Laws of Chemical Combination and Dalton’s Atomic Theory

• The concept of atoms was initially proposed by ancient Indian and Greek philosophers, with the word 'atom' derived from the Greek term meaning indivisible
• Modern understanding has evolved, revealing that atoms are divisible into subatomic particles

Laws of Chemical Combination

• Antoine Lavoisier proposed the Law of Conservation of Mass
• The Law of Conservation of Mass, states that mass is neither created nor destroyed in a chemical reaction, with the total mass of reactants equaling that of products
• Joseph Proust proposed the Law of Constant Proportion (Definite Proportions)
• The Law of Constant Proportion (Definite Proportions) states that a chemical compound always contains the same elements in the same proportion by mass
• In water, the mass ratio of hydrogen to oxygen is consistently 1:8

Dalton’s Atomic Theory

• John Dalton proposed the Atomic Theory in 1808.
• Matter consists of tiny particles called atoms
• Atoms are indivisible and indestructible according to the theory
• All atoms of a given element are identical in mass and properties
• Atoms of different elements have different masses and properties
• Compounds form through the combination of atoms of different elements in fixed, simple, whole-number ratios
• Chemical reactions involve the rearrangement of atoms, not their creation or destruction

Limitations of Dalton’s Atomic Theory

• The theory does not account for subatomic particles, contradicting the indivisibility of atoms, disproved by the discovery of protons, electrons, and neutrons
• It doesn’t account for isotopes; elements can have different atomic masses

Atoms

• The smallest unit of an element retains the properties of that element
• Atoms consist of subatomic particles: protons, neutrons, and electrons
• Protons and neutrons reside in the nucleus, with electrons orbiting around it
• Atomicity is 1 for single-element, monoatomic atoms

Valency

• Valence electrons are those in the atom's outermost orbit
• An atom's valency is determined by its ability to gain, lose, or share valence electrons to complete its octet

Atomic Mass

• An atom's mass is primarily due to protons and neutrons
• Electrons contribute negligible mass
• One atomic mass unit (amu) is defined as one-twelfth the mass of a carbon-12 atom

Molecules

• A molecule comprises two or more atoms chemically bonded together
• Molecules can be elements, and contain the same atoms (e.g., O₂) or compounds (e.g., H₂O)
• Molecules of elements contain atoms of the same (Homo atomic) types of elements
• Diatomic molecules have an atomicity, number of atoms present in the molecule, of 2, such as H2,O2,Cl2
• O3 is Triatomic with an atomicity of 3
• P4 is tetra atomic with an atomicity of 4
• Molecules of a compound contain atoms of two or more different types (Hetero atomic) of elements
• CO, HCl, NaCl are diatomic
• CO2,H2O are triatomic
• NH3 is tetra atomic
• The molecular formula represents the number and type of atoms in a molecule

Molecular Mass

• The sum of the atomic masses of all atoms in a molecule.
• To calculate add the atomic masses of the constituent atoms in the molecular formula
• The molecular mass of H2O = (2 × Mass of one H-atom) + (Mass of one O-atom) = (2 × 1) + 16 = 18u

Ions

• Atoms or groups of atoms with a net electric charge due to the loss or gain of electrons
• Cations are positively charged ions, e.g., Na⁺
• Anions are negatively charged ions, e.g., Cl⁻

Chemical Formulae

• Represent the composition of molecules and compounds
• Empirical formulas show the simplest whole-number ratio of atoms in a compound
• Molecular formulas show the actual number of atoms of each element in a molecule

Writing Chemical Formulae

• Write the symbol of the elements involved
• Write the valency of each element
• Cross-over the valency numbers to balance the charges

The lesson covers the laws of chemical combinations, including the Law of Conservation of Mass and the Law of Constant Proportion. It also discusses Dalton's Atomic Theory, which states that matter consists of indivisible and indestructible atoms. The concept of atoms was introduced by ancient philosophers.