At show clearly how bonding occured in (cao)
Calcium
oxide

They both combine with The help of valency, which is the combining capacity of elements

Explanation:

I find it easier to understand valency as part of the Bhor’s model of an atom.

In his model, it was stated that electrons are spread around the nucleus in such a way that there are rings around the model. Each ring has a n value. So, k has 1, l has 2, m has 3 and n has 4...

what you need to know to find the number of electrons in each ring is to solve the formula 2n squared

Then, the valency is the 8 minus the number of electrons in the outermost shell. Remember that 8 is max number I;outer most shell. If you try it out for oxygen, you will get the value of 2 remaining, which can be gained if combined with calcium.

Hope it helps

explanation:

let the different atoms be x and y. each with four protons and same mass will be represented as follows.

mass number is number of protons + number of neutrons.

            and  

since, it is given that one has a negative charge while the other is neutral. therefore, there will be increase in number of protons and the atom will be represented as follows.

according to periodic table, mass number of beryllium is 9 and it has atomic number 4 so, the atoms will be and .  

the protons and neutrons will be located inside the nucleus whereas the electrons will be revolving around the nucleus.