The Language of Chemistry

1. Introduction to Elements and Molecules

• Elements & Atoms: An element is a pure, simple form of matter that cannot be decomposed into simpler substances. Atoms are the smallest particles representing all properties of an element.
• Molecules: The smallest particle capable of independent existence. Molecules can be monoatomic (e.g., Helium), diatomic (e.g., Oxygen - O₂), tetratomic (e.g., Phosphorus - P₄), or octatomic (e.g., Sulphur - S₈).

2. Chemical Symbols

• Definition: A short form or abbreviation standing for the atom of a specific element.
• Evolution: Early alchemists used pictographic symbols. Dalton used circles with inner markings. Johann Berzelius suggested using the initial letters of elements.
• Modern System (IUPAC): Usually the first letter (capitalized) or first two letters (first capital, second small) of the element's English or Latin name (e.g., Copper is 'Cu' from Cuprum, Sodium is 'Na' from Natrium).
• Significance: A symbol represents the name of the element, one atom of that element, and a quantity of the element equal to its atomic mass.

3. Chemical Formula

• Definition: The symbolic representation of a molecule denoting the atoms of constituent elements.
• Significance: Represents one molecule of the compound, the exact number of different atoms present, the molecular mass of the compound, and the mass ratio of the elements present.

4. Valency

• Classic Definition: The combining capacity of an atom or radical, measured by the number of hydrogen atoms or double the number of oxygen atoms it can combine with.
• Modern Definition: The number of electrons an atom can lose, gain, or share to attain a stable electron configuration.
• Cations & Anions: Metals generally lose electrons to form positive ions (cations). Non-metals generally gain electrons to form negative ions (anions).
• Variable Valency: Some elements exhibit more than one valency by losing electrons from the penultimate shell (e.g., Iron can be Fe²⁺ or Fe³⁺). Suffixes like "ous" (lower) and "ic" (higher), or Roman numerals, are used to name them.

5. Radicals

• Definition: An atom or a group of atoms behaving as a single unit with a positive or negative charge.
• Types: Simple radicals consist of a single atom (e.g., Na⁺). Compound (polyatomic) radicals consist of multiple atoms (e.g., SO₄²⁻).
• Basic vs. Acidic: Basic radicals (cations) are contributed by a base during neutralization, while acidic radicals (anions) are contributed by an acid.

6. Writing Chemical Formulae & Naming Compounds

• Criss-Cross Method: Write the basic radical left, acidic right. Write valencies on top, divide by highest common factor, and interchange valencies as subscripts. Enclose polyatomic radicals in brackets if the valency is > 1.
• Naming Rules: Metal named first, then non-metal with an "-ide" suffix. For two non-metals, use prefixes (mono, di, tri). For oxygen-containing compounds, use suffixes like "-ite" (less oxygen) or "-ate" (more oxygen), and prefixes like "hypo-" or "per-".
• Trivial Names: Some compounds are known by common, widely accepted names rather than systematic rules (e.g., Ammonia for NH₃, Water for H₂O).

7. Chemical Equations

• Definition: The symbolic representation of a chemical reaction using symbols and formulae of reactants (left) and products (right).
• Law of Conservation of Mass: Matter can neither be created nor destroyed. A chemical equation must be balanced (equal number of atoms of each element on both sides) to comply with this law. Unbalanced equations are known as skeleton equations.
• Hit and Trial Method: Balancing is done by counting atoms of each element on both sides and multiplying by suitable integers, starting with the element occurring least frequently.
• Information Conveyed: Shows actual chemical changes, reactants/products involved, molecular quantities, chemical composition, relative masses, and proves the conservation of mass.
• Limitations & Improvements: A standard equation doesn't show physical state, time taken, heat absorption/evolution, concentration, or rate. It is made more informative by adding state symbols (s, l, g, aq), conditions above the arrow, and heat indicators (+ Heat / - Heat).

8. Atomic Mass, Molecular Mass & Empirical Formula

• Relative Atomic Mass (Atomic Weight): A number showing how many times one atom of an element is heavier than 1/12th the mass of a Carbon-12 atom. Expressed in atomic mass units (amu).
• Relative Molecular Mass (Molecular Weight): Obtained by adding together the relative atomic masses of all atoms present in a molecule.
• Percentage Composition: The percentage by weight of each element present in a compound. Calculated as: (Total weight of the element / Gram molecular weight of the compound) × 100.
• Empirical Formula: The simplest formula of a compound, representing the simplest whole-number ratio of atoms of different elements present in one molecule (e.g., Glucose C₆H₁₂O₆ has an empirical formula of CH₂O).