Chemistry · Periodic Classification Of Properties · NEET
Take the number of electrons in the outermost shell (valence electrons). For Group 1 (ns1) valence is 1, Group 2 (ns2) valence is 2, Group 13 valence is 3, Group 14 valence is 4. After Group 14 it is easier to use the second rule: valence = 8 minus outer electrons. So Group 15 (5 outer) has valence 3, Group 16 (6 outer) has valence 2, Group 17 (7 outer) has valence 1, and Group 18 (8 outer) has valence 0. NCERT states this exactly: valence is 'equal to the number of electrons in the outermost orbitals and/or equal to eight minus the number of outermost electrons.'
Early in a period an atom has few outer electrons, so it loses or shares them — valence equals that small count (1, 2, 3, 4). After the middle, the atom is closer to a full octet, so it gains or shares only the electrons it still needs — valence becomes 8 minus the outer count (3, 2, 1). The noble gas already has a full shell (ns2 np6), so it needs nothing and its valence is 0. This is why valence goes 1, 2, 3, 4, 3, 2, 1, 0 across Period 3 (Na to Ar).
Valence is just a whole number that tells how many bonds an atom forms — it has no plus or minus sign. Oxidation state is valence WITH a sign, decided by comparing electronegativity. NCERT gives the classic example: in OF2 oxygen is +2 (F is more electronegative), but in Na2O oxygen is -2 (Na is less electronegative). So the same element (oxygen) can have different oxidation states, but the term oxidation state is now often used in place of valence.
Use fixed values: H = +1, O = -2, and make the whole formula add to its charge (0 for a neutral molecule). In HNO3: (+1) + N + 3(-2) = 0, so N = +5. In NO: N + (-2) = 0, so N = +2. In N2 the element is alone, so N = 0. In NH4Cl the NH4+ ion has N + 4(+1) = +1, so N = -3. Decreasing order: HNO3 (+5) > NO (+2) > N2 (0) > NH4Cl (-3). This is exactly NEET 2018.
In transition elements the (n-1)d and ns electrons are close in energy, so a variable number of them can take part in bonding. That is why Mn shows +2 all the way to +7, and its states differ by one unit (V is +2, +3, +4, +5). Main-group elements mostly use only their outer s and p electrons, so their common oxidation states differ by two (like +2 and +4 in tin and lead) because a lone pair either bonds or stays out.
Elements in the same group have the same number of electrons in their outermost shell (the same valence-shell configuration, like ns1 for all alkali metals). Valence depends on this outer electron count, not on how many inner shells there are. So Li, Na, K all have valence 1, and O, S, Se all have valence 2. This is why groups are also called 'families' with similar chemistry.
The correct order of N-compounds in its decreasing order of oxidation states is
Among Group 16 elements, which one does NOT show the -2 oxidation state?
It is because of inability of ns2 electrons of the valence shell to participate in bonding that
Try the real previous-year questions from this chapter — each with the answer and a full solution.
For s-block and Groups 13-14 of the p-block, valence equals the number of outer electrons (1, 2, 3, 4). But for Groups 15-18 it is easier and correct to use valence = 8 minus outer electrons (3, 2, 1, 0). So it is not always the plain group number, especially on the right side of the table.
Yes. Many elements show more than one, especially transition metals (Mn shows +2 to +7) and heavy p-block elements (Pb shows +2 and +4). NCERT calls these 'variable oxidation states'. Main-group states usually differ by 2, transition-metal states usually differ by 1.
Oxidation state depends on which atom is more electronegative. Fluorine is more electronegative than oxygen, so in OF2 oxygen is assigned +2. Sodium is far less electronegative than oxygen, so in Na2O oxygen is -2. Same element, different partner, different sign.
Their normal (common) valence is 0 because their outer shell ns2 np6 is already complete, so they do not usually gain, lose or share electrons. Heavier noble gases like Xe can be forced to form compounds (XeF2, XeF6), but the standard periodic-trend valence taught for NEET is 0.
