The number of neutrons in an atom of ₁₁²³X is - 

Concept:-

• Atomic Number (Z): This number represents the number of protons in an atom's nucleus. It is fundamental to the identity of an atom because the atomic number defines the element. For example, an atom with 1 proton is always hydrogen, an atom with 2 protons is always helium, etc. In your notation 1123X, 11 is the atomic number.
• Atomic Mass or Mass Number (A): This value is generally the sum of the protons and neutrons in an atom. Atomic mass gives the average mass of all isotopes of an atom weighted by their natural abundance. For atoms expressed in the form AZX, like 1123X, A represents the atomic mass. In 1123X, the atomic mass is 23.
• Neutrons: Neutrons, along with the protons, make up an atom's nucleus. Neutrons have no electric charge but contribute to the atom's mass. The number of neutrons in an atom can vary, producing different isotopes of the same element.

Explantion:-

The convention to represent an atom is _A^ZX, where X is the chemical symbol, A is the mass number (the sum of protons and neutrons), and Z is the atomic number (the number of protons).

So in the atom you're mentioning - ₁₁²³X - the number 11 is the atomic number (Z, representing the number of protons), and 23 is the mass number (A, the total number of protons and neutrons).

The number of neutrons can be calculated as the difference between the mass number (A) and the atomic number (Z).
To calculate the number of neutrons in an atom, you subtract the atomic number (Z) from the atomic mass (A). So, the formula is Neutrons = A - Z.

Neutrons = A - Z Neutrons = 23 - 11 Neutrons = 12

Conclusion:-

So, the correct answer is: 12