How many neutrons and protons are there in the following nuclei? $^{13}_{6}C,\ ^{16}_{8}O,\ ^{24}_{12}Mg,\ ^{56}_{26}Fe,\ ^{88}_{38}Sr$

How many neutrons and protons are there in the following nuclei? ¹³₆C,¹⁶₈O,²⁴₁₂Mg,⁵⁶₂₆Fe,⁸⁸₃₈Sr

Exercises · 2.3

^{13}_{6}C,\ ^{16}_{8}O,\ ^{24}_{12}Mg,\ ^{56}_{26}Fe,\ ^{88}_{38}Sr

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Concept understanding — Atomic Notation

Atomic Notation: The ID Card of an Atom

Imagine you're at a huge stadium filled with people. To identify any one person, you'd need more than just a name — you'd need their jersey number, their team, maybe their position. An atom is the same way. Just saying "carbon" or "oxygen" tells you the kind of atom, but not the full story. Atomic notation is the shorthand that gives you the complete identity.

The Intuition: Three Numbers, One Symbol

Every atom is built from three key particles:

Protons (positive charge) — these define what element it is.
Neutrons (no charge) — these add mass and stability.
Electrons (negative charge) — these orbit the nucleus and determine chemical behaviour.

Atomic notation packs all this information around the element's symbol. Think of it as a label with two numbers attached:

^{A}_{Z}\text{X}

Where:

X is the element symbol (e.g., C for carbon, O for oxygen).
Z is the atomic number (number of protons).
A is the mass number (protons + neutrons).

Note

The atomic number $Z$ is what makes an atom that element. Change $Z$ , and you change the element entirely. Carbon always has $Z=6$ ; if you change it to $7$ , it's nitrogen.

The Precise Statement

For any atom represented as $^{A}_{Z}\text{X}$ :

Atomic number $Z$ = number of protons = number of electrons (in a neutral atom).
Mass number $A$ = number of protons + number of neutrons.
Number of neutrons $N = A - Z$ .

A neutral atom has equal protons and electrons. If the atom gains or loses electrons, it becomes an ion — but the notation for the nucleus stays the same.

Example: Carbon-12

The most common carbon atom is written as:

^{12}_{6}\text{C}

From this:

$Z=6$ → 6 protons, and in a neutral atom, 6 electrons.
$A=12$ → total nucleons (protons + neutrons) = 12.
Neutrons $N = 12 - 6 = 6$ .

Why Two Numbers? Why Not Just One?

If you only knew the mass number $A$ , you wouldn't know the element — because different elements can have the same mass number (e.g., $^{14}\text{C}$ and $^{14}\text{N}$ both have $A=14$ ). If you only knew the atomic number $Z$ , you wouldn't know how many neutrons are present — and that matters for isotopes.

Important

Isotopes are atoms of the same element (same $Z$ ) with different numbers of neutrons (different $A$ ). For example:

$^{12}_{6}\text{C}$ (6 neutrons)
$^{13}_{6}\text{C}$ (7 neutrons)
$^{14}_{6}\text{C}$ (8 neutrons) All are carbon, but they have different masses and some are radioactive.

A Quick Table for Clarity

Notation	Element	Protons ( $Z$ )	Neutrons ( $N$ )	Mass Number ( $A$ )
$^{1}_{1}\text{H}$	Hydrogen	1	0	1
$^{4}_{2}\text{He}$	Helium	2	2	4
$^{23}_{11}\text{Na}$	Sodium	11	12	23
$^{235}_{92}\text{U}$	Uranium	92	143	235

Common Mistakes to Avoid

Watch out

Don't confuse $A$ and $Z$ . $Z$ is always the smaller number (protons), $A$ is the larger (total nucleons).
Don't assume $A$ equals atomic mass. Atomic mass is an average of all isotopes, not the mass number of a single atom.
Don't forget that $Z$ defines the element. If you see $^{14}_{7}\text{N}$ , that's nitrogen — not carbon, even though $A=14$ is the same as carbon-14.

