MHT-CET Chemistry · Structure of Atom

Subatomic Particles, Isotopes, Isobars and Isoelectronic Species

An atom is built from protons, neutrons and electrons; the atomic number counts the protons and the mass number counts the nucleons, and from those two numbers the whole family of iso-terms (isotopes, isobars, isotones, isoelectronic) is just a matter of asking which count is being held fixed.

All Structure of Atom notes MHT-CET Chemistry strategy

Why this matters

About 14 PYQs, all EASY and almost all pure recall or a one-line electron count — the guaranteed free marks of this chapter. They split three ways: the definitions of the four iso-words (isotopes, isobars, isotones, isoelectronic), the recurring 'identify the isoelectronic pair / the odd one out' electron-count, and one weighted-average-mass calculation for chlorine. Master the two defining numbers and how to count electrons in an ion, and every question here is a single step.

Concept 1 of 6

The three subatomic particles and the nuclide notation

Intuition

Every atom is three particles. Protons (positive) and neutrons (neutral) sit in the tiny dense nucleus and carry nearly all the mass; electrons (negative) are almost massless and occupy shells outside. The nuclide symbol packs the two key counts around the element: mass number on top, atomic number at the bottom.

Definition

The three particles and the two numbers that describe a nuclide:

Proton — charge +1, mass about 1 u, in the nucleus. Its count is the atomic number.
Neutron — charge 0, mass about 1 u, in the nucleus. Protons and neutrons together are nucleons.
Electron — charge −1, mass about 1/1836 of a proton (nearly massless), in shells outside the nucleus.
The nuclide is written $^{A}_{Z}\text{X}$ : mass number $A$ (top) and atomic number $Z$ (bottom), with neutrons $= A - Z$ .

Particle	Charge	Relative mass	Location
Proton	$+1$	$\approx 1\ \text{u}$	Nucleus
Neutron	$0$ (neutral)	$\approx 1\ \text{u}$	Nucleus
Electron	$-1$	$\approx \tfrac{1}{1836}$ of a proton	Shells outside the nucleus Electrons are so light that the mass number counts only protons and neutrons — never electrons.

Nucleons (protons + neutrons) carry the mass; the atomic number

Z

fixes the element.

Practice this conceptself-check · 4 quick reps

Try it yourself

An element has mass number 40 and 21 neutrons. Write its nuclide symbol and give its atomic number.

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

1.
Which particle carries a charge of $-1$ and sits outside the nucleus?
2.
What is the collective name for protons and neutrons?
3.
In $^{23}_{11}\text{Na}$ , how many neutrons are present?
4.
Which number in $^{A}_{Z}\text{X}$ identifies the element?

From the bank · past-year question

Example 1Structure of AtomEASY

What is the representation of an element having mass number of 40 and 21 neutrons in it?

[Q75 · 3rd May 2nd Shift · 2023]

Mass number counts nucleons, not electrons

The mass number is protons plus neutrons only. Electrons are about

1/1836

of a nucleon, so they add nothing to the mass number — never include them.

Read the notation the right way up

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

the top number is the mass number

A

and the bottom is the atomic number

Z

. Swapping them gives the wrong neutron count

A-Z

Concept 2 of 6

Counting protons, neutrons and electrons in a species

Intuition

Once you have the atomic number, the electron count is fixed by the charge: a neutral atom has electrons equal to protons, an ion adds electrons for a negative charge and loses them for a positive one. This single count is what almost every question here really tests.

Definition

The counts, straight from $Z$ , $A$ and the charge:

Protons $= Z$ ; neutrons $= A - Z$ .
Electrons in a neutral atom $= Z$ .
Electrons in an ion $= Z - (\text{charge})$ : subtract the charge, so a $+$ ion has fewer electrons and a $-$ ion has more.
Example — $\text{Ca}$ has $Z = 20$ , so a neutral calcium atom has exactly 20 electrons, whereas $\text{K}^{+}$ ( $Z = 19$ ) has $19 - 1 = 18$ .

Electron count of a species

e^- = Z - (\text{charge}) \qquad N = A - Z

Zatomic number (protons)
Amass number (nucleons)
Nnumber of neutrons

Worked example

Which of these has exactly 20 electrons:

\text{K}^{+}

(

Z=19

\text{Ca}

(

Z=20

\text{Mg}

(

Z=12

\text{Cl}

(

Z=17

For an ion, electrons $= Z - \text{charge}$ ; for a neutral atom, electrons $= Z$ .
$\text{K}^{+}: 19 - 1 = 18$ ; $\text{Mg}: 12$ ; $\text{Cl}: 17$ .
$\text{Ca}$ is neutral with $Z = 20$ , so it has $20$ electrons.

