CDS Chemistry · Atomic Structure and Periodic Classification

Atomic Number, Isotopes and Isobars

Counting protons, neutrons and electrons from Z, A and the charge, and telling isotopes, isobars and isoelectronic species apart.

All Atomic Structure and Periodic Classification notes

Why this matters

Eight CDS questions, all EASY or MODERATE. The isotope–isobar swap is the favourite trap: four questions turn on which of the two shares the mass number.

Concept 1 of 3: Counting protons, neutrons and electrons

The atomic number Z is the number of protons, and it fixes which element you have. The mass number A counts protons plus neutrons. A neutral atom has as many electrons as protons; each positive charge removes one electron and each negative charge adds one.

Definition

The rules:

Atomic number Z = number of protons. It defines the element.
Mass number A = protons + neutrons = number of nucleons.
Neutrons = A − Z.
Electrons = Z for a neutral atom; Z − q for an ion of charge +q, Z + q for charge −q.
For a molecule or molecular ion, add the electrons of every atom, then adjust for the charge.

Mass number and electron count

A = Z + N \qquad e^{-} = Z - q

$A$ mass number (protons + neutrons)
$Z$ atomic number (protons)
$N$ number of neutrons
$q$ charge on the ion (+ or −)

Worked example

A sodium ion is written

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

. How many protons, neutrons and electrons does it have?

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

CDS · 2025 · CDS (I) 2025 — General Knowledge · Q60Moderate

Example 1 · Atomic Structure and Periodic Classification · Atomic Number, Isotopes and Isobars

Number of electrons present in the species

\mathrm{H_2^+}

\mathrm{He}

\mathrm{H_2}

and

\mathrm{O_2^+}

respectively are :

A positive ion has FEWER electrons

A charge of +q means q electrons have been lost. O₂⁺ has 16 − 1 = 15 electrons, not 17.

Drill 1 more on counting protons, neutrons and electrons

Concept 2 of 3: Isotopes and isobars

Isotopes are the same element with different weights: same Z, different A. Isobars are different elements that happen to weigh the same: same A, different Z. Keep the letters straight and these questions answer themselves.

Definition

The two terms:

Isotopes: same atomic number, different mass number (different neutrons). Same element, same chemistry, same place in the periodic table.
Isobars: same mass number, different atomic number. Different elements.
Hydrogen has three isotopes: protium (¹H), deuterium (²H) and tritium (³H). Only tritium is radioactive.
Chlorine has two stable isotopes, ³⁵Cl and ³⁷Cl, both with Z = 17.

Term	Same	Different	Example
Isotopes	Atomic number Z	Mass number A	³⁵Cl and ³⁷Cl (both Z = 17)Q
Isobars	Mass number A	Atomic number Z	⁴⁰Ar (Z = 18) and ⁴⁰Ca (Z = 20)Q CDS 2019 (II): 'isobars have the same atomic number' was the incorrect statement.
Hydrogen isotopes	Z = 1	A = 1, 2, 3	Protium, deuterium, tritium; tritium is radioactiveQ

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

CDS · 2020 · CDS (I) 2020 — General Knowledge · Q2Moderate

Example 2 · Atomic Structure and Periodic Classification · Atomic Number, Isotopes and Isobars

The elements of which of the following pairs are isobars?

Isobars share A, not Z

Isobars have the same mass number and different atomic numbers. 'Same atomic number, different mass number' describes isotopes. CDS has used this swap as the wrong statement more than once.

Hydronium is not an isotope

H₃O⁺ is an ion. The isotopes of hydrogen are protium, deuterium and tritium.

Drill 4 more on isotopes and isobars

Concept 3 of 3: Isoelectronic species

Isoelectronic species have the same number of electrons, even though they are different atoms or ions. Count the electrons in each and group them.

Definition

How to check:

Electrons = Z − charge for each species.
The 18-electron set: P³⁻, S²⁻, Cl⁻, Ar, K⁺, Ca²⁺.
The 10-electron set: N³⁻, O²⁻, F⁻, Ne, Na⁺, Mg²⁺, Al³⁺.

Electrons	Species
10	N³⁻, O²⁻, F⁻, Ne, Na⁺, Mg²⁺, Al³⁺
18	P³⁻, S²⁻, Cl⁻, Ar, K⁺, Ca²⁺Q CDS 2022 (II): Cl⁻ is not isoelectronic with Mg²⁺, which has only 10 electrons.

Practice this concept3 quick reps

The same idea in a real exam question:

CDS · 2022 · CDS (II) 2022 — General Knowledge · Q16Moderate

Example 3 · Atomic Structure and Periodic Classification · Atomic Number, Isotopes and Isobars

\mathrm{Cl^-}

is not isoelectronic with

Count the ion's electrons, not the atom's

Magnesium has 12 electrons, but Mg²⁺ has 10. So Mg²⁺ belongs with neon, not with the 18-electron ions such as Cl⁻ and K⁺.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

Counting protons, neutrons and electrons
Mass number and electron count
$A = Z + N \qquad e^{-} = Z - q$

Reference tables (2)

Isotopes and isobars3 rows

Term	Same	Different	Example
Isotopes	Atomic number Z	Mass number A	³⁵Cl and ³⁷Cl (both Z = 17)Q
Isobars	Mass number A	Atomic number Z	⁴⁰Ar (Z = 18) and ⁴⁰Ca (Z = 20)Q CDS 2019 (II): 'isobars have the same atomic number' was the incorrect statement.
Hydrogen isotopes	Z = 1	A = 1, 2, 3	Protium, deuterium, tritium; tritium is radioactiveQ

Isoelectronic species2 rows

Electrons	Species
10	N³⁻, O²⁻, F⁻, Ne, Na⁺, Mg²⁺, Al³⁺
18	P³⁻, S²⁻, Cl⁻, Ar, K⁺, Ca²⁺Q CDS 2022 (II): Cl⁻ is not isoelectronic with Mg²⁺, which has only 10 electrons.

Watch out for (4)

A positive ion has FEWER electrons
→ Counting protons, neutrons and electrons
Isobars share A, not Z
→ Isotopes and isobars
Hydronium is not an isotope
→ Isotopes and isobars
Count the ion's electrons, not the atom's
→ Isoelectronic species

Test yourself on Atomic Structure and Periodic Classification

10 past CDS questions from this chapter, timed at 10 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.

Take the Atomic Structure and Periodic Classification chapter test Drill all 8 questions