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✦Same element, different mass?

The water you drink contains a tiny amount of heavy water ( $\ce{D2O}$ ) — identical in every chemical way to ordinary water, but about 11% heavier. The difference? One extra neutron in each hydrogen nucleus. That single addition doesn't change the chemistry but completely changes the mass. That's isotopes in action.

Every atom is defined by two numbers: how many protons it carries (which tells you which element it is) and how many total nuclear particles it has (which tells you how heavy it is). Once you know these two numbers, you know the atom.

Atomic Number (Z)

The atomic number Z = number of protons in the nucleus.

Because every element has a unique proton count, Z is the atom's identity card. Hydrogen always has Z = 1, carbon always has Z = 6, and gold always has Z = 79 — no exceptions.

In a neutral atom, the positive charge of the nucleus (from protons) is exactly balanced by the negative charge of the electrons orbiting it:

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

If electrons are added or removed (making an ion), Z stays the same but the electron count changes.

Mass Number (A) and the Nucleon

Protons and neutrons together are called nucleons — they are the particles that make up the nucleus.

The mass number A = total number of nucleons:

$A = Z + n = \text{protons} + \text{neutrons}$

where $n$ is the number of neutrons. So if you know A and Z, you can always find the neutron count:

$n = A - Z$

Example: Sodium ( $\ce{Na}$ ) has Z = 11 and A = 23, so it has $23 - 11 = 12$ neutrons.

The Nuclear Notation

Any atom can be written compactly as $\ce{^A_Z X}$ , where X is the element symbol:

Top-left superscript = A (mass number)
Bottom-left subscript = Z (atomic number)

Examples:

$\ce{^1_1 H}$ — Hydrogen with 1 proton, 0 neutrons
$\ce{^{23}_{11} Na}$ — Sodium with 11 protons, 12 neutrons
$\ce{^{35}_{17} Cl}$ — Chlorine with 17 protons, 18 neutrons

Because Z is already fixed by the symbol (Na is always 11), you'll often see just the mass number written: $\ce{^{23}Na}$ or sodium-23.

\begin{aligned} Z &= \text{number of protons} = \text{number of electrons (neutral atom)}\\ A &= Z + n \quad \text{(mass number = protons + neutrons)}\\ n &= A - Z \quad \text{(neutron count)} \end{aligned}

Key definitions

Z = atomic number, A = mass number, n = neutron number

Isobars and Isotopes

Isobars are atoms of different elements with the same mass number (same A, different Z). For example, $\ce{^{14}_6 C}$ and $\ce{^{14}_7 N}$ both have A = 14, but one is carbon and the other is nitrogen. They are completely different elements — just happen to weigh the same.

Isotopes are atoms of the same element with different mass numbers (same Z, different A). Same element means same proton count, but different neutron counts make them heavier or lighter.

Think of it this way: same element with different weights = isotopes; different elements with the same weight = isobars.

The Hydrogen Isotopes: A Case Study

Hydrogen is the only element whose three isotopes have separate names:

Isotope	Symbol	Protons	Neutrons	Abundance
Protium	$\ce{^1_1 H}$	1	0	99.985%
Deuterium	$\ce{^2_1 H}$ (D)	1	1	0.015%
Tritium	$\ce{^3_1 H}$ (T)	1	2	trace

All three have Z = 1 — they are all hydrogen and all react chemically in the same way. The extra neutrons make deuterium and tritium heavier, but their chemistry is essentially identical to protium.

Carbon isotopes: $\ce{^{12}_6 C}$ , $\ce{^{13}_6 C}$ , $\ce{^{14}_6 C}$ — all with 6 protons and 6, 7, or 8 neutrons respectively. $\ce{^{14}C}$ (carbon-14) is radioactive and is used in carbon dating.

Chlorine isotopes: $\ce{^{35}_{17} Cl}$ (75%) and $\ce{^{37}_{17} Cl}$ (25%) — which is why the reported atomic mass of chlorine is ~35.5, a weighted average of the two.

Key point: Since chemical properties are determined by the number of electrons — which equals Z — all isotopes of a given element show the same chemical behaviour.

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Isotopes vs Isobars vs Isotones

Isotopes

Same Z (same element)
Different A (different neutron count)
Same chemical properties
Different physical properties (mass, radioactivity)
Example: ¹H, ²H, ³H

Isobars

Different Z (different elements)
Same A (same total nucleons)
Different chemical properties
Example: ¹²C and ¹²B, ¹⁴C and ¹⁴N

Isotones

Different Z and different A
Same neutron count (N = A − Z)
Less common in exams but important for nuclear physics
Example: ¹⁴C (N=8) and ¹⁵N (N=8)

Isotopes

Same Z (same element)
Different A (different neutron count)
Same chemical properties
Different physical properties (mass, radioactivity)
Example: ¹H, ²H, ³H

Isobars

Different Z (different elements)
Same A (same total nucleons)
Different chemical properties
Example: ¹²C and ¹²B, ¹⁴C and ¹⁴N

Isotones

Different Z and different A
Same neutron count (N = A − Z)
Less common in exams but important for nuclear physics
Example: ¹⁴C (N=8) and ¹⁵N (N=8)

✦JEE / NEET Exam InsightJEE / NEET

◆Finding neutrons: The most common calculation in this topic:

n = A - Z

. Given symbol and mass number, always calculate neutron count first.

