Describe the structure of common nuclear isotopes

Ontario Grade 11 Physics

Energy and Society

How to read isotope symbols and count the particles in an atom

Atoms are made of a central nucleus and electrons outside it. The nucleus contains protons and neutrons. Isotopes are forms of the same element with different numbers of neutrons. This lesson focuses on describing the structure of common isotopes and reading their symbols. Particle counts are counts, not measurements with SI units. Direction and a positive direction do not apply to this topic.

What you will learn

Describe the parts of an atom and where its protons, neutrons, and electrons are found.
Explain how isotopes of the same element are alike and different.
Use isotope notation to determine proton and neutron counts, and find electron counts when an atom is stated to be neutral.

1. Review: the parts of an atom

An element is a type of atom identified by its number of protons. A proton is a positively charged particle in the nucleus. A neutron is a particle in the nucleus with no electric charge. An electron is a negatively charged particle found outside the nucleus.

The nucleus is the central part of an atom. It contains protons and neutrons. Electrons are outside the nucleus. These locations describe the atom’s structure; they are not a scale drawing of how large each part is.

An atom is neutral when it has no overall electric charge. For a neutral atom, the number of electrons equals the number of protons. Use this relationship only when the atom is stated to be neutral.

N_p=Z

Protons and neutrons are in the nucleus; electrons are outside it.
The number of protons identifies the element.
Only for a neutral atom do the electron and proton counts match.

2. Isotopes and isotope notation

Isotopes are forms of the same element that have different numbers of neutrons. They have the same number of protons, so they are still atoms of the same element. Their different neutron counts give them different mass numbers.

The atomic number is the number of protons in a nucleus. It identifies the element. The mass number is the total number of protons and neutrons in one nucleus. It is a count, not a mass measured in kilograms.

In isotope notation, the mass number is written at the upper left of the element symbol and the atomic number at the lower left. In

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

,

A

is the mass number,

Z

is the atomic number, and

\mathrm{X}

is the element symbol. The neutron count is the mass number minus the atomic number.

To read a symbol, identify the element and read its atomic and mass numbers. The atomic number gives the proton count. Subtract it from the mass number to find the neutron count. Find the electron count only if the atom’s charge or neutrality is specified.

N_n=A-Z

Isotopes of one element have the same atomic number and different neutron counts.
Mass number is the total count of protons and neutrons.
The neutron count is the mass number minus the atomic number.

3. Comparing common isotopes

Carbon-12 and carbon-14 are both carbon. Each has atomic number 6, so each nucleus contains six protons. Carbon-12 has six neutrons; carbon-14 has eight. The neutron counts differ, but the proton counts stay the same.

The number in a name such as carbon-14 is the mass number. It does not mean that the atom has 14 protons. For carbon-14, the six protons and eight neutrons add to a mass number of 14.

The same method works for other elements. For example, hydrogen-1 and hydrogen-3 both have one proton. Their neutron counts are zero and two. When comparing isotopes, focus on what stays the same—the proton count—and what changes—the neutron count.

A=N_p+N_n

Carbon-12 and carbon-14 each have 6 protons.
Their neutron counts are 6 and 8, respectively.
The mass number equals the proton count plus the neutron count.

4. A dependable counting method

First, read the element symbol and the atomic number. The atomic number tells you the proton count. Next, read the mass number and subtract the atomic number to find the neutron count.

If the problem states that the atom is neutral, the electron count equals the proton count. If neutrality is not stated, do not assume that the electron and proton counts match.

Check your description by adding the proton and neutron counts. Their total must equal the mass number. Also check that the proton count equals the atomic number. These checks help prevent mixing up the two numbers in isotope notation.

N_p=Z,\quad N_n=A-Z

Use the atomic number for the proton count.
Subtract atomic number from mass number for the neutron count.
Use equal proton and electron counts only for a neutral atom.
Check that protons plus neutrons equal the mass number.

Particle counts in common neutral isotopes

Isotope	Protons	Neutrons	Electrons
$^{12}_{6}\mathrm{C}$	6	6	6
$^{14}_{6}\mathrm{C}$	6	8	6
$^{1}_{1}\mathrm{H}$	1	0	1
$^{3}_{1}\mathrm{H}$	1	2	1

Worked example

Example 1: Finding the particles in carbon-12

A neutral carbon atom is written as

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

. Find its proton, neutron, and electron counts.

Read the symbol
The system is one neutral carbon atom. Its mass number is 12 and its atomic number is 6. The unknowns are the counts of protons, neutrons, and electrons.
$A=12,\quad Z=6$
Count the particles
The atomic number gives six protons. The atom is neutral, so it has the same number of electrons. Subtract the atomic number from the mass number to find the neutrons.
$N_p=6,\quad N_e=6,\quad N_n=12-6=6$

Answer: The atom has 6 protons, 6 neutrons, and 6 electrons.

