can neither be created nor be destroyed. It can only be converted from one form to another.
2. Law of Constant Proportion
Two or more elements combine in fixed proportions (ratio) by their mass to form a compound.
Example
? Carbon (C) + Oxygen (O 2) Carbon dioxide (CO 2)
3g of carbon combine with 8g of oxygen to produce 11g of carbon dioxide.
? Similarly, 9g of water decomposes to produce 8g of oxygen and 1g of hydrogen.
Dalton’s Atomic Theory
1. Matter is made of tiny particles called atoms.
2. Atoms can neither be created or be destroyed.
3. All the atoms of an element have same mass and chemical properties.
4. Atoms of different element have different atomic mass and chemical properties.
5. Atoms combine in the ratio of whole numbers to form compounds.
6. The relative number and kinds of atoms are constant in a given compound.
Atoms
? The smallest constituent particles of matter that cannot be broken down further. E.g. Atom of oxygen is O,
nitrogen N.
? However, an atom is made up of protons, neutrons and electrons.
? Atoms are very small.
? Radius of hydrogen atom = 10
-10
m
? Radius of a water molecule = 10
-9
m
Symbols of elements
? Various elements have been assigned a symbol, which is specific to that element and is used refer to it in various
reactions.
? Dalton was the first scientist to use symbols
IUPAC or International Union For Pure and Applied Chemistry approves the symbols for elements.
Atomic Number – Number of protons or electrons in an uncharged atom is known as its atomic number.
Atomic mass – Number of protons and neutrons in an atom is called its atomic mass. Its unit is atomic mass unit (amu or
u).
One atomic mass unit is defined as the one twelfth (1/12
th
) the mass of one carbon-12 atom.
For e.g. The atomic mass of oxygen atom is 16. It means that it is 16 times heavier than one twelfth (1/12
th
) the mass of
one carbon-12 atom.
Some common elements and their atomic number and masses:
Molecule
The form of matter made up of two or more atoms that can exist independently. E.g. Oxygen and nitrogen can exist as
single atom. They combine with another atom of their own kind to form a molecule of Oxygen i.e. O 2 and nitrogen i.e. N 2.
Molecules of elements
1. Molecules of element – made of one or more atoms of the same element combined chemically to form a stable
substance.
Some elements usually non-metals are very reactive at normal conditions and hence react with more atom/s of their own
kind to form a molecule.
Atomicity – number of atoms in a molecule.
Monoatomic – A molecule that contain only one atom
Diatomic – A molecule that contain two atoms.
Triatomic – A molecule that contain 3 atoms.
Tetra-atomic - molecule that contain four atoms.
Element Symbol Atomic number Atomic mass Way to
remember 1 to
20 elements
Hydrogen H 1 1 High
Helium He 2 4 Heels
Lithium Li 3 7 Liye
Beryllium Be 4 9 Bena
Boron B 5 11 Bhi
Carbon C 6 12 Cal
Nitrogen N 7 14 Nani
Oxygen O 8 16 Or
Fluorine F 9 18 Fufa
Neon Ne 10 20 Nachenge
Sodium Na 11 23 Na
Magnesium Mg 12 24 Mang
Aluminium Al 13 27 Allah
Silicon Si 14 28 Se
Phosphorus P 15 31 Pepsi
Sulphur S 16 32 Soda
Chlorine Cl 17 35 Cola
Argon Ar 18 40 Aur
Potassium K 19 39 Kaju
Calcium Ca 20 40 Cake
Name Symbol of molecule Atomicity
Argon Ar Monoatomic
Helium He Monoatomic
Oxygen O 2 Diatomic
Chlorine Cl Monoatomic
Nitrogen N 2 Diatomic
Sodium Na Monoatomic
Aluminium Al Monoatomic
Copper Cu Monoatomic
Phosphorus P 4 Tetra-atomic
Ozone O 3 Tri-atomic
2. Molecules of Compounds
Compounds are made up of two or more type of elements combined chemically in fixed ratio by their masses.
E.g. Carbon dioxide (CO 2).
? Ratio mass of Carbon and Oxygen in CO 2 = 12/16*2 = 3:8
(16*2 because atomic mass of oxygen is 16 and there are two atoms of oxygen in CO 2)
? Water – H 2O; Ratio of hydrogen and oxygen in H 2O = 1*2/16 = 1:8
Ion
Charged species are called ions.
? Atoms are made up of electrons, protons and neutrons.
? Electrons are negatively charged; protons are positively charged while neutrons do not carry any charge.
? Protons and neutrons are present in the center, in a nucleus and electron revolve around the nucleus.
Fig. An atom of carbon
The number of electron and protons is same in an uncharged atom. For example, Sodium’s atomic number is 11 and an
uncharged atom of sodium contain 11 electrons as well as 11 protons.
However, an atom can gain electrons to become negatively charged and a loose electron to become positively charged.
Explanation – there are 11 electrons and 11 protons in a neutral sodium atom. If sodium loses one electron, the number of
proton remain 11 and the number of electrons reduces to 10. Since, there is one more positive charge than negative
sodium becomes positively charged.
