Question

Define valence shell on chemistry for 0

Answer 1

, a valence electron is an outer shell electron that is associated with an atom, and that can participate in the formation of a chemical bond if the outer shell is not closed; in a single covalent bond, both atoms in the bond contribute one valence electron in order to form a shared pair. The presence of valence electrons can determine the element's chemical properties, such as its valence—whether it may bond with other elements and, if so, how readily and with how many. For a main group element, a valence electron can exist only in the outermost electron shell; in a transition metal, a valence electron can also be in an inner shell.

An atom with a closed shell of valence electrons (corresponding to an electron configuration s2p6) tends to be chemically inert. Atoms with one or two more valence electrons than are needed for a "closed" shell are highly reactive due to the following reasons:
1) It requires relatively low energy (compared to the lattice enthalpy) to remove the extra valence electrons to form a positive ion.
2) Because of their tendency either to gain the missing valence electrons (thereby forming a negative ion), or to share valence electrons (thereby forming a covalent bond).

Similar to an electron in an inner shell, a valence electron has the ability to absorb or release energy in the form of a photon. An energy gain can trigger an electron to move (jump) to an outer shell; this is known as atomic excitation. Or the electron can even break free from its associated atom's valence shell; this is ionization to form a positive ion. When an electron loses energy (thereby causing a photon to be emitted), then it can move to an inner shell which is not fully occupied.

Valence energy levels correspond to the principal quantum numbers (n = 1, 2, 3, 4, 5 ...) or are labeled alphabetically with letters used in the X-ray notation (K, L, M, …).

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Answer 2

What is valency ?

=> Valency is the combining capacity of an atom with another atom (s) .

This is normally given by the number of Valence electrons or by 8 minus the Valence electrons (Only when the number of Valence electrons is more than 4) .

Valency of first 20 elements is given below.
1) Hydrogen (H) = 1

2) Helium (He) = 0

3) Lithium (Li) = 1

4) Beryllium (Be) = 2

5) Boron (B) = 3

6) Carbon (C) = 4

7) Nitrogen (N) = 3

8) Oxygen (O) = 2

9) Fluorine (F) = 1

10) Neon (Ne) = 0

11) Sodium (Na) = 1

12) Magnesium (Mg) = 2

13) Aluminium (Al) = 3

14) Silicon (Si) = 4

15) Phosphorus (P) = 3, 5

16) Sulphur (S) = 2

17) Chlorine (Cl) = 1

18) Argon (Ar) = 0

19) Potassium (K) = 1

20 ) Calcium (Ca) = 2

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