Alkali metals are relatively soft because the atoms contribute only one to a metallic bond

Valence Electron because the more valence electrons an atom can contribute the shared pool, the stronger the metallic bonds will be

0.21 mole of NaCl

Explanation:

From the question,

using Avogadro's Constant,

1 mole of every compound contains 6.02×10²³ molecules of that compound.

I.e

1 mole of NaCl ⇒ 6.02×10²³ molecules of NaCl

x moles of NaCl ⇒ 1.25××10²³ molecules of NaCl

Solving for x,

x moles of NaCl = (1 mole of NaCl)×(1.25×10²³ molecules of NaCl)/(6.02×10²³ molecules of NaCl)

x moles of NaCl = (1×1.25×10²³)/6.02×10²³

x = 0.208 mole of NaCl

x ≈ 0.21 mole of NaCl