What a chemical bond is
An atom with an incomplete valence shell has three ways to complete it: it can lose electrons, gain them, or share them with another atom.
Whichever way is cheaper
Sodium is 2, 8, 1. It could give away its single outer electron and be left with 2, 8 — the neon arrangement. Or it could collect seven more to reach 2, 8, 8. Both routes end in a full shell, but one asks it to move one electron and the other seven. Sodium loses the one.
Chlorine is 2, 8, 7, and faces the mirror image of the same choice. Gaining one electron gives 2, 8, 8; losing seven would empty the shell entirely. Chlorine gains the one.
Notice that neither atom is doing anything a chemist decided for it. Each simply takes the shortest route to a full outer shell.
Chemical bond
The force of attraction that arises between atoms, or between ions, when the electrons of their valence shells are rearranged so that the atoms become stable.
Two kinds of bond
How the atoms taking part behave splits chemical bonds into two types, and the rest of this chapter is largely about telling them apart.
Electrons are transferred outright from one atom to another. The atoms become charged ions and the charges hold them together.
Electrons are shared between the atoms. No charges appear; the shared pair belongs to both atoms at once.
Transfer or share — that single distinction is what separates salt from sugar, and it decides nearly every property the two have.