That can be written as
Na Na
+
+ e
-
Sodium produces sodium ion on losing one electron.
Types of ions
1. Cations – A positively charged ion is called cation. Cations are formed when an atom/molecule loses
electron. Metals usually form cations.
2. Anions – A negatively charged ion is called anion. Anions are formed when an atom/molecule gains
electrons.
Chemical Formula
The symbolic representation of the composition of elements in a compound is called its chemical formula.
*Valency – The combining capacity of an element of molecule is called its valency.
*Details are given in the next chapter.
Valency of some Ions
Rules to write a chemical formula:
? Valencies of the ion should be balanced (the compound should not have any charge as a whole).
? If a compound consists of a metal and a non-metal, name of symbol of the metal should be written
first followed by the name or symbol of the non-metal.
? If the compound is formed from polyatomic ion, the ions should be enclosed in a bracket before
writing the ratio.
Examples
1. Formula of sodium chloride
Name Sodium Chloride
Symbol Na Cl
Valency 1 1
Formula NaCl
Valency Metallic
ions
Symbol Non-metallic
ions
Symbol Polyatomic
ions
Symbol
1
Sodium Na
+
Hydrogen H
+
Ammonium NH4
+
Potassium K
+
Hydride H
-
Hydroxide OH
-
Silver Ag
+
Chloride Cl
-
Nitrate NO3
-
Copper Cu
+
Bromide Br
-
bicarbonate HCO3
-
Iodide I
-
2
Magnesium Mg
2+
Oxide O
2-
Carbonate CO3
2-
Calcium Ca
2+
Sulphide S
2-
Sulphite SO3
2-
Zinc Zn
2+
Sulphate SO4
2-
Iron (II) Fe
2+
Copper (II) Cu
2+
3
Aluminium Al
3+
Nitride N
3-
Phosphate PO4
3-
Iron (III) Fe
3+
2. Formula for Calcium chloride
Name Calcium Chloride
Symbol Ca Cl
Valency 2 1
Formula CaCl 2
3. Formula for Calcium oxide
Name Calcium Oxide
Symbol Ca O
Valency 2 2
Formula Ca 2O 2 but the formula is always written in the simplest ratio and 2/2 = 1:1 so Formula for calcium
chloride is CaO.
4. Formula for Ammonium carbonate
Name Ammonium Carbonate
Symbol NH4 CO3
Valency 1 2
Formula (NH) 2CO 3
Molecular Mass and Mole concept
Molecular mass – Sum of atomic masses of all the atoms present in a molecule.
For e.g. Atomic mass of Na2O = 2 x atomic mass of Na + atomic mass of oxygen
2 x 23 + 16
= 62u.
Formula unit mass – Sum of atomic masses of all the atoms present in a formula unit or empirical formula of
that compound.
Empirical formula of a compound represents the elements of a compound in their simplest ratio regardless of
their chemical composition.
E.g. Disulphur dioxide (S2O2) contains two atoms of both sulphur and oxygen.
Read More
FAQs on Chapter 3 Atoms and Molecules - Class 9
| 1. What are atoms and molecules? |  |
Atoms are the basic building blocks of matter. They are the smallest units of an element that retain its chemical properties. On the other hand, molecules are formed when two or more atoms chemically bond together. They can be made up of atoms of the same element or different elements.
| 2. How are atoms and molecules related? | |
Atoms are the individual units that make up molecules. Molecules are formed when atoms chemically combine with each other through bonding. This bonding may involve sharing of electrons (covalent bonding) or transfer of electrons (ionic bonding) between atoms. Therefore, molecules are composed of two or more atoms held together by chemical bonds.
| 3. What is the difference between an atom and a molecule? | |
The main difference between an atom and a molecule is their composition. An atom is the smallest unit of an element, whereas a molecule is formed when two or more atoms chemically bond together. Atoms can exist as independent entities, while molecules require the presence of multiple atoms.
| 4. How do atoms and molecules contribute to the properties of matter? | |
Atoms and molecules play a crucial role in determining the properties of matter. The arrangement and types of atoms or molecules present in a substance dictate its physical and chemical properties. For example, the number and arrangement of carbon, hydrogen, and oxygen atoms in a molecule of glucose determine its properties as a sugar. Similarly, the arrangement of atoms in a metal determines its conductivity and malleability.
| 5. Can atoms and molecules be seen with the naked eye? | |
No, atoms and molecules cannot be seen with the naked eye as they are extremely tiny. They are on a scale much smaller than the wavelength of visible light, making them invisible to us. However, their existence and behavior can be inferred and studied through various scientific techniques such as microscopy, spectroscopy, and electron microscopy.
Related Exams
About this Document
|
5.7K Views |
|
4.84/5 Rating |
|
Oct 24, 2024 Last updated |
Document Description: Chapter 3 Atoms and Molecules for Class 9 2024 is part of Class 9 preparation. The notes and questions for Chapter 3 Atoms and Molecules have been prepared according to the Class 9 exam syllabus. Information about Chapter 3 Atoms and Molecules covers topics like and Chapter 3 Atoms and Molecules Example, for Class 9 2024 Exam. Find important definitions, questions, notes, meanings, examples, exercises and tests below for Chapter 3 Atoms and Molecules.
