1. What is an Atom?
An atom is the smallest particle of an element that retains the properties of that element.
2. Subatomic Particles
| Particle | Symbol | Relative Mass | Relative Charge | Location |
|---|---|---|---|---|
| Proton | p⁺ | 1 | +1 | Nucleus |
| Neutron | n | 1 | 0 | Nucleus |
| Electron | e⁻ | 1/1836 | -1 | Orbitals/Shells |
3. Atomic Number (Z) and Mass Number (A)
- Atomic Number (Z): Number of protons in an atom.
- Mass Number (A): Total number of protons + neutrons.
Example:
- Carbon has 6 protons and 6 neutrons.
- Atomic number = 6, Mass number = 12
So, we write:
₆¹²C
4. Isotopes
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
Example:
- ₁₇³⁵Cl and ₁₇³⁷Cl are isotopes of chlorine.
Both have 17 protons, but 18 and 20 neutrons respectively.
🔹 Uses of Isotopes:
- Carbon-14: Carbon dating
- Cobalt-60: Cancer treatment
- Iodine-131: Thyroid treatment
5. Electronic Configuration
Electrons are arranged in energy levels or shells around the nucleus:
- 1st shell: holds max 2 electrons
- 2nd shell: holds max 8 electrons
- 3rd shell: holds max 8 electrons (for O level)
Examples:
| Element | Atomic Number | Electron Configuration |
|---|---|---|
| H | 1 | 1 |
| He | 2 | 2 |
| Li | 3 | 2,1 |
| C | 6 | 2,4 |
| O | 8 | 2,6 |
| Na | 11 | 2,8,1 |
| Cl | 17 | 2,8,7 |
6. Groups and Periods in the Periodic Table
- Group: Number of electrons in the outer shell
- Period: Number of electron shells
Example:
- Na (2,8,1) → Group 1, Period 3
7. Ions
When atoms lose or gain electrons, they become ions.
- Cation: A positive ion (loss of electrons)
- Anion: A negative ion (gain of electrons)
Examples:
- Na → Na⁺ + e⁻ (Cation)
- Cl + e⁻ → Cl⁻ (Anion)
8. Noble Gases and Stability
- Group 18 (Noble Gases) have full outer shells → very stable and unreactive
- Atoms tend to gain, lose, or share electrons to achieve noble gas configuration.
9. Relative Atomic Mass (Ar)
The average mass of all the isotopes of an element compared to 1/12 of a carbon-12 atom.
Example:
Chlorine has two isotopes:
- 75% ³⁵Cl
- 25% ³⁷Cl
Ar=[(75×35)+(25×37)]/100=[2625+925]/100=35.5
10. Common Misconceptions
- Electrons do not orbit in fixed paths like planets — the modern model uses orbitals.
- Mass number includes only protons and neutrons, not electrons.
- Isotopes have the same chemical properties, but different physical properties.
✍️ Practice Questions
- What is the electron configuration of:
- (a) Magnesium (Z=12)
- (b) Oxygen (Z=8)
- Calculate the relative atomic mass of boron given:
- 20% of boron is ¹⁰B
- 80% is ¹¹B
- How many protons, neutrons, and electrons are in:
- (a) ₈¹⁶O
- (b) ₁₁²³Na⁺
📌 Summary
- Atoms are made of protons, neutrons, and electrons.
- Atomic number = protons; Mass number = protons + neutrons.
- Isotopes are atoms with same protons but different neutrons.
- Electron configuration helps identify group and period in the periodic table.
- Ions form to achieve noble gas configuration.
- Relative atomic mass is weighted average of isotopes.
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