Chemistry — Atoms and Molecules

Overview

Atoms and Molecules form the foundational chapter for understanding all of chemistry at the upper-primary level. This topic appears consistently in KAR TET Paper II (Mathematics and Science) and tests your grasp of atomic structure, subatomic particles, electron arrangement, and how atoms combine to form molecules. Questions typically involve calculating atomic mass, identifying electron configurations, distinguishing between atoms and molecules, and applying valency concepts.

For TET purposes, you must be comfortable with Dalton's atomic theory, Thomson and Rutherford's models, Bohr's model, the concept of shells and subshells, and basic molecular formation rules. The pedagogy angle often asks how to teach abstract atomic concepts using models and analogies—making this topic both content-heavy and pedagogically significant.

Mastering this chapter also builds the base for understanding chemical reactions, bonding, and the periodic table—topics that frequently interconnect in the exam.

Key Concepts

• **Atom**: The smallest particle of an element that retains its chemical properties. Cannot be created or destroyed in a chemical reaction (law of conservation of mass).

• **Molecule**: The smallest particle of an element or compound that can exist independently. Formed when two or more atoms bond together (e.g., H₂, H₂O, CO₂).

• **Subatomic particles**: Protons (positive charge, in nucleus), neutrons (no charge, in nucleus), and electrons (negative charge, orbiting nucleus). Mass of proton ≈ mass of neutron ≈ 1 amu; electron mass is negligible (1/1836 of proton).

• **Atomic number (Z)**: Number of protons in the nucleus. Defines the element. For a neutral atom, Z = number of electrons.

• **Mass number (A)**: Total number of protons + neutrons. Written as A = Z + N, where N is the number of neutrons.

• **Isotopes**: Atoms of the same element with equal atomic number but different mass numbers (different neutrons). Example: Carbon-12, Carbon-14.

• **Electron shells (K, L, M, N...)**: Electrons occupy energy levels around the nucleus. Maximum electrons in a shell = 2n², where n is the shell number. K-shell holds 2, L-shell holds 8, M-shell holds 18.

• **Valency**: Combining capacity of an atom, determined by electrons in the outermost shell (valence electrons). Atoms combine to achieve a stable configuration (usually 8 electrons in outermost shell—octet rule, or 2 for the first shell—duplet rule).

Formulas / Key Facts

| Formula / Fact | Context | |----------------|---------| | Mass number (A) = Protons + Neutrons | Basic atomic composition | | Atomic number (Z) = Number of protons = Number of electrons (neutral atom) | Identifying elements | | Maximum electrons in shell = 2n² | n=1 gives 2; n=2 gives 8; n=3 gives 18 | | Relative atomic mass unit: 1 amu = 1/12 mass of C-12 atom | Standard for atomic mass | | Molecular mass = Sum of atomic masses of all atoms in molecule | For H₂O: 2(1) + 16 = 18 amu | | Valency = 8 − valence electrons (if valence electrons > 4) | For oxygen with 6 valence electrons: valency = 2 | | Valency = valence electrons (if valence electrons ≤ 4) | For sodium with 1 valence electron: valency = 1 | | Avogadro's number = 6.022 × 10²³ particles per mole | One mole of any substance |