Question

if na is Avogadro's number then number of electrons in 4.2g of nitrogen atom​

Answer 1

Answer ⇒ Option (a). 1.5 Na.

Explanation ⇒ Using the formula,

No. of atoms = mass/atomic mass × Avogadro's number.

∴ No. of atoms = 4.2/14 × Na.

∴ No. of atoms = 0.3 Na.

Now, 1 atoms of Nitrogen contains 7 electrons.

∴ 0.3 Na atoms of Nitrogen contains 7 × 0.3 Na electrons = 2.1 Na electrons.

∴ No. of Nitrogen electrons = 2.1 Na.

This will be the only answer. Now we can see there is no such options. Let see that what will be our answer in case of Valence electrons.

no. of valence electrons = 5.

∴ no. of valence electrons in 4.2 g of nitrogen = 5 × 0.3 Na = 1.5 Na.

Now, this is one of the option.

Hence, (a). is correct.

Remember that!!! Prefer the first part only. But most of the time in  case of options answer do not match, then check for free electrons or valence electrons.

If there will be no options, then check first method only.

Hope it helps.