na-23 has 11 protons. how many neutrons does it have?\n\na. 11\nb. 12\nc. 23\nd. 34

Explanation:

Step1: Recall atomic number concept

The atomic number of an element is equal to the number of protons. For sodium (Na), the atomic number is 11 as it has 11 protons.

Step2: Recall mass - number formula

The mass - number of an atom is the sum of the number of protons and neutrons. The mass - number of Na - 23 is 23. Let the number of neutrons be $n$, the number of protons be $p$, and the mass - number be $A$. Then $A=p + n$.

Step3: Calculate number of neutrons

We know that $A = 23$ and $p=11$. Substitute these values into the formula $n=A - p$. So $n=23 - 11=12$.

Answer:

B. 12