Answer:

\text{Ca}

— a neutral calcium atom has

20

electrons.

Practice this conceptself-check · 4 quick reps

Try it yourself

How many electrons does the sulphide ion

\text{S}^{2-}

have, given sulphur has

Z = 16

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

1.
Electrons in $\text{Ca}$ ( $Z = 20$ )?
2.
Electrons in $\text{K}^{+}$ ( $Z = 19$ )?
3.
Electrons in $\text{F}^{-}$ ( $Z = 9$ )?
4.
Neutrons in $^{40}_{20}\text{Ca}$ ?

From the bank · past-year question

Example 2Structure of AtomEASY

Which of the following species contain 20 electrons?

[Q88 · 16th May Shift 1 · 2023]

A neutral atom of Ca has 20 electrons, but Ca-based ions do not

The 20-electron species is neutral calcium, because electrons

= Z = 20

\text{Ca}^{2+}

would have only

18

. Check the charge before you count.

Add for negative, subtract for positive

For

\text{O}^{2-}

you add

2

electrons (

8 + 2 = 10

); for

\text{Na}^{+}

you subtract

1

(

11 - 1 = 10

). Getting the sign backwards is the single most common slip.

Concept 3 of 6

Isotopes, isobars, isotones and isoelectronic species

Intuition

Four 'iso-' words sound alike but each holds a different count fixed. Ask 'what is the same?' — protons, mass number, neutrons, or electrons — and the term names itself. This one distinction answers most of the definition questions in the subtopic.

Definition

The four families, sorted by which count is held equal:

Isotopes — same protons (same element, same $Z$ ), different neutrons and mass number. Same chemical properties, same periodic-table position. Example: $^{35}\text{Cl}$ and $^{37}\text{Cl}$ .
Isobars — same mass number $A$ , different elements (different $Z$ ). Example: $^{40}\text{Ar}$ and $^{40}\text{Ca}$ .
Isotones — same number of neutrons $(A - Z)$ , different $Z$ and $A$ . Example: $^{12}_{6}\text{C}$ and $^{11}_{5}\text{B}$ (both $6$ neutrons).
Isoelectronic species — same number of electrons, regardless of element or charge. Example: $\text{Na}^{+}$ , $\text{F}^{-}$ , $\text{O}^{2-}$ and $\text{Ne}$ (all $10$ ).

Term	What is the same	What differs	Example
Isotopes	Protons $Z$ (same element)	Neutrons / mass number	$^{35}\text{Cl}$ , $^{37}\text{Cl}$ Isotopes do NOT have equal neutrons — that is the false statement the bank plants.
Isobars	Mass number $A$	Element ( $Z$ )	$^{40}\text{Ar}$ , $^{40}\text{Ca}$
Isotones	Number of neutrons	$Z$ and $A$	$^{12}_{6}\text{C}$ , $^{11}_{5}\text{B}$
Isoelectronic	Number of electrons	Element and charge	$\text{Na}^{+}$ , $\text{F}^{-}$ , $\text{O}^{2-}$ , $\text{Ne}$

Ask 'what count is held fixed?' — protons, mass number, neutrons, or electrons.

Practice this conceptself-check · 4 quick reps

Try it yourself

^{14}_{6}\text{C}

and

^{16}_{8}\text{O}

: are they isotopes, isobars, isotones or isoelectronic?

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

1.
Two nuclides with the same mass number but different atomic numbers are called what?
2.
$^{35}\text{Cl}$ and $^{37}\text{Cl}$ differ only in the number of which particle?
3.
$^{12}_{6}\text{C}$ and $^{11}_{5}\text{B}$ share the same number of which particle?
4.
Do isotopes occupy the same position in the periodic table?

From the bank · past-year question

Example 3Structure of AtomEASY

Which from following pairs is an example of isotones?

[Q65 · 9th May Shift 1 · 2023]

'Isotopes have equal neutrons' is FALSE

Isotopes share protons, not neutrons — indeed they must differ in neutrons (that is what gives them different mass numbers). The statement 'they have equal number of neutrons' is the false one to spot.

Isotones vs isobars vs isotopes

Same neutrons -> isotones; same mass number -> isobars; same protons -> isotopes.

^{14}_{6}\text{C}

and

^{16}_{8}\text{O}

share neutrons, so they are isotones, not isobars.

