The valency of an element depends upon the
- (a)total number of protons in an atom
- (b)mass number of an atom
- (c)total number of neutrons in an atom
- (d)total number of electrons in the outer most shell of an atom
Correct — D, total number of electrons in the outer most shell of an atom. Valency is the combining capacity of an atom, and an atom combines in order to reach the stable eight-electron arrangement of the nearest noble gas. Only the electrons in the outermost shell — the valence electrons — take part, by being lost, gained or shared. So sodium, with one valence electron, has valency 1; magnesium, with two, has valency 2; and chlorine, with seven, gains one and also shows valency 1. For elements with more than four valence electrons the valency is eight minus that number.
- (a)total number of protons in an atom — The proton count is the atomic number, which fixes the element's identity and how many electrons a neutral atom has, but not its combining capacity directly. Sodium with 11 protons and chlorine with 17 both show a valency of one.
- (b)mass number of an atom — The mass number counts protons plus neutrons and describes the nucleus. Chemical combination is a business of the outer electrons, and the nucleus takes no part in it.
- (c)total number of neutrons in an atom — Neutrons are uncharged and do not enter into bonding. Isotopes of an element differ precisely in their neutron count yet have identical valency and identical chemistry.
Electrons occupy shells around the nucleus, and only the outermost shell is chemically active. An atom is stable when that shell holds eight electrons — two for helium — so atoms lose, gain or share electrons to get there. Valency is the number of electrons involved in that adjustment: it equals the number of valence electrons when they are few enough to be given away or shared, and eight minus the valence electrons when it is easier to accept a few. Noble gases already have full outer shells, so their valency is zero.
Because a neutral atom's electron count equals its proton count, the atomic number does decide the electron arrangement — which is why option (a) feels close. The distinction the question wants is that valency depends on the outermost shell only, not on the total. Carbon, with four valence electrons, is tetravalent; that single fact is the doorway to the whole of organic chemistry. Elements in the same group of the periodic table share their valence-electron count, and therefore their valency, which is why magnesium, calcium and barium behave alike.
- Valency is the combining capacity of an atom, decided by the electrons in its outermost shell.
- For elements with four or fewer valence electrons the valency equals that number; beyond four it is eight minus that number.
- Noble gases have complete outer shells, so their valency is zero.
- Isotopes of an element differ in neutron number but have the same valency and the same chemical behaviour.
- Elements in the same group of the periodic table have the same number of valence electrons and hence the same valency.
Only the outermost shell counts, which is option (d).
- Reading valency straight off the atomic number instead of the outermost shell.
- Forgetting the eight-minus rule for elements with more than four valence electrons.
- Assuming isotopes differ chemically — they differ only in mass.
NDA asks what valency depends on, for the valency of a named element, or for a formula built from two given valencies.
For an element with atomic number 35, which one of the following will be the correct number of electrons in its valence shell based on Bohr's model of an atom?
- (a) 1
- (b) 3
- (c) 5
- (d) 7
Answer(d) 7
Puts the same rule to work numerically, going from the atomic number to the outermost shell and from there to the combining capacity.
Which of the following sets of elements has the same valency?
- (a) Na, Mg, Ca
- (b) Na, Mg, Al
- (c) Mg, Ca, K
- (d) Mg, Ca, Ba
Answer(d) Mg, Ca, Ba
Shows the group rule that follows from this definition, since elements of one group share their valence-electron count.
- practice — not a real PYQ
An element has the electronic configuration 2, 8, 6. Its valency is
- (a)2
- (b)6
- (c)8
- (d)16
Answer(a) 2 — with six valence electrons the atom gains two to complete its octet, so eight minus six gives a valency of two.
- practice — not a real PYQ
The valency of the noble gases is
- (a)one
- (b)two
- (c)zero
- (d)eight
Answer(c) zero — their outermost shells are already complete, so they have no tendency to lose, gain or share electrons.