Atomic Structure
1. Atoms and Molecules
Atom
An ATOM is the SMALLEST particle of an element that can take part in a chemical reaction.
- Atoms are EXTREMELY small (radius ~10⁻¹⁰ m).
- An atom CANNOT be divided by ordinary chemical means.
Molecule
A MOLECULE is the SMALLEST particle of an element or compound that can EXIST INDEPENDENTLY.
Examples:
- Oxygen gas: O₂ (2 atoms of oxygen).
- Water: H₂O (2 atoms of hydrogen, 1 atom of oxygen).
- Hydrogen: H₂.
Difference Between Atom and Molecule
| Feature | Atom | Molecule |
|---|---|---|
| Definition | Smallest particle of element | Smallest particle of element or compound |
| Independent existence | NO (except noble gases) | YES |
| Composition | Single particle | Two or more atoms bonded |
| Example | H, O, N (not alone) | H₂, O₂, H₂O |
2. Dalton's Atomic Theory (1808)
John Dalton proposed the first MODERN atomic theory.
Main Postulates (Simplified)
- All matter is made of TINY PARTICLES called ATOMS.
- Atoms of the SAME element are IDENTICAL.
- Atoms of DIFFERENT elements are DIFFERENT.
- Atoms COMBINE in FIXED WHOLE NUMBER ratios to form COMPOUNDS.
- Atoms can neither be CREATED nor DESTROYED (Law of Conservation of Mass).
Limitations
- Later discovered that atoms of the same element may have DIFFERENT masses (ISOTOPES).
- Atom is NOT indivisible (it contains subatomic particles).
3. Subatomic Particles
An atom is made of THREE fundamental particles:
| Particle | Symbol | Charge | Mass (approx) | Location |
|---|---|---|---|---|
| Proton | p⁺ | +1 | 1 amu (1.67 × 10⁻²⁷ kg) | Nucleus |
| Neutron | n⁰ | 0 (neutral) | 1 amu | Nucleus |
| Electron | e⁻ | -1 | 1/1837 amu (negligible) | Shells around nucleus |
The Nuclear Model
- Nucleus: Central part of atom containing PROTONS and NEUTRONS.
- Electron shells/orbits: Regions around nucleus where ELECTRONS revolve.
- The nucleus is TINY but contains ALMOST ALL the mass.
4. Atomic Number and Mass Number
Atomic Number (Z)
The NUMBER OF PROTONS in the nucleus of an atom.
- Each element has a UNIQUE atomic number.
- In a NEUTRAL atom: Number of electrons = Number of protons = Z.
Examples: Hydrogen (Z = 1), Carbon (Z = 6), Oxygen (Z = 8), Iron (Z = 26).
Mass Number (A)
The TOTAL NUMBER of protons and neutrons in the nucleus. A = Z + N, where N = number of neutrons.
Notation
Element is written as: ᴬZX Example: ¹²₆C (Carbon: Z = 6, A = 12. Neutrons = 12 - 6 = 6).
Worked Example (ICSE 2024, 2 marks)
'An atom has atomic number 17 and mass number 35. Find the number of protons, electrons, and neutrons.'
Solution: Protons = Z = 17. Electrons = 17 (neutral atom). Neutrons = A - Z = 35 - 17 = 18.
5. Electronic Configuration
Electrons are arranged in SHELLS (energy levels) around the nucleus.
Shells
| Shell | Maximum Electrons (2n²) | n value |
|---|---|---|
| K | 2 | 1 |
| L | 8 | 2 |
| M | 18 | 3 |
| N | 32 | 4 |
Rules for Filling
- Electrons fill the INNERMOST shell FIRST.
- First shell (K) can hold up to 2 electrons.
- Second shell (L) up to 8 electrons.
- Third shell (M) up to 18, but for first 20 elements, the OUTERMOST shell holds max 8.
Electronic Configuration of First 20 Elements
| Element | Symbol | Z | Configuration |
|---|---|---|---|
| Hydrogen | H | 1 | 1 |
| Helium | He | 2 | 2 |
| Carbon | C | 6 | 2, 4 |
| Nitrogen | N | 7 | 2, 5 |
| Oxygen | O | 8 | 2, 6 |
| Sodium | Na | 11 | 2, 8, 1 |
| Chlorine | Cl | 17 | 2, 8, 7 |
| Calcium | Ca | 20 | 2, 8, 8, 2 |
Valence Electrons
Electrons in the OUTERMOST shell are called VALENCE ELECTRONS. They determine the CHEMICAL PROPERTIES and VALENCY of the element.
6. Isotopes
Atoms of the SAME element having the SAME atomic number but DIFFERENT mass numbers.
Examples:
- Hydrogen: Protium (¹H), Deuterium (²H), Tritium (³H).
- Carbon: ¹²C, ¹³C, ¹⁴C.
Isotopes have IDENTICAL chemical properties but slightly DIFFERENT physical properties.
7. ICSE Exam Focus
| Topic | Marks | Frequency |
|---|---|---|
| Subatomic particles (charge, location) | 2 marks | Very High |
| Atomic number, mass number calculations | 2-3 marks | Very High |
| Electronic configuration (1-20) | 3 marks | Very High |
| Isotopes | 2 marks | Medium |
| Dalton's theory (basic postulates) | 2 marks | Medium |
Common Mistakes
- Saying proton has negative charge (it is POSITIVE).
- Writing atomic number = mass number (they are DIFFERENT concepts).
- Filling electrons in wrong order (K shell fills FIRST, then L, then M).
- Not knowing that mass number = protons + neutrons.
Self-Test (5 Questions)
Q1. Which particle has the LEAST mass? (1 mark)
- A) Proton
- B) Neutron
- C) Electron
- D) All have same mass
Q2. 'An atom has Z = 11 and A = 23. Find the number of neutrons.' (2 marks)
Q3. Write the electronic configuration of chlorine (Z = 17). (2 marks)
Q4. 'What are isotopes? Give one example.' (2 marks)
Q5. 'In a neutral atom, which two particles are equal in number?' (1 mark)
Answers
A1. C) Electron (1/1837 amu — negligible compared to proton/neutron). A2. Neutrons = A - Z = 23 - 11 = 12. A3. 2, 8, 7. (K=2, L=8, M=7.) A4. Isotopes are atoms of the SAME element with same Z but different A. Example: Protium (¹H), Deuterium (²H), Tritium (³H) — all hydrogen isotopes. A5. Protons and electrons (both equal to atomic number Z).
