The Periodic Table
118 elements arranged in order of increasing atomic number. Developed by Dmitri Mendeleev (1869) based on atomic mass; revised using atomic number by Moseley.
A. Structure of the Periodic Table
Key Facts About the Table
- 118 elements currently known
- 7 Periods (horizontal rows) — the period number = number of electron shells the element has
- 18 Groups (vertical columns) — elements in the same group have the same number of valence electrons and similar chemical properties
- Groups 1–2 and 13–18: main group (s and p block) elements
- Groups 3–12: transition metals (d block)
- Metals are on the LEFT, non-metals on the RIGHT, metalloids along the staircase boundary
- Mendeleev arranged by atomic mass and left gaps for undiscovered elements — his predictions were later confirmed
⚡ MCQ Tip Mendeleev = atomic mass (1869). Modern table = atomic number. Period = number of shells. Group = number of valence electrons (for main group elements).
B. Key Groups & Their Properties
Group 1 — 1 valence e⁻
Alkali Metals
Soft, highly reactive metals. React vigorously with water → metal hydroxide + H₂ gas. Form +1 ions. Reactivity increases down the group.
Li, Na, K, Rb, Cs, Fr
Group 2 — 2 valence e⁻
Alkaline Earth Metals
Reactive metals, harder than Group 1. Form +2 ions. React with water (less vigorously than Group 1).
Be, Mg, Ca, Sr, Ba, Ra
Groups 3–12
Transition Metals
Good conductors, high melting points, form coloured compounds and complex ions, variable valency (oxidation states).
Fe, Cu, Zn, Cr, Ni, Mn, Ti
Group 17 — 7 valence e⁻
Halogens
Very reactive non-metals. Exist as diatomic molecules (F₂, Cl₂…). Form −1 ions (halides). Reactivity DECREASES down group. Good oxidising agents.
F, Cl, Br, I, At
Group 18 — full outer shell
Noble Gases
Completely unreactive (inert) — full outer shell of 8 electrons (He = 2). Monoatomic. Used in lighting, balloons, and welding.
He, Ne, Ar, Kr, Xe, Rn
C. Periodic Trends
How Properties Change Across & Down the Table
| Property | Across Period (→) | Down Group (↓) |
|---|---|---|
| Atomic Radius | Decreases → (more protons pull electrons closer) | Increases ↓ (more shells added) |
| Ionisation Energy | Increases → (harder to remove electrons) | Decreases ↓ (outer electrons further from nucleus) |
| Electronegativity | Increases → (F is highest) | Decreases ↓ |
| Metallic Character | Decreases → (metals on left, non-metals on right) | Increases ↓ |
| Non-metallic Character | Increases → | Decreases ↓ |
| Reactivity of Metals | Decreases → | Increases ↓ (K more reactive than Na) |
| Reactivity of Non-metals | Increases → | Decreases ↓ (F more reactive than Cl) |
Atomic Radius
→ Decreasesacross period (same shells, more protons)
↓ Increasesdown group (extra shell added)
Electronegativity
→ Increasesacross period — F is highest of all
↓ Decreasesdown group (further from nucleus)
⚡ MCQ Tip Fluorine (F) has the highest electronegativity of all elements. Caesium (Cs) is the most reactive alkali metal. Francium (Fr) is the most reactive metal overall. Noble gases are the LEAST reactive. Group 1 reactivity increases DOWN the group.
Quick MCQ Revision
| Fact | Answer |
|---|---|
| Mendeleev arranged by | Atomic mass (1869) — gaps left for undiscovered elements |
| Modern table arranged by | Atomic number (protons) |
| Period = | Number of electron shells |
| Group = | Number of valence electrons (main group) |
| Group 1 properties | Alkali metals — soft, very reactive, form +1 ions, react with water |
| Group 17 properties | Halogens — very reactive non-metals, form −1 ions, diatomic molecules |
| Group 18 properties | Noble gases — inert, full outer shell, used in lighting |
| Atomic radius across period | Decreases → (more protons, same shells) |
| Electronegativity down group | Decreases ↓ |
| Most electronegative element | Fluorine (F) — top right of table |