Which of the following statements are true?

The correct statement is: On Pauling scale, the electronegativity values of N and Cl are not the same.

Let us analyze each statement step-by-step to understand why this is correct and evaluate the others:

1. Electronegativity of Nitrogen (N) and Chlorine (Cl):
On the Pauling scale of electronegativity, nitrogen has a value of 3.04, whereas chlorine has a value of 3.16. Although they are very close and often rounded to 3.0 in simplified tables, they are not the same. Thus, this statement is true.

2. Melting points of Gallium (Ga) and Cesium (Cs):
Gallium has an unusually low melting point of about 29.76 °C (302.9 K) and can melt in the palm of a hand. Cesium also has a very low melting point of 28.44 °C (301.59 K). The statement claiming gallium has a very high melting point is false.

3. First ionization enthalpies of Na, Mg, Al, and Si:
Generally, first ionization enthalpy increases across a period. However, magnesium has a stable, fully-filled valence shell electronic configuration ($3s^2$) compared to aluminum ($3s^{2}3p^1$). This makes it harder to remove an electron from magnesium than from aluminum. Therefore, the actual order of first ionization enthalpies is:
$\mathrm{Na}<\mathrm{Al}<\mathrm{Mg}<\mathrm{Si}$
This makes the statement Si > Al > Mg > Na incorrect.