Ionisation potential,Can ionization energy is negative?,why ionization energy of nitrogen is greater than oxygen ?

Definition:-

The minimum energy required to remove the outermost electron of an atom in its gaseous state is called ionisation potential or ionization energy.

Can ionization energy is negative?

No ionization energy never be negative.

Ionization energy is always positive and it is the minimum amount to energy.

Periodic trend of ionization energy

In periodic table ionization energy increases for going  from left to right along a period.

Because in a particular period for going left to right one electron increases in each step and this valence electron is placed in a same shell  thus increases the effective nuclear charge due to this ionization energy  also increases.

In a group for going above to below  ionization energy decreases.

Because in a particular group foregoing above to below in every step the new new upcoming electron is placed the different shell which decreases the effective nuclear charge of this outermost electron that is why ionization energy decreases along the group from above to below.

Among metal and nonmetal we can say that metal have lowest ionization energy and non metal have highest amount of ionization energy.

Why metals are  low amount of ionization energy and non metals are high amount of ionization energy?

definition of ionization energy say that the minimum amount of energy required to remove  the outermost electron of an atom, now for metal the outermost electron is loosely bounded by the nucleus that is why metal have very low ionization energy. But in case of non metal the outermost electron are tightly bound it by the nucleus so to remove this tightly bounded electron from its valence shell required very high amount of energy and that is why non metals have high ionization energy.

why ionization energy of nitrogen is greater than oxygen ?

ionization energy depends upon the  electronic configuration ,higher the stability of electronic configuration higher will be its ionization energy.

the electronic configuration of nitrogen is 1s22s22p3. thus 2P orbital are half-filled and highly stabilized. to remove the outermost electron from 2P orbital required very high amount of energy and that is why the ionization energy of nitrogen is high.

 on the other hand the electronic configuration of oxygen is 1s22s22p4. this electronic configuration are not stabilized like nitrogen and that is why to remove the outermost electron of oxygen required less energy and that is why its ionization energy is low.