Periods and Groups in the Periodic Table Explained
Chemistry · Periodic Classification Of Properties · NEET
In the periodic table, periods are the horizontal rows (there are 7) and groups are the vertical columns (there are 18). Elements in the same group have the same number of outer electrons, so they behave in a similar way. Memory hook: "Period = across a Page (horizontal), Group = a Gang standing in a column."
Periods are horizontal rows (read across) and groups are vertical columns (read down). The modern table has 7 periods and 18 groups; elements in the same group share outer-shell electrons and behave alike.
Your doubts, answered
Is a period a row or a column? I always mix them up.
A period is a horizontal ROW. A group is a vertical COLUMN. Use the hook: you read a Period across the Page (left to right), and a Group is a Gang lined up top to bottom. The modern long-form table has 7 periods and 18 groups.
How many periods and groups are there in total?
There are 7 periods (numbered 1 to 7) and 18 groups (numbered 1 to 18). Period 1 is the shortest with just 2 elements (H and He). Periods get longer lower down because more sub-shells are being filled.
What does the period number actually tell me?
The period number equals the highest principal quantum number n (the outermost shell being filled). Example: Sodium has configuration [Ne]3s1, the outer shell is n=3, so sodium is in Period 3. This is the fastest way to check a period in NEET.
How do I find the group of a main-group element from its configuration?
For s-block: group number = number of valence electrons (ns electrons). For p-block: group number = 10 + (number of ns + np valence electrons). Example: an element with valence ns2 np2 is in group 10+4 = 14, the carbon family. This exact idea was tested for element Z=114 in NEET 2017.
Why do all elements in one group behave the same way?
Because they have the same number and same arrangement of electrons in their outermost shell. Chemistry is mostly decided by valence (outer) electrons. So Li, Na, K (all group 1, all ns1) all react strongly with water. This is why groups are also called families.
What is the difference between the old group names (IA, IIA) and the new 1-18 numbers?
They label the same columns. IUPAC now recommends numbering groups 1 to 18 straight across to avoid confusion with the old A/B system. For NEET, learn the 1-18 system but recognise that group 1 = alkali metals, group 17 = halogens, group 18 = noble gases.
⚠️ The NEET trap ✗ Element Z=114 has valence configuration ns2 np2, so students count 2 electrons and put it in group 2. ✓ For a p-block element, group = 10 + (ns + np valence electrons) = 10 + 4 = group 14 (carbon family). Only count 2 for the group if the element is s-block. 🧠 s-block: group = valence electrons. p-block: ADD 10. Do not use the raw electron count for p-block elements.
Real NEET questions
NEET 2017
The element Z = 114 has been discovered recently. It will belong to which of the following family/group and electronic configuration?
A · Halogen family, [Rn] 5f14 6d10 7s2 7p5
B · Carbon family, [Rn] 5f14 6d10 7s2 7p2 ✓
C · Oxygen family, [Rn] 5f14 6d10 7s2 7p4
D · Nitrogen family, [Rn] 5f14 6d10 7s2 7p6
Solution: Filling electrons for Z=114 gives [Rn] 5f14 6d10 7s2 7p2. The valence shell (highest n=7) has ns2 np2. For a p-block element, group = 10 + (2+2) = 14, which is the carbon family. Period number = 7 (highest n). So the answer is the carbon family with configuration ending 7s2 7p2.
NEET 2025
Which among the following electronic configurations belong to main group elements? A. [Ne]3s1 B. [Ar]3d3 4s2 C. [Kr]4d10 5s2 5p5 D. [Ar]3d10 4s1 E. [Rn]5f0 6d2 7s2
A · D and E only
B · A, C and D only
C · B and E only ✓
D · A and C only
Solution: Main group elements are s-block + p-block (groups 1-2 and 13-18). A = [Ne]3s1 is Na (s-block, main group); C = [Kr]4d10 5s2 5p5 is I (p-block, main group); B = V and D = Cu are d-block (transition); E is an actinide (inner-transition). By the standard NCERT definition main group = A and C (option D). The printed official 2025 key, however, marked (C) B and E only, so NEET awarded (C) - reproduce as keyed. This shows how carefully you must read block vs group in the exam.
Solved Periodic Classification Of Properties NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
How many groups and periods are in the modern periodic table?
7 periods (horizontal rows) and 18 groups (vertical columns).
Which group has the noble gases?
Group 18 (the last column) holds the noble gases like He, Ne, Ar. They have a full outer shell, so they are very unreactive.
Do the period number and the number of shells match?
Yes. The period number tells you how many electron shells the element uses, which equals the highest principal quantum number n of its valence electrons.
Why is period 1 so short?
Only the 1s sub-shell is available in the first shell, which holds a maximum of 2 electrons. So period 1 has just hydrogen and helium.
Are alkali metals a group or a period?
Alkali metals (Li, Na, K, Rb, Cs) are a group, specifically group 1. They share the ns1 valence configuration, which is why they react similarly.
