Chemistry · Periodic Classification Of Properties · NEET
Because chemistry is decided by the OUTERMOST electrons (valence electrons), and every element in a group has the same number of them. NCERT says it directly: elements in a vertical column form a group and 'exhibit similar chemical behaviour... because these elements have the same number and same distribution of electrons in their outermost orbitals.' The inner shells are full and mostly do not take part in reactions, so only the outer shell matters.
Look only at the OUTER shell. Li = 1s2 2s1 (outer = 2s1, one electron). Na = 1s2 2s2 2p6 3s1 (outer = 3s1, one electron). Both have exactly ONE electron in their outermost s-orbital (ns1). The extra inner electrons in Na are locked in filled shells and do not change the basic chemistry. So both lose that one electron easily and both are very reactive metals (Group 1).
Count the electrons in the outermost shell (highest n). For s- and p-block (main group) elements: outer ns electrons + np electrons = the last digit of the group number. Example: outer config ns2 np2 means 2+2 = 4 valence electrons, so Group 14 (carbon family). ns1 = Group 1, ns2 = Group 2, ns2 np5 = Group 17, ns2 np6 = Group 18.
For main-group (s and p block) elements, yes in the old system: Group 1 = 1 valence electron, Group 2 = 2, Group 13 = 3, up to Group 18 = 8 (except He). In modern IUPAC 1-18 numbering, for p-block use (group number - 10) to get valence electrons. Warning: this simple rule does NOT work cleanly for d-block (transition) elements, so only use it for main group.
As you go down, you add a whole new shell, but that new outer shell fills in the SAME pattern. So the value of n (period number) goes up, but the ns/np pattern stays identical. Li 2s1, Na 3s1, K 4s1 — same ns1, only n changes from 2 to 3 to 4. Same pattern = same type of chemistry, which is exactly why a group is called a 'family'.
It means not just the same COUNT but the same ARRANGEMENT in s and p orbitals. Oxygen is 2s2 2p4 and Sulphur is 3s2 3p4 — both are ns2 np4. They have 6 valence electrons arranged the same way, so both tend to gain 2 electrons and both commonly show a -2 state. Same distribution is the real reason for the family resemblance.
The element Z = 114 has been discovered recently. It will belong to which family/group and electronic configuration?
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
Try the real previous-year questions from this chapter — each with the answer and a full solution.
For main-group (s and p block) elements, yes: valence electrons are the electrons in the outermost shell (highest n). For transition metals, valence electrons include the outer ns plus the incomplete (n-1)d electrons, which is why the simple rule breaks for them.
Yes. Except helium, all Group 18 elements have the outer configuration ns2 np6 — a completely filled outer shell of 8 electrons. That shared full-shell configuration is why they are all stable and unreactive. Helium is ns2 (only 2 electrons) but is still placed in Group 18 because its shell is also full.
It is closely linked. The modern periodic law says properties are a periodic function of atomic number. As atomic number increases, the outer-shell configuration repeats at regular intervals, and each repeat lands elements with the same valence configuration into the same group. So group similarity is the visible result of that periodic law.
NEET often gives you a configuration and asks for the group, family, or block, or asks which configurations are main group. If you can read only the valence shell (ns/np pattern), you can instantly place the element and predict its chemistry without memorising every element.
