Atomic Structure – TS EAMCET Chemistry Notes
(Prepared for the TS EAMCET exam – concise, clear, and exam‑ready)
1. Simple Explanation
The atomic structure tells us how the atoms of all matter are built.
An atom contains:
| Part | Symbol | What it does | Charge |
|---|---|---|---|
| Nucleus | – | Holds the mass | 0 |
| Protons | p⁺ | Contribute to the element’s identity (atomic number)** | +1 |
| Neutrons | n⁰ | Add mass, no charge | 0 |
| Electrons | e⁻ | Surround the nucleus in shells, determine chemical behaviour | –1 |
Atomic number (Z) = number of protons = number of electrons in a neutral atom.
Mass number (A) = protons + neutrons.
2. Key Points
| Topic | Summary |
|---|---|
| Quantum Numbers | Four numbers describe each electron: • n (principal, shell) = 1,2,3… • ℓ (azimuthal, subshell) = 0–(n‑1) → s,p,d,f • mₗ (magnetic) = –ℓ…+ℓ • mₛ (spin) = ±½ |
| Shell Capacity | Maximum electrons per shell: 2n² (n = 1→2, 2→8, 3→18, 4→32, …). |
| Electron Configuration | Follows the Aufbau principle, Pauli exclusion, Hund’s rule. Use the order: 1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p… |
| Isotopes | Same element (same Z) but different neutron number → different A. |
| Ionisation Energy (IE) | Energy to remove one electron. Increases across a period, decreases down a group. |
| Atomic Radius | Distance from nucleus to outermost electron cloud. Decreases across a period, increases down a group. |
| Periodic Trends | 1. IE ↑ across a period, ↓ down a group. 2. Atomic radius ↓ across a period, ↑ down a group. 3. Electron affinity increases across a period, less clear down a group. |
| Common Notations | Bracket notation: 1s² 2s² 2p⁶… ; IUPAC notation: 1s²2s²2p⁶3s²3p⁶4s²3d¹⁰4p⁶5s²5p⁶. |
3. Worked Example
Problem
Find the electronic configuration and the element name for the ion ( \text{Fe}^{3+} ).
Step‑by‑Step
-
Identify the neutral element
Iron (Fe) has Z = 26.
Neutral Fe: 26 electrons. -
Write the full configuration of Fe
[ \text{Fe: } 1s^2,2s^2,2p^6,3s^2,3p^6,4s^2,3d^6 ] (Using the order 4s before 3d.) -
Remove three electrons for ( \text{Fe}^{3+} )
Electrons are removed from the outermost shell first:
– First remove 4s² (2 e⁻).
– Then remove 3d¹ (1 e⁻). -
Resulting configuration
[ \text{Fe}^{3+}: 1s^2,2s^2,2p^6,3s^2,3p^6,4s^0,3d^5 ] (The 4s orbital is empty; 3d has 5 electrons.) -
Check the number of electrons
( 2+2+6+2+6+0+5 = 23 ) electrons → correct for ( \text{Fe}^{3+} ) (Z = 26, +3 charge).
Answer: The electronic configuration of ( \text{Fe}^{3+} ) is
(1s^2,2s^2,2p^6,3s^2,3p^6,4s^0,3d^5), and the ion belongs to the element Iron (Fe).
4. Quick Revision Summary
- Atomic number (Z) = protons = electrons (neutral).
- Mass number (A) = protons + neutrons.
- Electron arrangement follows Aufbau, Pauli, Hund.
- Shell capacity = 2n².
- Quantum numbers: n (shell), ℓ (subshell), mₗ (orientation), mₛ (spin).
- Isotopes: same Z, different A.
- Ionisation energy ↑ across a period, ↓ down a group.
- Atomic radius ↓ across a period, ↑ down a group.
- Electron configuration written in increasing energy order.
Tip for EAMCET:
- Memorise the first 20‑30 elements’ configurations.
- Practice writing bracket notation for ions.
- Know the order of subshells (1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, …).
Good luck! 🌟