Chemistry · Periodic Classification Of Properties · NEET
Look at how many electrons are in the outermost shell. If it is 1, 2, or 3, the atom prefers to LOSE those electrons, so valency = number of outer electrons. If it is 4, 5, 6, or 7, the atom prefers to GAIN electrons to reach 8 (a full octet), so valency = 8 minus outer electrons. The atom always takes the shorter path to a stable octet.
A stable atom wants 8 electrons in its outer shell (the octet, like a noble gas). An atom with 6 outer electrons needs only 2 more to reach 8, so it gains 2 and its valency is 8 minus 6 = 2. Losing 6 electrons would take far more energy, so nature chooses the easier route of gaining a few.
5 is in the 4 to 7 range, so use 8 minus outer. Valency = 8 minus 5 = 3. For example, nitrogen has the configuration 1s2 2s2 2p3, which is 5 outer electrons, so its valency is 3 (as in NH3). This exact idea was tested in NEET 2018.
Metals have only 1, 2, or 3 electrons in the outer shell. Removing these few electrons is easy and leaves a full inner octet behind. So sodium (1 outer electron) has valency 1, magnesium (2) has valency 2, and aluminium (3) has valency 3. It is easier to lose 1-3 electrons than to gain 5-7.
Carbon sits exactly in the middle with 4 outer electrons. Both rules give the same answer here: outer electrons = 4, and 8 minus 4 = 4. So carbon has a valency of 4. It neither easily loses nor easily gains, so it mostly shares electrons (covalent bonding).
Noble gases already have 8 outer electrons (except helium with 2), so 8 minus 8 = 0. Their valency is zero, meaning they normally do not bond. This matches why noble gases are unreactive - their octet is already complete.
Magnesium reacts with an element (X) to form an ionic compound. If the ground state electronic configuration of (X) is 1s2 2s2 2p3, the simplest formula for this compound 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.
Valency is the number of electrons an atom loses, gains, or shares to complete its outer shell (reach a stable octet of 8 electrons).
If outer electrons are 1, 2, or 3, valency equals the number of outer electrons. If outer electrons are 4, 5, 6, or 7, valency equals 8 minus the number of outer electrons. If outer electrons are 8 (noble gas), valency is 0.
Because a filled outer shell of 8 electrons (the octet) is very stable, like the noble gases. Atoms bond in whatever way brings them closest to this 8-electron arrangement.
For main-group elements and simple compounds it works reliably and is directly tested (for example NEET 2018). Transition metals and heavier p-block elements can show variable valency due to d-orbitals and the inert pair effect, so treat those as special cases.
Valency is just a count (always a positive number) of bonds an atom can form. Oxidation state carries a plus or minus sign and can change with the compound. The next concept, oxidation state of oxygen in OF2 vs Na2O, shows exactly how the sign flips.
