|Table of contents|
|Laws of Chemical Combination|
|John Dalton's Atomic Theory|
|What is an Atom?|
|Modern-Day Symbols of Atoms|
|What is a Molecule?|
|Molecules of Compounds|
|What is an Ion?|
|Writing Chemical Formulae|
- This concept was based on philosophical considerations and did not have experimental validation at that time.
- It was later in the eighteenth century that Antoine L. Lavoisier and Joseph L. Proust conducted experiments to establish the laws of chemical combination.
- However, it was Maharishi Kanad who first suggested the concept of dividing matter into smaller particles.
The following two laws of chemical combination were established after much experimentation by Lavoisier and Joseph L. Proust.
Law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction.
The Law of Conservation of Mass, established by Lavoisier and Joseph L. Proust, addresses the question of mass change during a chemical reaction. An experiment is conducted using different pairs of chemicals (X and Y), observing the mass before and after mixing the solutions. The law asserts that mass remains constant in a chemical reaction—neither created nor destroyed.
- This means that the total mass of the reactants before the reaction is equal to the total mass of the products after the reaction.
- The experiment described in the passage demonstrates this law by showing that the mass of the flask remains the same before and after the reaction, regardless of the chemical pair used.
- Therefore, the correct answer is option C: The mass remains constant.
The law states that in a chemical substance, the elements are always present in definite proportions by mass.
For example, water always contains hydrogen and oxygen in the same proportion 1 : 8 by mass, whatever the source of water, from the river, well or rainwater.
John Dalton's Atomic Theory, emerging as a result of these laws, introduces the concept of atoms as the fundamental particles of matter.
Background on John Dalton
An atom is the defining structure of an element, which cannot be broken by any chemical means.
- This means that regardless of the source of the substance, the elements will always be present in the same ratio by mass.
- For example, water always contains hydrogen and oxygen in the same proportion of 1:8 by mass.
- This law helps in understanding the composition of chemical substances and their reactions.
- He represented one atom of an element with a symbol.
- Berzelius, on the other hand, suggested constructing symbols from one or two letters of the element's name.
- IUPAC is an international scientific organization that approves names, symbols, and units, but they did not pioneer the use of symbols for elements.
Atomic mass of some elements
A molecule may be defined as the smallest particle of an element or a compound that is capable of independent existence and shows all the properties of that substance.
1. Monoatomic Molecules: Elements like Argon (Ar) and Helium (He) consist of only one atom of that element in their molecules.
2. Diatomic Molecules: Nonmetals like Oxygen (O2), Hydrogen (H2), Nitrogen (N2), and Chlorine (Cl2) form molecules with two atoms of the same element, and this is known as diatomic molecules.
3. Polyatomic Molecules: Some elements, like Phosphorus (P4) and Sulphur (S8), can form molecules consisting of more than two atoms. For example, phosphorus forms tetra-atomic molecules (P4), and sulphur forms polyatomic molecules.
- It is the sum of the masses of all the subatomic particles within the atom.
- Option A correctly defines the atomic mass as the total mass of the electrons, neutrons, and protons in an atom.
- Option B is incorrect because it refers specifically to the mass of a carbon-12 atom, not the general atomic mass.
- Option C is incorrect because it refers to the average mass of a group of atoms, not the mass of a single atom.
- Option D is incorrect because it does not specify that the atomic mass includes all the subatomic particles.
As a teacher, it is important to explain to students that the atomic mass is a fundamental property of an atom and represents the total mass of its constituent particles.
The number of atoms constituting a molecule is known as its atomicity.
Atomicity of some non-metals
Atoms of different elements combine in definite proportions to form molecules of compounds.
- It is a measure of how many atoms are chemically bonded together within a molecule.
- The atomicity of a molecule determines its chemical properties and behavior.
- For example, oxygen gas (O2) has an atomicity of 2 because it consists of two oxygen atoms bonded together.
- Similarly, carbon dioxide (CO2) has an atomicity of 3 because it contains one carbon atom and two oxygen atoms bonded together.
- Therefore, the correct answer is Option A: The number of atoms in a molecule.
To write the chemical formulae of binary compounds (made up of two different elements), one can use the valencies of the ions involved. The process involves a "crossover" of the valencies of the combining atoms. Example
The molecular mass of a substance is the sum of the atomic masses of all the atoms in a molecule of that substance. It is expressed in atomic mass units (u).
Example 1: (a) Calculate the relative molecular mass of water (H₂O).
(b) Calculate the molecular mass of HNO₃.
Solution: (a) Atomic mass of hydrogen = 1u, oxygen = 16u. So, the molecular mass of water (2H + 1O) = 2 × 1 + 1 × 16 = 18u.
(b) Molecular mass of HNO₃ (H + N + 3O) = 1 + 14 + 3 × 16 = 63u.
- The formula for binary compounds involves a "crossover" of the valencies of the combining atoms.
- In the case of magnesium chloride, the valency of magnesium (2) is crossed over to become the subscript for chlorine.
- Therefore, the chemical formula for magnesium chloride is MgCl?, indicating that there are two chlorine atoms for every magnesium atom.
As a teacher, it is important to explain the concept of valency and how it influences the chemical formula of binary compounds. Students should understand that the valency of an element determines its combining capacity and how it interacts with other elements to form compounds.
Formula unit mass, calculated similarly to molecular mass, is applicable to substances with ions as constituent particles. It represents the sum of the atomic masses of all atoms in a formula unit of a compound.
Example 2: Calculate the formula unit mass of CaCl₂.
Solution: Atomic mass of Ca + (2 × atomic mass of Cl) = 40 + 2 × 35.5 = 40 + 71 = 111u.
|1. What is John Dalton's Atomic Theory?|
|2. What is the difference between an atom and a molecule?|
|3. What is an ion?|
|4. What are the laws of chemical combination?|
|5. How are chemical formulae written?|