Drill 3 more on isotopes, isobars, isotones and isoelectronic species

Concept 4 of 6

Identifying isoelectronic species by counting electrons

Intuition

The bank's favourite recurring question: pick the isoelectronic pair, or spot the odd one out. There is no theory — just count electrons in each species and find the match. The count is

Z

minus the charge, so

\text{O}^{2-}

8 + 2 = 10

\text{Na}^{+}

11 - 1 = 10

, and they match.

Definition

Two species are isoelectronic when they have the same number of electrons:

Compute electrons for each: $e^- = Z - (\text{charge})$ .
The $10$ -electron set (neon core) is the one PYQs use most: $\text{O}^{2-}$ , $\text{F}^{-}$ , $\text{Ne}$ , $\text{Na}^{+}$ , $\text{Mg}^{2+}$ , $\text{Al}^{3+}$ all have $10$ .
The odd-one-out is usually a neutral atom slipped in beside its ions — e.g. neutral $\text{Na}$ has $11$ , not $10$ .

Isoelectronic test

e^-_1 = Z_1 - q_1 \;\overset{?}{=}\; Z_2 - q_2 = e^-_2

Zatomic number of the species
qcharge (with sign; subtract it)
e^-resulting electron count

Worked example

Identify the isoelectronic pair: (a)

\text{Ne}

and

\text{O}^{2-}

, (b)

\text{Cl}^{-}

and

\text{Ca}

, (c)

\text{Ar}

and

\text{F}^{-}

Count electrons: $\text{Ne} = 10$ ; $\text{O}^{2-} = 8 + 2 = 10$ — a match.
$\text{Cl}^{-} = 17 + 1 = 18$ , but $\text{Ca} = 20$ — not equal.
$\text{Ar} = 18$ , but $\text{F}^{-} = 9 + 1 = 10$ — not equal.

Answer:(a)

\text{Ne}

and

\text{O}^{2-}

, both with

10

electrons.

Practice this conceptself-check · 4 quick reps

Try it yourself

Which one is NOT isoelectronic with the other three:

\text{Ne}

\text{O}^{2-}

\text{Na}

\text{Na}^{+}

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

1.
Is $\text{Mg}^{2+}$ ( $Z=12$ ) isoelectronic with $\text{Ne}$ ?
2.
How many electrons in $\text{Al}^{3+}$ ( $Z = 13$ )?
3.
Are $\text{O}^{2-}$ and $\text{Na}^{+}$ isoelectronic?
4.
Is neutral $\text{Na}$ isoelectronic with $\text{Ne}$ ?

From the bank · past-year question

Example 4Structure of AtomEASY

Identify isoelectronic pair from following.

[Q95 · 22 April Shift II · 2025]

The neutral atom hidden among its ions

When the options are

\text{Ne}

\text{O}^{2-}

\text{Na}^{+}

and

\text{Na}

, the odd one out is the neutral

\text{Na}

(

11

electrons). Always apply the charge before comparing.

Same electrons, not same protons

Isoelectronic is about electron count, so

\text{Cl}^{-}

and

\text{Ca}

(

18

20

) are not a pair even though both are 'near argon'. Recount

Z - q

each time — do not eyeball by element.

Drill 3 more on identifying isoelectronic species by counting electrons

Concept 5 of 6

Average atomic mass and isotope abundance ratio

Intuition

An element's periodic-table mass is the abundance-weighted average of its isotope masses. The same relation runs backwards: given the average and the two isotope masses, you can solve for the abundance ratio. Chlorine (average

35.5

from

^{35}\text{Cl}

and

^{37}\text{Cl}

) is the standard example.

Definition

The weighted-average mass, used both ways:

Forwards — average $= \sum (\text{isotope mass} \times \text{fraction})$ , where the fractions sum to $1$ .
Backwards — let the abundance of one isotope be $x\%$ and solve. For chlorine: $\dfrac{35x + 37(100 - x)}{100} = 35.5$ gives $x = 75$ , so the ratio $^{35}\text{Cl} : {}^{37}\text{Cl} = 75 : 25 = 3 : 1$ .

Weighted average atomic mass

\bar{m} = \frac{m_1 x + m_2 (100 - x)}{100}

\bar{m}average atomic mass
m_1, m_2the two isotope masses
xpercentage abundance of isotope 1

Worked example

Copper has two isotopes of masses

63

and

65

and an average atomic mass of

63.5

. Find the percentage abundance of each isotope.

Let the abundance of the mass- $63$ isotope be $x\%$ , so the mass- $65$ isotope is $(100 - x)\%$ .
$\dfrac{63x + 65(100 - x)}{100} = 63.5 \Rightarrow 63x + 6500 - 65x = 6350$ .
$-2x = -150 \Rightarrow x = 75$ , so mass- $63$ is $75\%$ and mass- $65$ is $25\%$ .

