Atomic number and mass number

Every atom carries two whole numbers that work like an identity card — its atomic number and its mass number. The atomic number tells you which element you are looking at; the mass number tells you roughly how heavy that atom's nucleus is. Together with the count of protons, neutrons and electrons you already know from the "Atomic Structure" chapter, these two numbers let you describe any atom completely, work out any one subatomic-particle count from the other two, and read the standard $^{A}_{Z}\text{X}$ notation used to label atoms in chemistry. This article stays inside the CISCE Class VIII syllabus: only whole-number counts of protons, neutrons and electrons are used, and ideas like isotopes or relative atomic mass are left for a later class.

The core insight

Atomic number $Z$ counts only protons and fixes which element an atom is, while mass number $A$ counts protons plus neutrons and fixes roughly how heavy its nucleus is, so neutrons $= A - Z$ .

The core idea

Atomic number ( $Z$ ) is the number of protons in the nucleus of an atom. It is the single defining property of an element — every atom of a given element has exactly the same atomic number, and no two different elements share one. Since a neutral atom has equal numbers of protons and electrons, the atomic number also gives you the electron count for a neutral atom:

Z = \text{number of protons} = \text{number of electrons (in a neutral atom)}

where: $Z$ is the atomic number, a positive whole number that is unique to each element (for example, every carbon atom anywhere has $Z = 6$ ).

Mass number ( $A$ ) is the sum of the number of protons and the number of neutrons in the nucleus (electrons are far too light to count toward it):

A = (\text{number of protons}) + (\text{number of neutrons})

where: $A$ is the mass number, always a whole number equal to or greater than $Z$ , since it includes everything $Z$ includes plus the neutrons on top. Combining the two relations, if you know an atom's mass number and atomic number you can always recover the number of neutrons by subtraction:

\text{number of neutrons} = A - Z

where: the subtraction only works when $A$ and $Z$ describe the same atom — mixing up numbers from two different atoms gives a meaningless result. Chemists write an atom's identity compactly using both numbers together: the mass number as a left superscript and the atomic number as a left subscript, both placed before the element's symbol, as in $^{23}_{11}\text{Na}$ for sodium. Here the atomic number 11 says sodium has 11 protons (and 11 electrons, since the atom is neutral), and the mass number 23 says the total of protons and neutrons is 23, so the number of neutrons is $23 - 11 = 12$ . It helps to keep the two numbers straight by remembering what each one is built from. Atomic number counts just one kind of particle — protons — so it directly identifies the element: every oxygen atom, anywhere, has atomic number 8, and nothing else does. Mass number counts two kinds of particle added together — protons plus neutrons — so it reflects how heavy the nucleus is rather than which element it is. This is also why mass number is always at least as large as atomic number.

Figure

A labelled nucleus diagram showing eight protons and eight neutrons packed together with atomic number and mass number called out beside it

Derivation AFinding the number of neutrons from mass and atomic number

The goal is to show, with nothing beyond the two defining relations above, exactly why subtracting atomic number from mass number always hands you the number of neutrons in an atom — and to fix the subtraction firmly enough that you never mix it up with adding or with subtracting in the wrong order, since this single step is the one most Class VIII exam questions actually test.

Step 1. Start from the definition of mass number: it is protons plus neutrons added together. Writing this with symbols, $A = p + n$ , where $p$ is the number of protons and $n$ is the number of neutrons.

Step 2. Recall the definition of atomic number: it is simply the number of protons, so $Z = p$ . This means every occurrence of $p$ in the mass-number equation can be replaced by $Z$ .

Step 3. Substitute $Z$ for $p$ in the equation from Step 1: $A = Z + n$ . This is exactly the same relationship as before, just written using the atomic number instead of the raw proton count.

Step 4. Rearrange to isolate $n$ by subtracting $Z$ from both sides: $A - Z = n$ , so $n = A - Z$ . Because $A$ is always greater than or equal to $Z$ (mass number includes the protons of atomic number plus the neutrons), this subtraction never gives a negative answer for a real atom.

Worked example

Question: An atom of chlorine has mass number 35 and atomic number 17. Find the number of protons, neutrons and electrons in this neutral atom, and write it in the standard $^{A}_{Z}\text{X}$ notation.

Step 1. Read off the number of protons directly from the atomic number: since $Z = 17$ , the atom has $17$ protons.

Step 2. Find the number of electrons. The atom is neutral, so the number of electrons equals the number of protons: $17$ electrons.

Step 3. Find the number of neutrons using $n = A - Z$ : $n = 35 - 17 = 18$ neutrons.

Step 4. Write the standard notation with the mass number as superscript and atomic number as subscript, both before the symbol Cl: $^{35}_{17}\text{Cl}$ .

This chlorine atom has 17 protons, 18 neutrons and 17 electrons, written as $^{35}_{17}\text{Cl}$ .

Competitive stretch

Suppose the same chlorine atom lost one electron to become a positively charged ion. Would its atomic number, mass number, or neutron count change? None of them would — atomic number and mass number are properties of the nucleus (protons and neutrons), which is untouched when only an electron is removed. Only the electron count drops to $16$ , making the particle a $\text{Cl}^{+}$ ion rather than a neutral atom, while $Z = 17$ , $A = 35$ and neutrons $= 18$ stay exactly as before.

Common traps

Confusing atomic number with mass number — atomic number counts only protons; mass number counts protons and neutrons together.
Applying "number of electrons = number of protons" to an atom that isn't neutral — that rule only holds for a neutral atom.
Finding neutrons by using the mass number alone instead of subtracting the atomic number — remember $n = A - Z$ , not $A$ by itself.
Swapping which number goes on top and which goes on the bottom in $^{A}_{Z}\text{X}$ notation — mass number is always the superscript (top), atomic number is always the subscript (bottom).

Ask the tutor: want a quick drill calculating protons, neutrons and electrons from atomic number and mass number for a few different elements?

If mass number and atomic number are so closely related, why do we need both

Because they tell you different things. Atomic number alone tells you which element an atom is, since it is simply the proton count and every atom of an element shares it. But two atoms with the same atomic number can still have different numbers of neutrons, so mass number is needed separately to say how many total nuclear particles — and therefore roughly how heavy — a particular atom is. You need atomic number to identify the element and mass number to know its neutron count, so both numbers together give the complete picture.

Does mass number include the mass of the electrons too

No. Mass number only adds up the number of protons and the number of neutrons; electrons are left out of the count entirely because they are so much lighter than protons and neutrons that their contribution to an atom's total mass is negligible in this chapter's whole-number picture. So even though a real atom does have electrons contributing a tiny amount of mass, for the purposes of defining mass number in Class VIII chemistry, only protons and neutrons are counted.

Can two different elements ever have the same atomic number

No, never. Atomic number is the very definition of which element an atom is — it is precisely the number of protons, and every element is defined by having its own unique proton count. If two atoms have the same atomic number, they are, by definition, atoms of the same element, no matter how many neutrons or electrons they might otherwise have. Two atoms with the same atomic number but different mass numbers are still the same element (this deeper idea, called isotopes, is studied in a later class).

Atomic number and mass number

The core idea

Worked example

Competitive stretch

Common traps

Before this

Where this leads