Atoms with a low electronegativity like calium might bond eith

Atom with low electronegativity like calcium might combine with atom with higher electronegativity like fluorine, chlorine etc.

Explanation:

Electronegativity:

It is the ability of an atom to attract the electrons towards itself.

There are two bonds are formed on the basis of electronegativity.

Ionic bond:

It is the bond which is formed by the transfer of electron from one atom to the atom of another element.  

Both bonded atoms have very large electronegativity difference. The atom with large electronegativity value accept the electron from other with smaller value of electronegativity.

For example:

calcium fluoride is ionic compound. The electronegativity of fluorine is 3.98 and for calcium is 1. There is large difference is present. That's why electron from calcium is transfer to the fluorine. Calcium becomes positive and fluorine becomes negative ion.

Covalent bond:

It is formed by the sharing of electron pair between bonded atoms.  

The atom with larger electronegativity attract the electron pair more towards it self and becomes partial negative while the other atom becomes partial positive.

For example:

In water the electronegativity of oxygen is 3.44 and hydrogen is 2.2. That's why electron pair attracted more towards oxygen, thus oxygen becomes partial negative and hydrogen becomes partial negative.

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explanation: