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 |

**Key atomic models to remember:**

• Dalton: Atoms are indivisible solid spheres
• Thomson: Plum-pudding model (electrons embedded in positive sphere)
• Rutherford: Nuclear model (small dense nucleus, electrons around it)
• Bohr: Electrons in fixed circular orbits/shells with definite energy

Worked Examples

**Example 1: Finding protons, neutrons, and electrons**

*Problem*: An atom has atomic number 11 and mass number 23. Find the number of protons, neutrons, and electrons.

*Solution*:

• Protons = Atomic number = 11
• Electrons = Protons (for neutral atom) = 11
• Neutrons = Mass number − Atomic number = 23 − 11 = 12

*Answer*: 11 protons, 12 neutrons, 11 electrons (This is sodium)

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**Example 2: Electron configuration**

*Problem*: Write the electron configuration of chlorine (atomic number 17).

*Solution*:

• Total electrons = 17
• K-shell (n=1): Maximum 2 → Fill 2
• L-shell (n=2): Maximum 8 → Fill 8
• M-shell (n=3): Remaining = 17 − 2 − 8 = 7

*Answer*: 2, 8, 7 *Valence electrons* = 7, so *valency* = 8 − 7 = 1

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**Example 3: Calculating molecular mass**

*Problem*: Calculate the molecular mass of calcium carbonate (CaCO₃). Given: Ca = 40, C = 12, O = 16.

*Solution*:

• CaCO₃ contains: 1 Ca + 1 C + 3 O
• Molecular mass = 40 + 12 + (3 × 16) = 40 + 12 + 48 = 100 amu

*Answer*: 100 amu

Common Mistakes

• **Confusing atomic number with mass number** → Remember: Atomic number = protons only; mass number = protons + neutrons. The smaller number is usually atomic number.

• **Forgetting that electrons equal protons only in neutral atoms** → In ions, electrons ≠ protons. A cation has fewer electrons; an anion has more.

• **Applying 2n² blindly to all shells** → While the formula gives maximum capacity, actual filling follows specific rules. M-shell can hold 18 but often has only 8 before N-shell starts filling in lighter elements.

• **Calculating valency incorrectly for elements with more than 4 valence electrons** → Students often state valency = valence electrons for all cases. Correct approach: if valence electrons > 4, valency = 8 − valence electrons.

• **Thinking isotopes are different elements** → Isotopes have the same atomic number (same element) but different mass numbers. Carbon-12 and Carbon-14 are both carbon.

• **Mixing up atom and molecule** → O is an oxygen atom; O₂ is an oxygen molecule. Hydrogen exists naturally as H₂ (diatomic molecule), not as single H atoms.

Quick Reference

• **Atom**: smallest unit of element; **Molecule**: smallest unit that exists independently
• **Proton (+1), Neutron (0), Electron (−1)** — protons and neutrons in nucleus, electrons in shells
• **Atomic number (Z)** = protons = electrons (neutral); **Mass number (A)** = protons + neutrons
• **Shell capacity**: K=2, L=8, M=18 (use 2n²)
• **Valency**: combining capacity; equals valence electrons if ≤4, equals (8 − valence electrons) if >4
• **Isotopes**: same Z, different A (same element, different neutrons)
• **Molecular mass** = sum of atomic masses of constituent atoms