🔑 Key Terms and Concepts

• Atom – Smallest particle of an element that carries the element’s properties (Greek: indivisible).
• Atomic structure – Arrangement of protons, neutrons and electrons within an atom.
• Proton – Positively charged particle in the nucleus; relative mass 1, charge +1.
• Neutron – Neutral particle in the nucleus; relative mass 1, charge 0.
• Electron – Negatively charged particle outside the nucleus; relative mass ≈ 1/1840, charge –1.
• Nucleus – Central core containing protons and neutrons; most of the atom’s mass.
• Atomic number (Z) – Number of protons (= number of electrons in a neutral atom).
• Mass number (A) – Sum of protons + neutrons (nucleon number).
• Isotopes – Atoms of the same element with same protons but different neutrons.
• Electronic configuration – Arrangement of electrons in shells around the nucleus.
• Shell (energy level) – Region at a fixed distance from the nucleus where electrons move.
• Nuclide notation – Symbol showing mass number (top left) and atomic number (bottom left).
• Relative atomic mass (RAM) – Average mass of an atom relative to ¹⁄₁₂ of a carbon-12 atom.
• Valency – Combining power based on outer-shell electrons.
• Nucleons – Protons and neutrons in the nucleus.

1. Introduction to Atomic Structure

Atoms are the fundamental units that make up all matter. Understanding atomic structure helps scientists predict the behaviour of substances and design new materials.

Dalton’s Atomic Theory (1803)

Main assumptions:

1. Matter is made up of tiny particles called atoms.
2. Atoms can neither be created nor destroyed.
3. Atoms of the same element are identical; atoms of different elements differ.
4. Compounds form by combination of atoms in simple whole-number ratios.

Modern modifications:

• Atoms can be created or destroyed in nuclear reactions (fusion/fission).
• Some elements have isotopes (different masses).
• Atoms are made of sub-atomic particles.
• Atoms of different elements may combine in many ratios.

2. Sub-atomic Particles

Discovery Timeline

Properties of Sub-atomic Particles

3. Electronic Configuration

Electrons occupy shells (energy levels) around the nucleus.

• 1st shell (K): maximum 2 electrons
• 2nd shell (L): maximum 8 electrons
• 3rd shell (M): maximum 8 electrons (for the first 20 elements)
• 4th shell (N): starts filling after the 3rd has 8

Electronic Configurations of the First 20 Elements

Stable atoms:

• Duplet – 2 electrons in outer shell (He)
• Octet – 8 electrons in outer shell (Ne, Ar)

4. Atomic Number and Mass Number

Atomic Number (Z)

• Number of protons in an atom
• Also equal to the number of electrons in a neutral atom

Mass Number (A)

• Sum of protons + neutrons
• Also called nucleon number

Nuclide Notation

Written as ^A_ZX

• Hydrogen: ¹₁H
• Oxygen: ¹⁶₈O
• Potassium: ³⁹₁₉K → 19 protons, 20 neutrons, 19 electrons; config 2:8:8:1

5. Isotopes

Definition: Atoms of the same element with the same number of protons but different numbers of neutrons (same atomic number, different mass numbers).

Examples of Isotopes

Applications of Isotopes

• Carbon dating – Carbon-14 determines age of ancient objects.
• Medicine – Iodine-131 (thyroid), Cobalt-60 (radiotherapy), Technetium-99 (imaging).
• Agriculture – Phosphorus-32 tracks nutrient uptake in plants.
• Industry – Sodium-24 detects pipe leaks; tritium tracks water movement.
• Energy – Uranium-235 used in nuclear fission for electricity.

6. Relative Atomic Mass (RAM)

Definition: The average mass of one atom of an element relative to ¹⁄₁₂ the mass of one carbon-12 atom.

Because most elements have isotopes, the relative atomic mass is rarely a whole number (e.g. chlorine ≈ 35.5).

7. End-of-Topic Questions

Section A: Multiple Choice

1. Which description corresponds to a proton?
   A) Relative mass = 1, charge = +1
   B) Relative mass = 1, charge = 0
   C) Relative mass = 1/1840, charge = –1
   D) Relative mass = 4, charge = +2
2. What are nucleons?
   A) Neutrons and electrons
   B) Neutrons and protons
   C) Electrons and protons
   D) Protons, neutrons and electrons
3. Most atoms are neutral because they have:
   A) Only neutrons in the nucleus
   B) Equal numbers of electrons and protons
   C) Neutrons with zero charge
   D) The number of electrons balancing the number of protons
4. Isotopes of an element:
   A) Contain the same number of protons but different numbers of neutrons
   B) Have the same number of neutrons but different protons
   C) Have different numbers of electrons
   D) Have electrons added to the nucleus
5. The atomic number of element T is 9. Its electronic configuration is:
   A) 2:4:3
   B) 2:5:2
   C) 2:7
   D) 2:6:1
6. The number of neutrons in an atom is calculated by:
   A) Atomic number – Mass number
   B) Mass number – Atomic number
   C) Mass number + Atomic number
   D) Atomic number × Mass number

Section B: Structured Questions

1. Define the terms: (a) atomic number (b) mass number (c) isotope.
2. An atom of element X has mass number 27 and atomic number 13.
   (a) How many protons, neutrons and electrons does it have?
   (b) Write its electronic configuration.
   (c) Write its nuclide notation.
3. Explain why isotopes of the same element have similar chemical properties but different physical properties.
4. List three applications of isotopes in everyday life or industry.
5. Calculate the relative atomic mass of chlorine if 75% is ³⁵Cl and 25% is ³⁷Cl.

8. Suggested Answers

Section A

1. A
2. B
3. D (or B)
4. A
5. C (2:7)
6. B

Section B

1. (a) Atomic number = number of protons in the nucleus.
   (b) Mass number = number of protons + neutrons.
   (c) Isotopes = atoms of the same element with same number of protons but different numbers of neutrons.
2. (a) Protons = 13, Neutrons = 27 – 13 = 14, Electrons = 13
   (b) 2:8:3
   (c) ²⁷₁₃X
3. Chemical properties depend on the number of electrons (especially outer electrons). Isotopes have the same number of electrons, so similar chemistry. Physical properties (mass, density, melting point) depend on mass; different numbers of neutrons give different masses.
4. Any three: carbon dating (C-14), medical imaging/treatment (I-131, Co-60, Tc-99), detecting pipe leaks (Na-24), tracking plant nutrients (P-32), nuclear energy (U-235).
5. RAM = (35 × 75 + 37 × 25) / 100 = (2625 + 925) / 100 = 3550 / 100 = 35.5