Chemistry · Structure Of Atom · NEET
Atomic number (Z) is defined as the number of protons in the nucleus. In a NEUTRAL atom, the number of electrons happens to equal the number of protons, so electrons = Z too. But the identity of the element is fixed only by protons. If the atom becomes an ion, its electrons change but the atomic number stays the same. So the safe rule: Z = protons always, Z = electrons only when the atom is neutral.
Atomic number (Z) counts only protons. Mass number (A) counts protons AND neutrons together. Z tells you WHICH element it is (it never changes for a given element). A tells you how heavy that particular atom is, and it can differ for atoms of the same element (these are isotopes). Quick check: A is always bigger than or equal to Z, because A includes Z plus the neutrons.
Use the formula: neutrons = mass number minus atomic number, or n = A minus Z. Example: for an atom with A = 23 and Z = 11 (sodium), neutrons = 23 minus 11 = 12. Never subtract electrons here. Neutrons only depend on A and Z, so ions do not change the neutron count.
An element is written as (A on top-left, Z on bottom-left) X. The TOP number is the mass number A (protons + neutrons). The BOTTOM number is the atomic number Z (protons). Example: for lutetium written as Lu with 175 on top and 71 on the bottom, A = 175 and Z = 71. Many students swap them, so remember: bottom is the smaller number (protons), top is the bigger number (total nucleons).
No. When an atom gains or loses electrons to form an ion, only the electron count changes. The number of protons stays the same, so the atomic number Z does NOT change, and the mass number A does not change either. Example: Na (11 protons, 11 electrons) becomes Na+ (11 protons, 10 electrons). Z is still 11 for both.
Mass number A is just a COUNT of protons plus neutrons, so it must be a whole number (you cannot have half a neutron). Atomic mass (the value on the periodic table, like 35.5 for chlorine) is a weighted AVERAGE of all the isotopes' masses, so it comes out as a decimal. Do not confuse mass number (a count) with atomic mass (an average). NEET uses mass number for proton-neutron problems.
The number of protons, neutrons and electrons in the atom lutetium (written as Lu with mass number 175 on top and atomic number 71 on the bottom), respectively, are:
Magnesium reacts with an element X to form an ionic compound. If the ground-state electronic configuration of X is 1s2 2s2 2p3, the simplest formula for this compound is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Atomic number (Z) is the number of protons in the nucleus, and it decides which element the atom is. Mass number (A) is the total number of protons plus neutrons, and it tells you how heavy the atom is.
Mass number = protons + neutrons, so A = Z + n. Rearranged, neutrons = mass number minus atomic number, n = A minus Z.
Yes. Atoms of the same element have the same Z but can have different numbers of neutrons, so they have different mass numbers A. These are called isotopes (for example carbon-12 and carbon-14).
No. Mass number is a whole-number count of protons plus neutrons for one atom. Atomic mass is the weighted average mass of all natural isotopes, so it is usually a decimal like 35.5 for chlorine.
Because Z equals the number of protons, and the number of protons is what defines the element. Changing the protons would change the element itself. Gaining or losing electrons (making an ion) or neutrons (making an isotope) does not change Z.