Introduction of Chapter 3 Atoms and Molecules in English is available as part of
our Class 9 preparation & Chapter 3 Atoms and Molecules in Hindi for Class 9
courses. Download more important topics, notes, lectures and mock test series for Class 9
Exam by signing up for free. Class 9: Chapter 3 Atoms and Molecules - Class 9
Description
Full syllabus notes, lecture & questions for Chapter 3 Atoms and Molecules - Class 9 - Class 9 | Plus excerises question with solution to help you revise complete syllabus | Best notes, free PDF download
Information about Chapter 3 Atoms and Molecules
In this doc you can find the meaning of Chapter 3 Atoms and Molecules defined & explained in the simplest way possible.
Besides explaining types of Chapter 3 Atoms and Molecules theory,
EduRev gives you an ample number of questions to practice Chapter 3 Atoms and Molecules tests, examples and also practice Class 9 tests.
Download as PDF
|
Explore Courses for Class 9 exam
|
|
Signup for Free!
Signup to see your scores go up within 7 days! Learn & Practice with 1000+ FREE Notes, Videos & Tests.
Related Searches
ppt
,Extra Questions
,Semester Notes
,Previous Year Questions with Solutions
,Chapter 3 Atoms and Molecules - Class 9
,video lectures
,Chapter 3 Atoms and Molecules - Class 9
,Free
,Summary
,Important questions
,Chapter 3 Atoms and Molecules - Class 9
,mock tests for examination
,Exam
,Sample Paper
,study material
,Objective type Questions
,MCQs
,practice quizzes
,past year papers
,Viva Questions
,shortcuts and tricks
;
Additional Information about Chapter 3 Atoms and Molecules for Class 9 Preparation
Chapter 3 Atoms and Molecules Free PDF Download
The Chapter 3 Atoms and Molecules is an invaluable resource that delves deep into the core of the Class 9 exam.
These study notes are curated by experts and cover all the essential topics and concepts, making your preparation more efficient and effective.
With the help of these notes, you can grasp complex subjects quickly, revise important points easily,
and reinforce your understanding of key concepts. The study notes are presented in a concise and easy-to-understand manner,
allowing you to optimize your learning process. Whether you're looking for best-recommended books, sample papers, study material,
or toppers' notes, this PDF has got you covered. Download the Chapter 3 Atoms and Molecules now and kickstart your journey towards success in the Class 9 exam.
Importance of Chapter 3 Atoms and Molecules
The importance of Chapter 3 Atoms and Molecules cannot be overstated, especially for Class 9 aspirants.
This document holds the key to success in the Class 9 exam.
It offers a detailed understanding of the concept, providing invaluable insights into the topic.
By knowing the concepts well in advance, students can plan their preparation effectively.
Utilize this indispensable guide for a well-rounded preparation and achieve your desired results.
Chapter 3 Atoms and Molecules Notes
Chapter 3 Atoms and Molecules Notes offer in-depth insights into the specific topic to help you master it with ease.
This comprehensive document covers all aspects related to Chapter 3 Atoms and Molecules.
It includes detailed information about the exam syllabus, recommended books, and study materials for a well-rounded preparation.
Practice papers and question papers enable you to assess your progress effectively.
Additionally, the paper analysis provides valuable tips for tackling the exam strategically.
Access to Toppers' notes gives you an edge in understanding complex concepts.
Whether you're a beginner or aiming for advanced proficiency, Chapter 3 Atoms and Molecules Notes on EduRev are your ultimate resource for success.
Chapter 3 Atoms and Molecules Class 9 Questions
The "Chapter 3 Atoms and Molecules Class 9 Questions" guide is a valuable resource for all aspiring students preparing for the
Class 9 exam. It focuses on providing a wide range of practice questions to help students gauge
their understanding of the exam topics. These questions cover the entire syllabus, ensuring comprehensive preparation.
The guide includes previous years' question papers for students to familiarize themselves with the exam's format and difficulty level.
Additionally, it offers subject-specific question banks, allowing students to focus on weak areas and improve their performance.
Study Chapter 3 Atoms and Molecules on the App
Students of Class 9 can study Chapter 3 Atoms and Molecules alongwith tests & analysis from the EduRev app,
which will help them while preparing for their exam. Apart from the Chapter 3 Atoms and Molecules,
students can also utilize the EduRev App for other study materials such as previous year question papers, syllabus, important questions, etc.
The EduRev App will make your learning easier as you can access it from anywhere you want.
The content of Chapter 3 Atoms and Molecules is prepared as per the latest Class 9 syllabus.
|
© EduRev
|
Education Revolution
|
Follow Us
|
Signup to see your scores
go up
within 7 days!
within 7 days!
Takes less than 10 seconds to signup