Chemistry · Structure Of Atom · NEET
A (the mass number) is the TOP-left number. Z (the atomic number) is the BOTTOM-left number. NCERT writes it as ᴬZ X. Easy trick: A is the bigger number, so it sits on top like the roof. Z counts only protons, so it is smaller and sits at the bottom. Example: in ¹²₆C, A = 12 (top) and Z = 6 (bottom).
Z = atomic number = number of protons only. A = mass number = protons + neutrons (the total number of nucleons). So A is always equal to or bigger than Z, because you add neutrons on top of the protons. To get neutrons: neutrons = A − Z. This subtraction is tested very often in NEET.
Put the element letter in the middle. Write the mass number A at the top-left and the atomic number Z at the bottom-left. For chlorine with 17 protons and mass number 35, you write ³⁵₁₇Cl. Both numbers go on the LEFT side of the letter, never on the right (the right side is only for charge).
The charge goes at the TOP-RIGHT of the symbol, after the letter. Example: ⁴⁰₂₀Ca²⁺ means calcium with mass number 40, atomic number 20, and a +2 charge. The left side always holds A and Z; the right side always holds the charge. Charge does NOT change A or Z — it only tells you how many electrons were gained or lost.
No. Adding or removing electrons makes an ion, but it never touches the nucleus. So protons (Z) and neutrons (A − Z) stay exactly the same for Ca and Ca²⁺. Only the electron count changes: Ca²⁺ has 2 fewer electrons than neutral Ca. This is a common NEET trap.
Read the top number as the mass number and the bottom number as the atomic number. So ¹⁷⁵₇₁Lu has A = 175 and Z = 71. That means 71 protons, 71 electrons (neutral atom), and 175 − 71 = 104 neutrons. This exact symbol appeared in NEET 2020.
The number of protons, neutrons and electrons in ¹⁷⁵₇₁Lu, respectively, are:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
A = mass number (protons + neutrons), Z = atomic number (number of protons), and X = the chemical symbol of the element. This is the standard NCERT notation for a nuclide.
A is always bigger than or equal to Z, because A = protons + neutrons while Z = protons only. The extra neutrons make A larger. That is why A sits on top.
Subtract the atomic number from the mass number: neutrons = A − Z. For example, ²³₁₁Na has 23 − 11 = 12 neutrons.
The charge is written at the top-right of the element symbol, for example ¹⁶₈O²⁻. The mass number and atomic number stay on the left side and do not change for an ion.
No. Ions form by gaining or losing electrons, which does not affect the nucleus. So both A (mass number) and Z (atomic number) stay the same; only the electron count changes.