Answer:

75\%

of mass

63

and

25\%

of mass

65

Practice this conceptself-check · 3 quick reps

Try it yourself

An element has isotopes of masses

10

and

11

in the ratio

1 : 4

. Find its average atomic mass.

Practice — Level 1 (3 reps)

Quick reps to lock in the method. Try each, then check.

1.
$^{35}\text{Cl}$ and $^{37}\text{Cl}$ give an average of $35.5$ . What is their abundance ratio?
2.
Two isotopes of mass $20$ and $22$ occur $90\%$ and $10\%$ . Average mass?
3.
Two isotopes of mass $12$ and $14$ occur $1 : 1$ . Average mass?

From the bank · past-year question

Example 5Structure of AtomEASY

Chlorine has two isotopes

^{35}

Cl and

^{37}

Cl with average atomic mass of 35.5. What is the ratio of their relative abundance respectively?

[Q59 · 9th May Shift 2 · 2023]

Weight by abundance, don't just average

The plain mean of

35

and

37

36

, but chlorine's true

35.5

is lower because the lighter

^{35}\text{Cl}

is far more abundant (

75\%

). Always multiply each mass by its fraction first.

Match the ratio order to the isotopes

For chlorine the

3 : 1

^{35}\text{Cl} : {}^{37}\text{Cl}

— the lighter, more abundant isotope first. A distractor offers

1 : 3

; keep the order aligned with the masses.

Concept 6 of 6

Isotope counts, hydrogen-like species and radioactivity

Intuition

A few pure-recall facts round out the subtopic: how many natural isotopes a common element has, what makes a species 'hydrogen-like', and which elements are radioactive. Each is a one-line memory item the bank recycles.

Definition

The recall facts the bank tests here:

Number of isotopes — nitrogen has 2 natural isotopes ( $^{14}\text{N}$ , $^{15}\text{N}$ ); hydrogen has $3$ (protium, deuterium, tritium); carbon has $3$ ( $^{12}\text{C}, {}^{13}\text{C}, {}^{14}\text{C}$ ).
Hydrogen-like (hydrogenic) species have exactly one electron: $\text{H}$ , $\text{He}^{+}$ , $\text{Li}^{2+}$ , $\text{Be}^{3+}$ . Neutral $\text{He}$ has $2$ electrons, so it is not hydrogen-like.
Radioactivity — heavy elements such as $\text{At}$ (astatine), $\text{Po}$ (polonium) and $\text{Rn}$ (radon) are radioactive; the noble gas $\text{Ar}$ (argon) is stable and not radioactive.

Fact	Answer	Watch out for
Natural isotopes of nitrogen	$2$ ( $^{14}\text{N}$ , $^{15}\text{N}$ )	Hydrogen has $3$ , not nitrogenQ
Hydrogen-like species	One electron only	Neutral $\text{He}$ has $2$ e $^-$ , so it is excludedQ
Not radioactive	$\text{Ar}$ (argon)	$\text{At}$ , $\text{Po}$ , $\text{Rn}$ are all radioactiveQ

Three independent recall items — learn the exception in each.

Practice this conceptself-check · 4 quick reps

Try it yourself

Which is NOT a hydrogen-like species:

\text{He}

\text{He}^{+}

\text{Li}^{2+}

\text{Be}^{3+}

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

1.
How many natural isotopes does nitrogen have?
2.
Is $\text{Li}^{2+}$ a hydrogen-like species?
3.
Which of At, Po, Rn, Ar is NOT radioactive?
4.
Is neutral helium hydrogen-like?

From the bank · past-year question

Example 6Structure of AtomEASY

Which of the following is NOT hydrogen-like species?

[Q81 · Shift 1 · 2022]

Hydrogen-like means one electron, not 'near hydrogen'

The test is a single electron, so

\text{He}^{+}

\text{Li}^{2+}

and

\text{Be}^{3+}

qualify but neutral

\text{He}

(

2

^-

) does not. The neutral noble gas is the planted wrong answer.

Argon is the stable one

Among

\text{At}

\text{Po}

\text{Rn}

and

\text{Ar}

, only argon is a stable, non-radioactive noble gas; the other three are radioactive heavy elements.

