Salt comes from electrostatic interaction of Cl and one electron of Na. Because Na atom has full 2s and 2p shells, it has only one electron on the next one (3s). So this electron "feels" the +11 charge of nucleus being well shielded by 10 inner electrons, thus leaving a charge about +1 only. Therefore, Coulomb attraction force keeping this electron "on a leash" is rather loose.
Cl atom, on the other side, has 5 electrons on the outer 2p shell above the last closed shell (2s) and can accept 1 more (as allowed by Shroedinger equation). These 5 electrons are at about the same distance from the +9 nuclear charge, so each of those electrons "feels" nucleus as only partially screened by his electron "neighborhood", namely with remaining charge as +9(nucleus)-4(inner shells electrons)-4/2(from 4 neighbors screening 50% each), so the balance is +9-4-4/2=+3.
There are 2 consequences for Cl outer electrons have such strong interaction with only partially screened nucleus:
1) being pulled stronger, they come closer to nucleus (thus are attracted even stronger as the Coulomb force is inversely proportional to square of distance) - and, indeed, Cl atom is almost twice smaller than Na.
Second consequence is much more important because it explains why chlorine is such active chemical:
2) as any other atom with less bonded outer electron passes by Cl atom, then partly screened strong Coulomn force of Cl immediately pulls this electron out of weakier "partner" to add to existing 5 ones. Thus Cl becomes -1 charged ion as a whole. Now it has 6 strongly bonded electrons on outer shell and still does not mind to take a couple more, but Shroedinger equation prohibits taking "more than you need". Poor partner atom happen to be close (say, Na) now lost its outer electron and thus became +1 charged ion. It now has no choice as fall "in close love" with the chlorine ion as dictates by Coulomb equation (alike charges attract) - and, indeed, he does. Vivat: we made a NaCl (salt), so we can finally take a steak and salt it (our body cells being originated in salty ocean water about 2-3 billion years ago, still keep very similar to that early days environment inside us. Thus from time to time we need to refill Na+ deficit (which is driven by the ability of Na+ to bond with many unwanted chemicals to drive them out of our body via urine)). Red vine or good beer is also good with a steak, and then some Discovery or History channel, or some mild - tempered movie (not to awake, because during digestion blood runs mostly around stomach leaving little for brain which thus falls asleep).