Atomic Structure
1. Dalton's Atomic Theory (1808)
John Dalton proposed the first SCIENTIFIC theory of matter.
Main postulates:
- All matter is made of TINY, indivisible particles called ATOMS.
- Atoms of the SAME element are IDENTICAL in all respects.
- Atoms of DIFFERENT elements have DIFFERENT masses and sizes.
- Atoms COMBINE in simple whole-number ratios to form COMPOUNDS.
- Atoms can neither be CREATED nor DESTROYED in a chemical reaction.
Limitations (later disproved):
- Atoms ARE divisible into subatomic particles.
- Isotopes show that atoms of the SAME element can have DIFFERENT masses.
2. Subatomic Particles
| Particle | Symbol | Charge | Mass | Location |
|---|---|---|---|---|
| Electron | e⁻ | —1 | 9.1 × 10⁻³¹ kg (≈ 1/1840 u) | Around nucleus (shells) |
| Proton | p⁺ | +1 | 1.672 × 10⁻²⁷ kg (≈ 1 u) | In the NUCLEUS |
| Neutron | n⁰ | 0 (NEUTRAL) | 1.674 × 10⁻²⁷ kg (≈ 1 u) | In the NUCLEUS |
Discovery
| Particle | Scientist | Year | Experiment |
|---|---|---|---|
| Electron | J.J. THOMSON | 1897 | Cathode ray tube |
| Proton | E. GOLDSTEIN | 1886 | Canal rays (anode rays) |
| Neutron | James CHADWICK | 1932 | Bombarding beryllium with alpha particles |
3. Atomic Number (Z)
The number of PROTONS in the nucleus of an atom.
Z = Number of protons = Number of electrons (in a NEUTRAL atom)
Example: Carbon has atomic number 6 → 6 protons, 6 electrons.
4. Mass Number (A)
The total number of PROTONS and NEUTRONS in the nucleus.
A = Number of protons + Number of neutrons
A = Z + N (where N = number of neutrons)
Example: Carbon-12: A = 12, Z = 6, N = 12 — 6 = 6
'Standard notation: ᴬ_ᴢX (e.g., ¹²_₆C). A at top left, Z at bottom left.'
5. Electronic Configuration
Electrons are arranged in SHELLS (energy levels) around the nucleus.
| Shell | n | Maximum electrons | Formula |
|---|---|---|---|
| K | 1 | 2 | 2n² |
| L | 2 | 8 | 2n² |
| M | 3 | 18 | 2n² |
| N | 4 | 32 | 2n² |
Rules for filling:
- Electrons fill the LOWEST energy shell FIRST (2, 8, 8 rule for first 20 elements).
- The outermost shell can have a MAXIMUM of 8 electrons.
- The inner shells must be FULLY filled before filling the next shell.
6. Electronic Configuration of First 20 Elements
| Element | Symbol | Atomic No. | Electronic Configuration | Valence Electrons |
|---|---|---|---|---|
| Hydrogen | H | 1 | 1 | 1 |
| Helium | He | 2 | 2 | 2 (full) |
| Lithium | Li | 3 | 2, 1 | 1 |
| Beryllium | Be | 4 | 2, 2 | 2 |
| Boron | B | 5 | 2, 3 | 3 |
| Carbon | C | 6 | 2, 4 | 4 |
| Nitrogen | N | 7 | 2, 5 | 5 |
| Oxygen | O | 8 | 2, 6 | 6 |
| Fluorine | F | 9 | 2, 7 | 7 |
| Neon | Ne | 10 | 2, 8 | 8 (full) |
| Sodium | Na | 11 | 2, 8, 1 | 1 |
| Magnesium | Mg | 12 | 2, 8, 2 | 2 |
| Aluminium | Al | 13 | 2, 8, 3 | 3 |
| Silicon | Si | 14 | 2, 8, 4 | 4 |
| Phosphorus | P | 15 | 2, 8, 5 | 5 |
| Sulphur | S | 16 | 2, 8, 6 | 6 |
| Chlorine | Cl | 17 | 2, 8, 7 | 7 |
| Argon | Ar | 18 | 2, 8, 8 | 8 (full) |
| Potassium | K | 19 | 2, 8, 8, 1 | 1 |
| Calcium | Ca | 20 | 2, 8, 8, 2 | 2 |
7. Valence Electrons and Valency
Valence electrons: Electrons in the OUTERMOST shell. Valency: The COMBINING capacity of an element.
| Valence Electrons | Valency | Examples |
|---|---|---|
| 1 | 1 | H, Li, Na, K |
| 2 | 2 | Be, Mg, Ca |
| 3 | 3 | B, Al |
| 4 | 4 | C, Si |
| 5 | 3 | N, P (tend to gain 3) |
| 6 | 2 | O, S (tend to gain 2) |
| 7 | 1 | F, Cl (tend to gain 1) |
| 8 | 0 | Ne, Ar (inert/noble gases) |
Common Mistakes and Fixes
| Mistake | Fix |
|---|---|
| 'Mass number = number of protons' | Mass number = PROTONS + NEUTRONS. Atomic number = PROTONS only |
| 'Electrons are in the nucleus' | Electrons ORBIT the nucleus in shells. Protons and neutrons are in the NUCLEUS |
| 'The M shell can hold 8 electrons for first 20 elements' | The M shell CAN hold up to 18, but for first 20 elements, it gets at most 8 before N starts filling |
| 'Valency is the number of valence electrons' | For metals, valency = number of valence electrons. For non-metals, valency = 8 — valence electrons |
ICSE Exam Focus (6–8 marks)
- 2-mark questions: Define atomic number, mass number, valency
- 3-mark questions: Write electronic configuration of given elements
- 4-mark questions: Calculate protons, neutrons, electrons from A and Z
- 6-mark questions: Diagram-based questions — draw atomic structure
Self-Test
Q1. An atom has atomic number 12 and mass number 24. Find protons, neutrons, and electrons. A1. Protons = Z = 12. Electrons = 12. Neutrons = A — Z = 24 — 12 = 12.
Q2. Write the electronic configuration of chlorine (Z = 17). A2. Cl = 2, 8, 7 (K=2, L=8, M=7).
Q3. What is the valency of magnesium? (Z = 12) A3. Mg configuration = 2, 8, 2. Valence electrons = 2. Since Mg is a metal, it LOSES 2 electrons → valency = 2.
Q4. Who discovered the electron? Describe the experiment. A4. J.J. Thomson in 1897, through cathode ray tube experiments. He observed rays that were DEFLECTED by electric and magnetic fields, and calculated the charge-to-mass ratio.
Q5. State one postulate and one limitation of Dalton's atomic theory. A5. Postulate: Atoms combine in simple whole-number ratios. Limitation: Atoms are divisible (subatomic particles exist).
Q6. Why is helium (Z=2) chemically inert? A6. Helium has electronic configuration 2 (K shell full). Its VALENCE shell is COMPLETE with 2 electrons. It has NO tendency to gain, lose, or share electrons.