◆Isotopes vs Isobars — the classic trap: Isotopes = same Z (same element). Isobars = same A (different elements). Mix these up and you lose easy marks.

◆Isotopes have the same chemical properties because chemical behaviour is determined by electron configuration, which is set by Z — not by A.

◆The chlorine anomaly: Cl has atomic mass ~35.5 — not a whole number. This is because it is a mixture of

\ce{^{35}Cl}

(75%) and

\ce{^{37}Cl}

(25%). Weighted average =

0.75 \times 35 + 0.25 \times 37 = 35.5

◆Ion notation:

\ce{^{23}_{11}Na^+}

has 11 protons, 12 neutrons, but only 10 electrons (lost one to form the 1+ cation). Z and A never change when an atom becomes an ion.

Quick Check

Q1.An atom has 17 protons and mass number 35. How many neutrons does it have?

✦Same element, different mass?

Atomic Number (Z)

The atomic number Z = number of protons in the nucleus.

Because every element has a unique proton count, Z is the atom's identity card. Hydrogen always has Z = 1, carbon always has Z = 6, and gold always has Z = 79 — no exceptions.

In a neutral atom, the positive charge of the nucleus (from protons) is exactly balanced by the negative charge of the electrons orbiting it:

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

If electrons are added or removed (making an ion), Z stays the same but the electron count changes.

Mass Number (A) and the Nucleon

Protons and neutrons together are called nucleons — they are the particles that make up the nucleus.

The mass number A = total number of nucleons:

$A = Z + n = \text{protons} + \text{neutrons}$

where $n$ is the number of neutrons. So if you know A and Z, you can always find the neutron count:

$n = A - Z$

Example: Sodium ( $\ce{Na}$ ) has Z = 11 and A = 23, so it has $23 - 11 = 12$ neutrons.

The Nuclear Notation

Any atom can be written compactly as $\ce{^A_Z X}$ , where X is the element symbol:

Top-left superscript = A (mass number)
Bottom-left subscript = Z (atomic number)

Examples:

$\ce{^1_1 H}$ — Hydrogen with 1 proton, 0 neutrons
$\ce{^{23}_{11} Na}$ — Sodium with 11 protons, 12 neutrons
$\ce{^{35}_{17} Cl}$ — Chlorine with 17 protons, 18 neutrons

Because Z is already fixed by the symbol (Na is always 11), you'll often see just the mass number written: $\ce{^{23}Na}$ or sodium-23.

\begin{aligned} Z &= \text{number of protons} = \text{number of electrons (neutral atom)}\\ A &= Z + n \quad \text{(mass number = protons + neutrons)}\\ n &= A - Z \quad \text{(neutron count)} \end{aligned}

Key definitions

Z = atomic number, A = mass number, n = neutron number

Isobars and Isotopes

Isotopes are atoms of the same element with different mass numbers (same Z, different A). Same element means same proton count, but different neutron counts make them heavier or lighter.

Think of it this way: same element with different weights = isotopes; different elements with the same weight = isobars.

The Hydrogen Isotopes: A Case Study

Hydrogen is the only element whose three isotopes have separate names:

Isotope	Symbol	Protons	Neutrons	Abundance
Protium	$\ce{^1_1 H}$	1	0	99.985%
Deuterium	$\ce{^2_1 H}$ (D)	1	1	0.015%
Tritium	$\ce{^3_1 H}$ (T)	1	2	trace

Chlorine isotopes: $\ce{^{35}_{17} Cl}$ (75%) and $\ce{^{37}_{17} Cl}$ (25%) — which is why the reported atomic mass of chlorine is ~35.5, a weighted average of the two.

Key point: Since chemical properties are determined by the number of electrons — which equals Z — all isotopes of a given element show the same chemical behaviour.

Loading simulator…

Isotopes vs Isobars vs Isotones

Isotopes

Same Z (same element)
Different A (different neutron count)
Same chemical properties
Different physical properties (mass, radioactivity)
Example: ¹H, ²H, ³H

Isobars

Different Z (different elements)
Same A (same total nucleons)
Different chemical properties
Example: ¹²C and ¹²B, ¹⁴C and ¹⁴N

Isotones

Different Z and different A
Same neutron count (N = A − Z)
Less common in exams but important for nuclear physics
Example: ¹⁴C (N=8) and ¹⁵N (N=8)

Isotopes

Same Z (same element)
Different A (different neutron count)
Same chemical properties
Different physical properties (mass, radioactivity)
Example: ¹H, ²H, ³H

Isobars

Different Z (different elements)
Same A (same total nucleons)
Different chemical properties
Example: ¹²C and ¹²B, ¹⁴C and ¹⁴N

Isotones

Different Z and different A
Same neutron count (N = A − Z)
Less common in exams but important for nuclear physics
Example: ¹⁴C (N=8) and ¹⁵N (N=8)

Quick Check

Q1.An atom has 17 protons and mass number 35. How many neutrons does it have?