When am atom loses electron it becomes

It becomes an ion.

Orginally, the number of electrons of an atom should be same as the number of protons, and the atom should be electrically neutral, as each proton brings a positive charge and each electron brings a negative charge. However, when an atom loses or gains electrons, the charge would be positive or negative, depending on what the change is. In this way, the atom is no longer an atom, but an ion.

The number of electrons gained or lose by an atom depends on their group number in the periodic table. They'll gain or loose the least electrons in order to have an octet of electrons (or duplet). Which means, their outermost shell electrons would be 8, or 2 for the first shell.

One exam is fluorine. It is in the group VII, it has an outer most shell electron of 7. In order to make fluoride ion, one electron is gained, so that the outer most shell will have 8 electrons. Therefore, fluoride ion has a negative charge, as one electron brings a negative charge.