Periodic Table — Periods and Groups
~9 min read
- Modern law: Properties are a periodic function of atomic number (Moseley). Replaces Mendeleev's atomic-mass version.
- Structure: 7 periods (rows), 18 groups (columns). 118 elements known.
- Key groups: Group 1 alkali metals, Group 2 alkaline earth, Group 17 halogens, Group 18 noble gases.
The periodic table organises every element by atomic number into periods and groups, with each column sharing chemical character. CDS/OTA tests its history, key groups and trends in properties.
From Mendeleev to the Modern Table
- Mendeleev (1869): arranged elements by atomic mass. Left blank gaps for undiscovered elements (predicted eka-aluminium = gallium, eka-silicon = germanium).
- Henry Moseley (1913): arranged by atomic number — modern periodic law: "properties of elements are a periodic function of atomic number".
- 118 elements known; up to Uranium (Z=92) occur naturally, rest are synthetic.
Periods and Groups
- 7 periods (horizontal rows). Period n has the principal quantum number n filled.
- 18 groups (vertical columns). Elements in the same group have the same valence configuration → similar chemistry.
- Blocks: s (groups 1-2), p (13-18), d (3-12 transition), f (lanthanoids and actinoids, placed separately).
Important Groups
| Group | Name | Members | Character |
|---|---|---|---|
| 1 | Alkali metals | Li, Na, K, Rb, Cs, Fr | Soft, low MP, react violently with water, valency 1 |
| 2 | Alkaline earth metals | Be, Mg, Ca, Sr, Ba, Ra | Harder than alkali, valency 2 |
| 17 | Halogens | F, Cl, Br, I, At | Highly reactive non-metals, valency 1 (gain electron) |
| 18 | Noble gases | He, Ne, Ar, Kr, Xe, Rn | Inert (full octet), monatomic gases |
Hydrogen is placed in Group 1 but does not strictly belong — shows properties of both alkali metals and halogens.
Periodic Trends
| Property | Across a period → | Down a group ↓ |
|---|---|---|
| Atomic size | Decreases | Increases |
| Metallic character | Decreases | Increases |
| Electronegativity | Increases | Decreases |
| Ionisation energy | Increases | Decreases |
Fluorine is the most electronegative element; caesium (excluding Fr) is the most electropositive.
CDS/OTA PYQ Examples
Q: The modern periodic law states that properties of elements are a periodic function of their:
(a) Atomic mass (b) Atomic number (c) Valency (d) Atomic radius
Answer: (b) Atomic number — Moseley.
Q: Noble gases belong to group:
(a) 1 (b) 17 (c) 18 (d) 2
Answer: (c) Group 18.
Q: Most electronegative element is:
(a) Oxygen (b) Fluorine (c) Chlorine (d) Nitrogen
Answer: (b) Fluorine.
Q: Alkali metals belong to group:
(a) 1 (b) 2 (c) 17 (d) 18
Answer: (a) Group 1 — Li, Na, K, Rb, Cs.
Q: Which of the following is a halogen?
(a) Sodium (b) Calcium (c) Chlorine (d) Argon
Answer: (c) Chlorine — Group 17.
Drill Periodic Table — Periods and Groups for CDS/OTA
CDS/OTA-pattern items on Periodic Table — Periods and Groups with answer keys and explanations.
Start Free Mock TestFrequently Asked Questions
Why was Mendeleev's table revolutionary?
It revealed periodic recurrence of chemical properties and predicted properties of unknown elements (gallium, germanium, scandium), all later confirmed.
Why are noble gases inert?
Their valence shell is completely filled (2 for He, 8 for the rest). They have no tendency to gain, lose or share electrons under ordinary conditions.
Why is hydrogen placed separately?
It has one electron, like alkali metals (Group 1), but needs one electron to complete its shell, like halogens (Group 17). Modern tables place it tentatively in Group 1.