what is the difference between valency 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
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 aluminum 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 co valency.