Drill 2 more on isotope counts, hydrogen-like species and radioactivity

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 (3)

Counting protons, neutrons and electrons in a species
Electron count of a species
$e^- = Z - (\text{charge}) \qquad N = A - Z$
Identifying isoelectronic species by counting electrons
Isoelectronic test
$e^-_1 = Z_1 - q_1 \;\overset{?}{=}\; Z_2 - q_2 = e^-_2$
Average atomic mass and isotope abundance ratio
Weighted average atomic mass
$\bar{m} = \frac{m_1 x + m_2 (100 - x)}{100}$

Reference tables (3)

The three subatomic particles and the nuclide notation3 rows

Particle	Charge	Relative mass	Location
Proton	$+1$	$\approx 1\ \text{u}$	Nucleus
Neutron	$0$ (neutral)	$\approx 1\ \text{u}$	Nucleus
Electron	$-1$	$\approx \tfrac{1}{1836}$ of a proton	Shells outside the nucleus Electrons are so light that the mass number counts only protons and neutrons — never electrons.

Nucleons (protons + neutrons) carry the mass; the atomic number

Z

fixes the element.

Isotopes, isobars, isotones and isoelectronic species4 rows

Term	What is the same	What differs	Example
Isotopes	Protons $Z$ (same element)	Neutrons / mass number	$^{35}\text{Cl}$ , $^{37}\text{Cl}$ Isotopes do NOT have equal neutrons — that is the false statement the bank plants.
Isobars	Mass number $A$	Element ( $Z$ )	$^{40}\text{Ar}$ , $^{40}\text{Ca}$
Isotones	Number of neutrons	$Z$ and $A$	$^{12}_{6}\text{C}$ , $^{11}_{5}\text{B}$
Isoelectronic	Number of electrons	Element and charge	$\text{Na}^{+}$ , $\text{F}^{-}$ , $\text{O}^{2-}$ , $\text{Ne}$

Ask 'what count is held fixed?' — protons, mass number, neutrons, or electrons.

Isotope counts, hydrogen-like species and radioactivity3 rows

Fact	Answer	Watch out for
Natural isotopes of nitrogen	$2$ ( $^{14}\text{N}$ , $^{15}\text{N}$ )	Hydrogen has $3$ , not nitrogenQ
Hydrogen-like species	One electron only	Neutral $\text{He}$ has $2$ e $^-$ , so it is excludedQ
Not radioactive	$\text{Ar}$ (argon)	$\text{At}$ , $\text{Po}$ , $\text{Rn}$ are all radioactiveQ

Three independent recall items — learn the exception in each.

Watch out for (12)

Mass number counts nucleons, not electrons
→ The three subatomic particles and the nuclide notation
Read the notation the right way up
→ The three subatomic particles and the nuclide notation
A neutral atom of Ca has 20 electrons, but Ca-based ions do not
→ Counting protons, neutrons and electrons in a species
Add for negative, subtract for positive
→ Counting protons, neutrons and electrons in a species
'Isotopes have equal neutrons' is FALSE
→ Isotopes, isobars, isotones and isoelectronic species
Isotones vs isobars vs isotopes
→ Isotopes, isobars, isotones and isoelectronic species
The neutral atom hidden among its ions
→ Identifying isoelectronic species by counting electrons
Same electrons, not same protons
→ Identifying isoelectronic species by counting electrons
Weight by abundance, don't just average
→ Average atomic mass and isotope abundance ratio
Match the ratio order to the isotopes
→ Average atomic mass and isotope abundance ratio
Hydrogen-like means one electron, not 'near hydrogen'
→ Isotope counts, hydrogen-like species and radioactivity
Argon is the stable one
→ Isotope counts, hydrogen-like species and radioactivity

Mastery check — 5 interleaved questions

Try each one before clicking. Questions are interleaved across the concepts above, not grouped — interleaving sharpens transfer.

Example 1Structure of AtomEASY

{}^{14}_6\text{C}

and

{}^{16}_8\text{O}

are examples of

[Q78 · May Shift 1 · 2021]

Example 2Structure of AtomEASY

Which from following species does not have number of electrons similar to other three species?

[Q93 · 2nd May Shift 1 · 2023]

Example 3Structure of AtomEASY

Which from following elements is NOT radioactive?

[Q86 · 12th May Shift 1 · 2024]

Example 4Structure of AtomEASY

Which of the following statements is correct regarding isobars?

[Q80 · 4th May Shift 1 · 2023]

Example 5Structure of AtomEASY

Which species is NOT isoelectronic with neon (10 e

^-

[Q92 · 10th May Shift 1 · 2024]

Drill every past-year question on this subtopic

14 questions from the bank — paginated, with cart and Word-export support.

Drill the 14 questions

Related notes

Isotopes and Isoelectronic Species (NDA Chemistry) →