KEAM – Chemistry
Classification of Elements & Periodic Properties
1. Simple Explanation
| Concept | What it means | Why it matters |
|---|---|---|
| Classification of Elements | Dividing all known elements into groups based on common physical & chemical characteristics. | Helps predict behaviour, reactivity, and placement in the periodic table. |
| Periodic Properties | Quantitative attributes (size, energy, charge) that change systematically across periods (rows) and groups (columns). | Allows us to anticipate trends and compare elements without memorising every fact. |
2. Classification of Elements
| Category | Typical Elements | Key Features | Example |
|---|---|---|---|
| Metals | Li, Fe, Cu, Pb | Good conductors, malleable, shiny, lose electrons → positive ions | Iron (Fe) |
| Non‑Metals | C, N, O, S | Poor conductors, brittle, gain electrons → negative ions | Oxygen (O) |
| Metalloids | Si, Ge, As | Properties between metals & non‑metals | Silicon (Si) |
Sub‑categories
| Group | Elements | Representative Properties |
|---|---|---|
| Alkali Metals | Li, Na, K, Rb, Cs, Fr | Highly reactive, one valence electron, low ionisation energy |
| Alkaline Earth Metals | Be, Mg, Ca, Sr, Ba, Ra | Two valence electrons, less reactive than alkali metals |
| Transition Metals | Sc–Zn (except the end‑members), Ti, Fe, Cu, Zn, etc. | Variable oxidation states, high melting points |
| Post‑Transition Metals | Al, Ga, In, Sn, Tl, Pb, Bi | Good conductors, lower melting points than transition metals |
| Halogens | F, Cl, Br, I, At | Seven valence electrons, high electronegativity, form salts |
| Noble Gases | He, Ne, Ar, Kr, Xe, Rn | Eight valence electrons (except He), extremely low reactivity |
| Lanthanides | La–Lu | f‑block, magnetic, used in glass & catalysts |
| Actinides | Ac–Lr | Radioactive, f‑block, used in nuclear energy |
Quick Tip – The first column (period 1) is a special case: H (metal/non‑metal) and He (noble gas).
3. Periodic Properties & Trends
| Property | Trend Across a Period (left → right) | Trend Down a Group (top → bottom) | Qualitative Explanation |
|---|---|---|---|
| Atomic Radius | Decreases | Increases | Electrons are added to higher shells (down) → size grows; nuclear charge pulls electrons tighter (across) → size shrinks |
| Ionisation Energy (IE) | Increases | Decreases | Harder to remove electron when nuclear charge is high; easier when outer shell is far away |
| Electron Affinity (EA) | Usually increases (more negative) | Generally increases | Electrons are more strongly attracted to nucleus with higher charge |
| Electronegativity (Pauling) | Increases | Decreases | Similar to EA – higher nuclear charge demands more electronegativity |
| Metallic Character | Decreases | Increases | Metals lose electrons more easily; non‑metals gain electrons more readily |
| Valence | Constant within a period | Constant within a group | Determines typical oxidation states |
Note – Exceptions exist (e.g., s‑block elements have lower IE than p‑block despite being on the right).
4. Key Points (Bullet List)
- Six main categories: metals, non‑metals, metalloids, alkali metals, alkaline earth metals, transition metals, post‑transition metals, halogens, noble gases, lanthanides, actinides.
- Atomic number (Z) = sequential placement in the periodic table.
- Period = horizontal row; reflects the number of electron shells.