Atomic Number Z, Mass Number A and Nuclide Notation
Physics · nuclei · NEET
Atomic number Z is the number of protons in a nucleus, and mass number A is the total number of protons plus neutrons (A = Z + N). A nucleus is written in nuclide notation as (A on top, Z at bottom) X, for example 197/79 Au. Memory hook: Z counts the "charge protons", A counts "All nucleons", and N = A - Z gives the neutrons.
Nuclide notation places mass number A at the top and atomic number Z at the bottom of the symbol; neutrons are found from N = A - Z. Example: 197/79 Au has 79 protons and 118 neutrons.
Your doubts, answered
Is the atomic number the number of protons or electrons?
Atomic number Z is defined as the number of protons in the nucleus. In a neutral atom the number of electrons equals Z, but the definition of Z is always based on protons. This matters in nuclear physics because the nucleus has no electrons, so we count protons, not electrons.
What is the difference between mass number and atomic mass?
Mass number A is a whole-number count of nucleons (protons + neutrons), so it has no units. Atomic mass is the actual measured mass of the atom in atomic mass units (u), and it is not a whole number (for example, chlorine has atomic mass about 35.45 u). For NEET, A is always an integer; atomic mass is not.
How do I find the number of neutrons from nuclide notation?
Use N = A - Z. Read the top number as A (mass number) and the bottom number as Z (atomic number), then subtract. Example: for 197/79 Au, N = 197 - 79 = 118 neutrons.
Does the nucleus contain electrons?
No. The nucleus contains only protons and neutrons (together called nucleons). Electrons orbit outside the nucleus. This is why an alpha particle (a helium nucleus) is made of 2 protons and 2 neutrons only, with no electrons.
What is a nucleon?
A nucleon is the common name for a proton or a neutron. The mass number A equals the total number of nucleons in the nucleus. So A is sometimes called the nucleon number.
⚠️ The NEET trap ✗ For an alpha particle students count 2 electrons, 2 protons and 2 neutrons, thinking a helium species must carry electrons. ✓ An alpha particle is a bare helium nucleus 4/2 He. It has only 2 protons and 2 neutrons. The nucleus never contains electrons. 🧠 When a question says nucleus, particle, or alpha, count nucleons only. Electrons live outside the nucleus, so never add them to A or Z.
Real NEET questions
NEET 2019
An alpha-particle consists of:
A · 2 protons and 2 neutrons only ✓
B · 2 electrons, 2 protons and 2 neutrons
C · 2 electrons and 4 protons only
D · 2 protons only
Solution: Step 1: An alpha-particle is the nucleus of a helium atom, written as 4/2 He. Step 2: The atomic number Z = 2, so there are 2 protons. Step 3: The mass number A = 4, so the number of neutrons N = A - Z = 4 - 2 = 2. Step 4: A nucleus contains no electrons. So an alpha-particle has 2 protons and 2 neutrons only. Correct option: A.
A is the mass number, the total count of protons and neutrons (nucleons) in the nucleus. It is written at the top-left of the element symbol.
What does Z stand for in nuclide notation?
Z is the atomic number, the number of protons in the nucleus. It is written at the bottom-left of the element symbol and decides which element it is.
How is a nuclide written?
A nuclide is written as (A on top, Z at bottom) X, where X is the chemical symbol. For example, gold is 197/79 Au, meaning A = 197 and Z = 79.
How many neutrons are in 197/79 Au?
N = A - Z = 197 - 79 = 118 neutrons. There are also 79 protons, giving 197 nucleons in total.
Why is the atomic number important for identifying an element?
The atomic number Z equals the number of protons and is unique for each element. Two nuclei with the same Z are the same element even if their neutron numbers differ (these are isotopes).
