The electrons present in the outermost shell of an atom are known as valence electrons or valency electrons.
In other words, electrons of an atom which take part in chemical reactions are called valence electrons.
Valence electrons decide the valency or the combining capacity of the atom.
VALENCY OF ELEMENTS
Electrons in an atom are arranged in shells/orbitals.
Valence electrons are those electrons which are present in the outermost orbit of the atom.
According to the Bohr-bury scheme the outermost shell can contain a maximum of 8 electrons.
A little chemical activity is observed when the outermost shell is completely filled or it’s combining capacity can be said to become zero.
Noble gases have a completely filled outermost shell and that’s why they are least reactive.
Other element’s reactivity depends upon their ability to attain the noble gas configuration.
The capacity of an atom of an element to from chemical bonds is known as its valency. The valency of an element is decided by the “number of valence” in its atom.
RELATION BETWEEN VALENCY AND VALENCE ELECTRONS
The valency of an element is either equal to the number of valency electrons in its atom or equal to the number of electrons required to complete eight electrons in the valence shell.
The capacity of an atom is described by the total number of electrons lost, gained or shared to complete its octet and it also determines the valency of the atom.
HOW TO DETERMINE VALENCY:
In general, if the atom contains 1, 2, or 3 electrons in its outermost shell, then valency is equal to the valence electrons.
When the number of electrons in the outermost shell of an atom is close to its full capacity, i.e., 5, 6 or 7 then the valency is equal to the number of electrons by adding which the shell may become complete or 8 minus valence electrons.
The number of electrons in the outermost shell of hydrogen is 1, and in magnesium, it is 2. Therefore the valency of hydrogen is 1 as it can easily lose 1 electron and become stable.
On the other hand, that of magnesium is 2 as it can lose 2 electrons easily and also attain the stability.
But valency is not only determined when an atom loses an electron. For example, fluorine has 7 electrons in its outermost orbital. It is hard to lose 7 electrons and so it completes its octet by gaining 1 electron. Since it gains 1 electron, its valency is 1.
In the periodic table, the elements in the same group have the same valency. For example, all the elements in group 8 have 8 electrons and completely filled orbitals So, the valency of all the elements in this group is zero.
HOW VALENCY IS DIFFERENT FROM OXIDATION NUMBER:
Oxidation number is the hypothetical charge of an atom in a molecule or ion.
It is a measure of its apparent capacity to gain or lose electrons within that species.
Valency is different from oxidation number, and it has NO SIGN.
For example, the valency of nitrogen is 3, whereas it can have oxidation numbers from -3 to +5.
CAUSE OF CHEMICAL COMBINATION
The atoms combine with one another to achieve the inert gas electron arrangement and become more stable.
An atom can achieve the inert gas (or noble gas) electron arrangement in three ways:
By losing one or more electrons (to another atom)
By gaining one or more electrons (from another atom)
By sharing one or more electrons (with another atom)