5.5 Electronegativity

Each element has a measurable tendency to give away or hang onto electrons, called the element's electronegativity.

Element	Symbol	Electronegativity
Fluorine	F	3.98
Oxygen	O	3.44
Chlorine	Cl	3.16
Nitrogen	N	3.04
Sulfur	S	2.58
Carbon	C	2.55
Hydrogen	H	2.20
Copper	Cu	1.90
Iron	Fe	1.83
Zinc	Zn	1.65
Magnesium	Mg	1.31
Calcium	Ca	1.00
Sodium	Na	0.93
Potassium	K	0.82

When two atoms come together whose electronegativities are very different, such as sodium and chlorine, they cannot form a covalent bond. The chlorine atom wants an electron in the worst way, and the sodium atom has one that it's desperate to get rid of. So they transfer the sodium's valence electron to the chlorine, releasing much energy, and become ions of Na⁺ and Cl^-.

Carbon and hydrogen, by contrast, have electronegativities that are very close together. They easily form covalent bonds, because they both want each other's electrons almost equally, so the easiest thing is to share them.

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