Chemistry · Periodic Classification Of Properties · NEET
Hydrogen only makes ONE bond. An element bonds to hydrogen using its unpaired electrons, and it can share at most 4. So from Group 14 onward the outer shell is more than half full, and the number of hydrogen atoms it can hold = 8 minus its group's outer electrons. That is why it climbs 1 to 4 (CH4) and then drops 3, 2, 1 (NH3, H2O, HF). It makes a hill shape.
Oxygen can accept 2 electrons or share 2 bonds, so it can pull out ALL the outer electrons of an element. The highest oxide valence therefore equals the group number (old numbering), rising 1, 2, 3, 4, 5, 6, 7. Example: Na2O (1), MgO (2), Al2O3 (3), up to Cl2O7 (7). It makes a ramp, not a hill.
For a main-group element, highest oxide valence = number of outer (valence) electrons = group number. So carbon (4 outer) gives CO2, nitrogen (5) gives N2O5, sulphur (6) gives SO3, chlorine (7) gives Cl2O7. Just count outer electrons, that is the oxygen combining power.
For groups 14 to 17 use the rule: hydrogen valence = 8 minus outer electrons. Nitrogen (5 outer): 8-5 = 3, so NH3. Oxygen (6): 8-6 = 2, so H2O. Fluorine (7): 8-7 = 1, so HF. This is why the hydride valence falls after Group 14.
No, not always. Only up to Group 14 they can match (carbon: CH4 and CO2, both valence 4). After that they differ. Nitrogen has valence 3 in NH3 but valence 5 in N2O5. This mismatch is a common NEET trick, so treat hydride valence and oxide valence separately.
Yes, the main valence stays the SAME down a group because the number of outer electrons does not change. Group 1 is always valence 1, Group 16 hydrides are always valence 2 (H2O, H2S, H2Se). The period (left-right) is where valence changes, not the group.
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:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is the number of hydrogen atoms that combine with one atom of the element. For groups 14 to 17 it equals 8 minus the number of outer electrons (for example nitrogen: 8-5 = 3, so NH3).
It equals the number of outer (valence) electrons, which is the old group number. It rises across a period from 1 (Na2O) up to 7 (Cl2O7).
Hydrogen forms only one bond and an atom shares at most 4, so hydride valence rises to 4 then falls (1-2-3-4-3-2-1). Oxygen can remove all outer electrons, so oxide valence keeps rising to 7.
Carbon. It shows valence 4 in both CH4 (hydride) and CO2 (oxide), because Group 14 sits at the top of the hydride hill (4) which also equals its outer-electron count.
No. The number of outer electrons is fixed within a group, so the characteristic valence stays the same down a group. It only changes as you move across a period.
