Chemistry · Periodic Classification Of Properties · NEET
For metals (Group 1), reactivity means LOSING an electron. Going down, the atom gets bigger, the outer electron is far from the nucleus, so it leaves easily - reactivity increases. For halogens (Group 17), reactivity means GAINING an electron. Going down (F to I), the atom gets bigger, so the nucleus pulls a new electron less strongly. So halogens become LESS reactive down the group: F > Cl > Br > I. NCERT states this order directly.
Every halogen has the valence shell ns2 np5, where n is the period number. F is 2s2 2p5, Cl is 3s2 3p5, Br is [Ar]3d10 4s2 4p5, I is [Kr]4d10 5s2 5p5. They have 7 valence electrons and need only 1 more to complete the octet (ns2 np6, the noble-gas shell). That is why they are placed in Group 17.
Valence is often 'eight minus the number of outer electrons' for non-metals. Halogens have 7 outer electrons, so 8 - 7 = 1. They gain or share exactly one electron to reach the octet. That is why they form ions with charge -1 (like Cl-) and compounds like SiBr4, NaCl, HCl - always combining in a 1:1 ratio per halogen atom for its bonding.
Fluorine is very small. Its electrons are packed tightly in the compact 2p subshell, so a new incoming electron feels strong repulsion from the electrons already there. This makes fluorine's electron gain enthalpy LESS negative than chlorine's, even though fluorine is smaller. Also, fluorine only shows -1 oxidation state (never positive) because it is the most electronegative element. This is a very common NEET trap.
By magnitude (most negative first) the order is Cl > F > Br > I. You might expect F to be first because it is smallest, but F is anomalous due to electron-electron repulsion in its small 2p shell. So chlorine releases the most energy when gaining an electron. This exact order was tested in ReNEET 2026.
Halogens are non-metals (astatine is a rare radioactive element with some metalloid character). NCERT says chemical reactivity is highest in the Group 17 non-metals within a period. They gain electrons instead of losing them, they are poor conductors, and they form acidic oxides - all non-metal behaviour.
Among the following, the correct trend in electron gain enthalpy (most negative first) is:
The element Z = 114 has been discovered recently. It will belong to which family/group and electronic configuration?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Fluorine is the most reactive halogen and the strongest oxidising agent. It is so reactive it can even displace oxygen from water. Reactivity decreases down the group: F > Cl > Br > I.
Fluorine is the most electronegative element, so it always pulls electrons toward itself. It only shows the -1 oxidation state. Other halogens (Cl, Br, I) can show positive states like +1, +3, +5, +7 in their oxoacids.
Seven. The configuration is ns2 np5 (2 + 5 = 7). They need just one more electron to complete the stable octet ns2 np6, which is why they are highly reactive non-metals.
Atomic radius increases down the group (F < Cl < Br < I) because a new shell is added at each step. This larger size is the main reason reactivity and electron gain enthalpy decrease down the group.
The word halogen means 'salt producer'. Halogens react with metals to form salts, for example sodium + chlorine forms sodium chloride (NaCl, common salt). They gain one electron to become -1 halide ions.
