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→ Table of electronegativities
Atoms with a small electronegativity abandon easily their outer electrons, atoms with a big electronegativity catch easily foreign electrons.
If the difference of the electronegativities of two atoms exceeds 2, an ionic bond is made.
For example, $E.n.(Cl)-E.n.(Na) = 2.1 $, so sodium chloride is an ionic substance.
Two atoms with electronegativities greater than or equal to 2 combine to a covalent bond.
For example, $E.n.(H), E.n.(O) > 2$, so water is a molecular substance.
Atoms with electronegativities less than 2 combine to metallic bonds.
For example, $E.n.(Na)=0.9 2$, therefore is sodium a metal.
The limit of 2 that we have given ourselves is vague. Near to this limit, intermediate cases are all to common, for example, - in a sample of iron(III)chloride $(\Delta(E.n.)$ $=$ $1.2)$ there are molecules of $FeCl_3$, but also $Fe^{3+}$ and $Cl^-$ ions! - gold ($E.n.=2.4$) is a metal!