Across a period, effective nuclear charge increases as electron shielding remains constant. This pulls the electron cloud closer to the nucleus, strengthening the nuclear attraction to the outer-most electron, and is more difficult to remove (requires more energy).

Since electrons repel each other, it is slightly easier to remove the electron from the paired set in the oxygen atom than it is to remove an unpaired electron from the nitrogen atom.

As mentioned above, the characteristic chemical property of a metal atom is to lose one or more of its electrons to form a positive ion. However, certain metals lose electrons much more readily than others. In particular, cesium (Cs) can give up its valence electron more easily than can lithium (Li).

Is it easier to form a positive ion from an element which has a high ionization energy or from an element that has a low ionization energy? Explain. It is easier to form a positive ion from an element which has a low ionization energy, because it’s easier to remove an electron (to make the ion).

The alkaline-earth metals, the next group to the right, have higher ionization energies ranging from 214.9 in beryllium to 120.1 kcal/mole in barium.

Melting point, temperature at which the solid and liquid forms of a pure substance can exist in equilibrium. As heat is applied to a solid, its temperature will increase until the melting point is reached.

Knowing the melting point of a chemical is very important for its storage & transport. In addition to that, melting point is often used to predict the partition behavior of a chemical between solid and gas phases. A higher melting point indicates greater intermolecular forces and therefore less vapour pressure.