How do I know if its an ionic bond or a covalent bond?

Ionic vs covalent.....

As for "ionic" vs "covalent"..... It's not an "either-or". The terms "ionic" and "covalent" refer to ideal bonds, bonds which seldom occur in nature. There are no 100% ionic bonds, and relatively few 100% covalent bonds. Real bonds lie along a continuum between these two hypothetical extremes and have characteristics of both. This is why it is more useful to talk about the "percent ionic character" of a bond rather than saying it is "ionic" or "covalent".

Most of the few 100% covalent bonds are found in the homonuclear, diatomic molecules of the elements (H2, N2, O2, F2, Cl2, Br2, I2) as well as molecules like P4 and S8. There are no 100% ionic bonds. But there are bonds which are very polar and have many of the characteristics of the hypothetical ionic bond. The percent ionic character of a bond is a function of the electronegativity difference, ΔEN. Bonds with the greatest ionic character will be between elements with the greatest difference in electronegativity.

Percent ionic character = 100[1 - e^(-ΔEN²/4)]

Compound .... percent ionic character

CsF ................ 92%

CaO ............... 77%

NaCl................ 71%

AlCl3 ............... 45%

CuCl2 ............. 33%

H2O ................ 32%

SnCl2 .............. 30%

PCl3 ................ 21%

CH4 .................. 3%

Don't be misled by statements that metal-nonmetal bonds are "ionic". Clearly, you can see that many bonds between metals and nonmetals have significant covalent character.

Therefore, the bonds in compounds which involve nonmetals have significantly greater covalent character. Bonds between the alkali metals (and some alkaline earth metals) and highly electronegative elements tend to have high ionic character. But for the transition metals and poor metals, their bonds with nonmetals will often have greater covalent character than ionic character. Therefore, you cannot say that all metal-nonmetal bonds are "ionic." That simply isn't true.

Metal / nonmetal: ionic. Nonmetal / nonmetal: covalent.

On a covalent bond the outer electron shell is filled on an ionic bond the outer shell in not filled to capacity. This is my best recall. Chemistry was many years ago.