The Bottom Line

Atomic notation $^{A}_{Z}\text{X}$ is the atom's ID card. It tells you:

What element (via $Z$ )
Which isotope (via $A$ )
How many neutrons (via $A-Z$ )

Once you can read this notation, you can decode the entire nuclear structure of any atom in a single glance.

Atomic notation, writing an element with its mass number and atomic number, is introduced early in the NCERT Class 11 Chemistry chapter on Structure of Atom, and "atomic notation definition, formula and examples" is a common search among students building their fundamentals for CBSE boards, JEE Main, and NEET. This notation is also used constantly in the Class 12 Physics Nuclei chapter, making it one of the most cross-referenced basics in PCM/PCB exam preparation.

For any nucleus written as $^{A}_{Z}X$ , the atomic number $Z$ gives the number of protons, and the neutron count is $A - Z$ . Applying this to the given nuclei: $^{13}_{6}C$ has 6 protons and 7 neutrons; $^{16}_{8}O$ has 8 protons and 8 neutrons; $^{24}_{12}Mg$ has 12 protons and 12 neutrons; $^{56}_{26}Fe$ has 26 protons and 30 neutrons; $^{88}_{38}Sr$ has 38 protons and 50 neutrons.

The idea: reading a nuclear symbol

Every nucleus is written in the standard notation $^{A}_{Z}X$ , where:

$X$ is the chemical symbol of the element.
$Z$ is the atomic number — the number of protons. This defines which element it is.
$A$ is the mass number — the total number of protons plus neutrons.

The number of neutrons $N$ is simply the difference: $N = A - Z$ .

That’s the entire principle. There is no hidden complexity — it’s just careful subtraction once you’ve correctly identified $Z$ and $A$ from the superscript and subscript.

Watch out

A common mistake is to confuse the mass number $A$ with the atomic mass (which is not an integer). Here, $A$ is always a whole number — it counts nucleons, not grams. Also, never assume the number of neutrons equals the number of protons; that’s only true for a few light stable nuclei.

Step-by-step for each nucleus

1. $^{13}_{6}C$

The subscript is $Z = 6$ , so there are 6 protons.

The superscript is $A = 13$ , so neutrons = $13 - 6 = 7$ .

2. $^{16}_{8}O$

$Z = 8$ → 8 protons.

$A = 16$ → neutrons = $16 - 8 = 8$ .

3. $^{24}_{12}Mg$

$Z = 12$ → 12 protons.

$A = 24$ → neutrons = $24 - 12 = 12$ .

4. $^{56}_{26}Fe$

$Z = 26$ → 26 protons.

$A = 56$ → neutrons = $56 - 26 = 30$ .

5. $^{88}_{38}Sr$

$Z = 38$ → 38 protons.

$A = 88$ → neutrons = $88 - 38 = 50$ .

Tip

Notice that for $^{16}_{8}O$ and $^{24}_{12}Mg$ , the number of protons equals the number of neutrons. This is a special case — these are “doubly magic” or light stable nuclei where $N = Z$ . For heavier elements like iron and strontium, neutrons outnumber protons, which is the general trend for stability.

✓Final answer

^{13}_{6}C

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Q.How many neutrons and protons are there in the following nuclei? 613C, 816O, 1224Mg, 2656Fe, 3888Sr^{13}_{6}C,\ ^{16}_{8}O,\ ^{24}_{12}Mg,\ ^{56}_{26}Fe,\ ^{88}_{38}Sr613​C, 816​O, 1224​Mg, 2656​Fe, 3888​Sr

The idea: reading a nuclear symbol

Step-by-step for each nucleus

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Q.How many neutrons and protons are there in the following nuclei? $^{13}_{6}C,\ ^{16}_{8}O,\ ^{24}_{12}Mg,\ ^{56}_{26}Fe,\ ^{88}_{38}Sr$

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