Valence electrons are the electrons present in the outermost shell of an atom. They take part in chemical bonding by being lost, gained, or shared during reactions. These electrons help atoms achieve a stable configuration, usually like noble gases. Elements having the same number of valence electrons show similar chemical behaviour and are placed in the same group in the periodic table.
Examples:
- Sodium (Na) → Electronic configuration: 2, 8, 1, Valence electrons = 1
- Oxygen (O) → Electronic configuration: 2, 6, Valence electrons = 6
Characteristics of Valence Electrons
Valence electrons have some important features that explain their role in the chemical behavior of elements.
- Valence electrons are present in the outermost shell (valence shell) of an atom, so they are farthest from the nucleus.
- They are loosely held by the nucleus compared to inner electrons, so they can be easily lost, gained, or shared.
- These electrons are directly involved in the formation of chemical bonds.
- The number of valence electrons determines the reactivity of an element. For example, elements with 1 valence electron are highly reactive.
- Elements having the same number of valence electrons show similar chemical properties and are placed in the same group in the periodic table.
- The maximum number of valence electrons in an atom is generally 8 (except helium which has 2).
Determination of Valence Electrons
The number of valence electrons in an atom can be determined by using its electronic configuration or its position in the periodic table.
1. By Electronic Configuration
- Write the electronic configuration of the element.
- The number of electrons in the outermost shell gives the number of valence electrons.
Examples:
- Sodium (Na) , Electronic Configuration: 2, 8, 1 → Valence electrons = 1
- Oxygen (O), Electronic Configuration: 2, 6 → Valence electrons = 6
- Magnesium (Mg), Electronic Configuration: 2,8,2→ Valence electrons = 2
2. By Position in Periodic Table
- For main group elements, the number of valence electrons is equal to the group number.
Examples:
- Group 1 , 1 valence electron (Na, K)
- Group 2 , 2 valence electrons (Mg, Ca)
- Group 17 , 7 valence electrons (Cl, F)
- Group 18 , 8 valence electrons (noble gases, except He = 2)
Valency
Valency is the combining capacity of an atom, that is, the number of electrons an atom loses, gains, or shares to form chemical bonds and achieve a stable electronic configuration. Atoms try to complete their outermost shell (octet) to become stable like noble gases.
Relation with Valence Electrons
Valency depends on the number of valence electrons:
- If valence electrons are 1, 2, 3, or 4, Valency = same number .
- If valence electrons are 5, 6, or 7, Valency = 8 − valence electrons.
- If valence electrons = 8 (complete shell), Valency = 0.
Examples:
- Sodium (Na): 2, 8, 1 → Valency = 1
- Oxygen (O): 2, 6 → Valency = 2 (8 − 6)
- Chlorine (Cl): 2, 8, 7 → Valency = 1 (8 − 7)
- Neon (Ne): 2, 8 → Valency = 0
Valence electrons of few elements
Atoms combine with other atoms by losing, gaining or sharing valence electrons to attain a stable electronic configuration. For transition elements, valence electrons may vary and are not strictly equal to group number.
Atomic Number | Element | Valence Electrons |
|---|---|---|
1 | Hydrogen (H) | 1 |
2 | Helium (He) | 2 |
3 | Lithium (Li) | 1 |
4 | Beryllium (Be) | 2 |
5 | Boron (B) | 3 |
6 | Carbon (C) | 4 |
7 | Nitrogen (N) | 5 |
8 | Oxygen (O) | 6 |
9 | Fluorine (F) | 7 |
10 | Neon (Ne) | 8 |
11 | Sodium (Na) | 1 |
12 | Magnesium (Mg) | 2 |
13 | Aluminium (Al) | 3 |
14 | Silicon (Si) | 4 |
15 | Phosphorus (P) | 5 |
16 | Sulphur (S) | 6 |
17 | Chlorine (Cl) | 7 |
18 | Argon (Ar) | 8 |
19 | Potassium (K) | 1 |
20 | Calcium (Ca) | 2 |