what is electrovalency and covalency
The total number of orbitals available in valence shell is known as covalency whether orbitals are completely filled or empty . For example, the electronic configuration of Boron (At No 5) is 1s2 2s2 2p1 . So, there are only two shells. The second shell contains one 2s orbitals and three 2p orbitals resulting total four orbitals in second shell. Therefore, Boron is restricted to a maximum covalency of 4 since only four (one s and three p) orbitals are available for bonding.
The electrons present in the last shell determine the valency of a particular element.
If the number of valence electrons ≤4: valency = number of valence electrons
If the number of valence electrons >4: valency = 8 - number of valence electrons.
You can observe the variation in the periodic table
|Group in periodic table||Number of valency electrons||valency|
In group, valency remains same while in periodic first it increase and then decrease
For your convenience, I am providing two examples to show how valency of any element can be found out.
If you consider the element aluminium and try to write its electronic configuration, then it is 2, 8, 3 the valency is equal to 3.
However, for element oxygen having electronic configuration 2, 6 the valency is 8 - 6 = 2.
The electronic configuration of Boron is 1s2 2s2 2p1 at ground state. The valence shell contains 3 electrons. So its valency is 3. By loosing 3 electrons, it gets noble gas configuration of helium.
Hence, in our example, Boron has 3 valence electrons with maximum 4 covalency.
Electrovalency is the actual transfer of the electrons from the metallic atom to the non metallic atom , so that each of the two atoms can occupy their nearest noble gas configuration. Complete transfer of electrons is responsible for the formation of cations and anions which combine together by the electrostatic force of attraction. Thus electrovalency term we take in the ionic compounds .
For example in the NaCl the two atoms held together by the electrostatic force of attraction . Two atoms of different electronegativity can form electrovalent bond.
covalency on the other hand takes place between the atoms which are of nearly same electronegativity, where the sharing of the electrons takes place between the two atoms and no complete transfer of electron . For example in the H2 molecule ,two hydrogen atoms share their electrons and attain the noble gas configuration.