Atomic number and placing elements

The atomic number is defined as the number of protons in the nucleus of an atom. It is unique to each element and never changes.

The atom has 8 protons, so its atomic number is directly 8. No calculation is needed — the atomic number equals the proton count.

The atomic number is 8, which corresponds to oxygen on the periodic table.

The question asks for the atomic number given that the atom has 8 protons. Atomic number = number of protons.

16 would be the mass number if the atom had 8 neutrons as well as 8 protons ($8+8=16$). The question only gives proton count, not mass number.

6 is the atomic number of carbon (6 protons) and 10 is the atomic number of neon (10 protons). Neither matches an atom with 8 protons.

Since atomic number = number of protons, the answer is 8.

The number of neutrons is found using: $\text{Number of Neutrons}=\text{Mass Number}-\text{Atomic Number}$

Substituting the given values: $\text{Neutrons}=23-11=12$

The atom has 12 neutrons. This element is sodium (Na), which has atomic number 11.

Mass number = protons + neutrons. So neutrons = mass number − atomic number = $23-11$.

11 is the atomic number (number of protons), not the number of neutrons. Confusing protons and neutrons is a common mistake.

23 is the mass number itself, not just the neutrons. 34 would result from adding $23+11$ instead of subtracting — an arithmetic error.

$23-11=12$ neutrons is the correct answer.