Basic Units: Atoms and Molecules

Atoms and molecules are the fundamental building blocks of matter. Understanding their structure and properties is crucial in the field of science. Let's delve into the details of atoms and molecules.

Atoms:
Atoms are the smallest units of matter that retain the chemical properties of an element. They are composed of three subatomic particles: protons, neutrons, and electrons.

- Protons: Positively charged particles found in the nucleus of an atom. They determine the atomic number of an element.
- Neutrons: Neutral particles also present in the nucleus. They contribute to the atomic mass of an element.
- Electrons: Negatively charged particles that orbit the nucleus in specific energy levels or shells.

Molecules:
Molecules are formed when two or more atoms chemically combine. They can be composed of the same type of atoms (e.g., oxygen molecule - O2) or different types of atoms (e.g., water molecule - H2O).

- Chemical Bonds: Atoms in a molecule are held together by chemical bonds. There are three types of chemical bonds: covalent, ionic, and metallic.
- Covalent Bond: Formed when atoms share electrons. This bond is common in molecules composed of non-metals.
- Ionic Bond: Formed when atoms transfer electrons. This bond is typical in compounds composed of metals and non-metals.
- Metallic Bond: Found in metals, where electrons are delocalized and shared among atoms.

Key Concepts:
- Atomic Number: The number of protons in an atom determines the atomic number, which is unique to each element.
- Atomic Mass: The sum of protons and neutrons in an atom gives the atomic mass or mass number.
- Valence Electrons: The electrons in the outermost shell of an atom are known as valence electrons. They determine the chemical properties and reactivity of an element.
- Molecular Formula: Represents the number and types of atoms in a molecule. It provides information about the composition of a compound.

Conclusion:
Atoms and molecules are the fundamental units of matter. Atoms are composed of protons, neutrons, and electrons, while molecules are formed by the chemical combination of atoms. Understanding the structure and properties of atoms and molecules is essential for comprehending the behavior and interactions of different substances in the field of science.

Verification of “Law of Conservation of mass” A solution of sodium chloride and silver nitrate are taken separately in the two limbs of an 'H' shaped tube. The tube is sealed and weighed precisely. The two reactants are made to react by inverting the tube. The following reaction takes place. AgNO3 (aq) + NaCl (aq) • AgCl (s) + NaNO 3 (aq) The whole tube is kept undisturbed for sometime so that the reaction is complete. When the tube is weighed again it is observed that: Weight before the reaction = Weight after the reaction Limitation of “Law of definite proportion” This law does not hold good when the compound is obtained by using different isotopes of the combining elements . Q.1 Why chemical reactions are in accordance with the Law of conservation of mass? Q.2 Calculate the ratio of atoms present in 5 g of magnesium and 5 g of iron. [Atomic mass of Mg=24 u, Fe=56 u] Law of Conservation of mass: proposed by the French chemist Antoine Lavoisier (1774) Mass can neither be created nor destroyed in a chemical reaction. For any chemical process in a closed system, the mass of the reactants must be equal the mass of the products. C + O2 = CO212g +32g = 44g 2. John Daltons Atomic Theory [ for postulates ,refer NCERT text book article 3.1.2 -page no.32 ] Using his theory, Dalton rationalized the various laws of chemical combination which were in existence at that time. However, he assumed that the simplest compound of two elements Q.1 In what respect does Dalton’s Atomic theory hold good even today? Q.2 Which of the following is not the postulate of Dalton’s Atomic theory of matter ? a) Each element is made up of extremely small particles called atoms. b) Atoms of a given element are identical in chemical properties but have different physical properties. c) Atoms cannot be created nor destroyed. d) Compounds are formed by the chemical union of atoms of two or more elements in fixed proportion . 3. Atoms ,Molecules, Ions Chemical Formula Atom Molecule An atom is the smallest particle of an element which can take part in a chemical reaction. It may or may not exist freely. The smallest particle of matter (element or compound) which can exist in a free state. Each atom of an element shows all the properties of the element. The properties of a substance are the properties of its molecules. • MOLECULES OF ELEMENT : The molecules of an element are constituted by the same type of atoms. • MOLECULES OF COMPOUND: Atoms of different elements join together in definite proportions to form molecules of compounds.(hetero atomic molecules) • ATOMICITY : The number of atoms contained in a molecule of a substance (element or compound) is called its atomicity. Element Formula Atomicity Ozone O3 3 Phosphorus P4 4 Sulphur S8 8 Oxygen O 2O2 • Based upon atomicity molecules can be classified as follows. Monoatomic molecules: Noble gases helium, neon and argon exist as He Ne and Ar respectively. Diatomic molecules: H2 , O2 , N2 ,Cl2, CO , HCl . Triatomic molecules: O3 ,CO2 , NO2 . • SYMBOLS • The abbreviation used to represent an element is generally the first letter in capital of the English name of element. capital of the English name of element. Oxygen • O Nitrogen • N • When the names of two or more elements begin with the same initial letter, the initial letter followed by the letter appearing later in the name is used to symbolize the element Barium • Ba Bismuth • Bi ELEMENT LATIN NAME SYMBOL Sodium Natrium Na Copper Cuprum Cu Potassium Kalium K Iron Ferrum Fe Mercury Hydragyrum Hg Tungsten Wolfram W eg: NH4+ - Ammonium Ion ; CO3 2 - Carbonate ion Valency :The number of electrons which an atom can lose , gain or share to form a bond. OR It is the combining capacity of an atom of the element. [ for valency of various cations anions ,refer NCERT text book table 3.6 , page no. 37 ] • Chemical Formula: A chemical formula is a short method of representing chemical elements and compounds. Writing a Chemical Formula -CRISS-CROSS rule RULE 1 [a] write the correct symbols of two elements. Ex : Aluminium & Oxygen Al O [ b] above each symbol, write the correct valence Al3+ O2 - [c] Criss-cross the valence and drop the algebraic sign. Al2O3 RULE 2 When the subscript is number 1, subscript is not written. Ex. Sodium Chloride - Na1+ Cl1 - --- NaCl RULE 4 > When there are multiple numbers of an individual polyatomic ion , parentheses must be used to separate the polyatomic ion from the subscirpt. Ex. Ammonium Sulphate- - NH4 1+ SO4 2-....... (NH4 )2 SO44. Mole Concept The mole (mol) is the amount of a substance that contains as many elementary entities as there are atoms in exactly 12.00 grams of 12C The Avogadro constant is named after the early nineteenth century Italian scientist Amedeo Avogadro. GRAM MOLECULAR MASSGram molecular mass is the mass in grams of one mole of a molecular substance. Ex: The molecular mass of N2 is 28, so the gram molecular mass of N2 is 28 g. ATOMIC MASS UNIT An atomic mass unit or amu is one twelfth of the mass of an unbound atom of carbon-12. It is a unit of mass used to express atomic masses and molecular masses. Also Known As: Unified Atomic Mass Unit MOLECULAR MASS : A number equal to the sum of the atomic masses of the atoms in a molecule. The molecular mass gives the mass of a molecule relative to that of the 12C atom, which is taken to have a mass of 12. Examples: The molecular mass of C2H6 is approximately 30 or [(2 x 12) + (6 x 1)] . Therefore the molecule is about 2.5 times as heavy as the 12C atom or about the same mass as the NO atom with a molecular mass of 30 or (14+16) .