The valency of an atom will be one if there is a sufficient gap between first and second _.

Explanation

The valency of an atom will be one if there is a sufficient gap between the first and second ionization energies. Ionization energy is the energy required to remove an electron from an atom in its gaseous state. The first ionization energy is the energy required to remove the first electron from the atom, and the second ionization energy is the energy required to remove the second electron from the atom. If there is a large gap between the first and second ionization energies, it means that it is much more difficult to remove the second electron than the first electron. This is because the first electron is loosely bound to the atom, but the second electron is more tightly bound. An atom with a large gap between the first and second ionization energies will tend to have a valency of one. This is because the atom will be more likely to lose one electron than two electrons. So the answer is ionization energy.