Question

what is the atomic number of this element​

Answer 1

Explanation:

The atomic number is the number of protons in the nucleus of an atom. The number of protons define the identity of an element (i.e., an element with 6 protons is a carbon atom, no matter how many neutrons may be present).

Answer 2

Question:-

what is the atomic number of this element

Answer:-

The mass number of an atom is its total number of protons and neutrons. Atoms of different elements usually have different mass numbers , but they can be the same. For example, the mass number of argon atoms and calcium atoms can both be 40.

Given:-

$\sf\fbox{Formula}$

Atomic number = $Number of protons$

Thus the atomic number of Na atom = $number of electrons$

number of protons = $11.$