Niels Bohr was a scientist that presented the world with his renowned model of an atom. Unlike JJ Thompson, he said that electrons were present in orbital shells around a nucleus. The protons (positively charged particles) and neutrons (neutral) were in the nucleus. The electron has a negative charge, so due to the theory of electrostatic attraction, opposite charges attract and electrons are thus bound in orbit.
On gaining energy, an electron "jumps" and moves over to the next consecutive shell. Each shell is associated with a fixed energy amount. He correctly pointed out that the electrons in the last shell (valence shell) are the ones which determine the chemical properties of the element. Why? This is because the last shell electrons, called the valence electrons participate in the chemical reactions that take place. Another thing to keep in mind is the maximum number of electrons in a shell is accomodated by this formula: n^2, where n is the shell number.
Na + Cl -----> NaCl. But it does not happen directly. Na has atomic number 11, so there are 3 shells: 2,8,1. Cl has atomic number 17 and has 3 shells: 2,8,7. Electrons tend to attain a stable configuration by attaining 8 electrons in the last shell. Therefore, it is evident that Na has an excess electron and Cl deficient of one electron. Na donates the electron to Cl and Cl takes the electron from Na, thereby forming an ionic bond. So the reaction takes place.