Check: These are counts of particles, so no SI units apply. Six protons plus six neutrons gives mass number 12. The proton and electron counts match because the atom is neutral.

Worked example

Example 2: Comparing two hydrogen isotopes

Compare neutral hydrogen-3,

^{3}_{1}\mathrm{H}

, with neutral hydrogen-1,

^{1}_{1}\mathrm{H}

.

Identify the element
Both symbols have atomic number 1. Each atom therefore has one proton and is hydrogen.
$Z=1$
Find the neutron counts
For each isotope, subtract the atomic number from the mass number. Hydrogen-3 has two neutrons, while hydrogen-1 has none.
$N_{n,3}=3-1=2,\quad N_{n,1}=1-1=0$
Find the electron counts
Both atoms are stated to be neutral. Each therefore has one electron, matching its one proton.
$N_{e,3}=N_{e,1}=1$

Answer: Hydrogen-3 has 1 proton, 2 neutrons, and 1 electron. Hydrogen-1 has 1 proton, 0 neutrons, and 1 electron. They are isotopes because they have the same proton count but different neutron counts.

Check: For hydrogen-3, 1 proton plus 2 neutrons gives mass number 3. For hydrogen-1, 1 plus 0 gives mass number 1. The counts are whole numbers, and the electron counts match the proton counts because both atoms are neutral.

Worked example

Example 3: Identifying an isotope from particle counts

A neutral atom has 8 protons and 10 neutrons. Identify its element, mass number, electron count, and isotope symbol.

Identify the element
The proton count is the atomic number. The element with atomic number 8 is oxygen.
$Z=N_p=8$
Find the mass number
The mass number is the total number of protons and neutrons. Add the two counts.
$A=8+10=18$
Determine the electrons and symbol
The atom is neutral, so it has eight electrons. Write the mass number above the atomic number beside the oxygen symbol.
$N_e=8,\quad ^{18}_{8}\mathrm{O}$

Answer: The atom is neutral oxygen-18. It has 8 protons, 10 neutrons, and 8 electrons. Its isotope symbol is

^{18}_{8}\mathrm{O}

.

Check: Eight protons plus ten neutrons gives mass number 18, and eight protons identify oxygen. The electron count matches the proton count because the atom is neutral. The particle counts are consistent.

Common mistakes and how to avoid them

Treating the mass number as the number of protons.

Correction: The atomic number gives the proton count. The mass number is the total number of protons and neutrons.

Thinking isotopes of the same element have different proton counts.

Correction: Isotopes of the same element have the same proton count and different neutron counts.

Assuming every atom has equal numbers of protons and electrons.

Correction: That equality applies to a neutral atom. Check whether the atom is stated to be neutral.

Adding the atomic number to the mass number to find neutrons.

Correction: Subtract the atomic number from the mass number. The mass number already includes the protons.

Lesson summary

The nucleus contains protons and neutrons; electrons are outside it.
The atomic number is the proton count and identifies the element.
The mass number is the total number of protons and neutrons.
Subtract the atomic number from the mass number to find neutrons.
Isotopes of one element have the same proton count and different neutron counts.
For a neutral atom, the electron count equals the proton count.

Check your understanding

Question 1

How many neutrons are in an atom represented by

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

?

11
12
23
34

Show answer and explanation

12

Subtract the atomic number from the mass number: 23 minus 11 equals 12 neutrons.

Question 2

Which feature makes two atoms isotopes of the same element?

They have the same number of neutrons and different numbers of protons.
They have the same number of protons and different numbers of neutrons.
They have the same mass number and different numbers of protons.
They have different numbers of protons and electrons.

Show answer and explanation

They have the same number of protons and different numbers of neutrons.

The proton count identifies the element. Isotopes of that element have different neutron counts.

Question 3

A neutral atom has 7 protons and 8 neutrons. Which statement is correct?

It has 7 electrons and mass number 15.
It has 8 electrons and mass number 15.
It has 7 electrons and mass number 8.
It has 15 electrons and mass number 7.

Show answer and explanation

It has 7 electrons and mass number 15.

A neutral atom has 7 electrons when it has 7 protons. Its mass number is 7 plus 8, or 15.

Key terms

Atom: A basic unit of an element, with a nucleus and electrons outside it.
Atomic number: The number of protons in an atom’s nucleus.
Electron: A negatively charged particle found outside the nucleus.
Isotope: A form of an element with a particular neutron count; isotopes of one element have different neutron counts.
Mass number: The total number of protons and neutrons in one nucleus.
Neutron: A particle in the nucleus with no electric charge.
Nucleus: The central part of an atom, containing protons and neutrons.
Proton: A positively charged particle in the nucleus.

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About this lesson and its review

Published by DoAssignment. This reviewed lesson follows Ontario Grade 11 Physics (SPH3U), expectation D3.9. It is a study resource, not an official curriculum publication.

Before publication, content is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability. Errors can still occur, so corrections are welcomed.

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