Giving and accepting electrons

When two atoms (or molecules) are at a sufficient distance, there is no interaction between them (indifference).

When two atoms (or molecules) come close enough, the first thing that takes place is a repulsion between its outermost electronic clouds.

For two atoms (or molecules) to join or react,they have to overcome this initial repulsion. Therefore, atoms (or molecules) have to collide with sufficient speed and proper orientation to surpass the aforementioned repulsion.

In energetic terms it is as if the system formed by the two atoms (or molecules) would have to follow a profile that has a small climb at the beginning. If the speed or orientation of atoms (or molecules) is not appropriate, atoms (or molecules) repel each other and nothing else happens.

If the speed and orientation are adequate, the initial repulsion is defeated and other forces begin to act that cause the reaction between atoms (or molecules)take place.

In this case the atom on the right is one of those that has a small ionization potential (for example an atom of the two first rows of the periodic system) and the one on the left, which has a large, very negative electronic affinity (for example an atom from the penultimate row of the periodic system). Under these circumstances, the atom on the right gives way easily an electron to the atom on the left and the reaction between the two atoms produces two ions.

The ion on the left is negative because by accepting an electron from the atom on the right it has one more electron that exceeds its number of protons in the nucleus, which has not been altered. The initial balance (equal number of electrons and protons) has changed in favor of one more electron.

The ion on the right is positive because the atom it comes from has given up an electron to the atom on the left. By having one less electron, the initial balance (equal number of electrons and protons) has been altered, resulting in the number of protons in the nucleus (which has not been altered) exceeding in one to the number of electrons.

Once the ions are formed, their opposite charges cause them to attract each other strongly. A true bond is not formed between the ions but they remain very close by very powerful electrostatic forces that act over long distances. To separate the ions completely, a great force will be needed and a high energy has thus to be provided to the ionic system. For this reason, ionic compounds, such as common salt for example, are very difficult to melt.