Mass number, atomic number and isotopes

What you'll learn

Why a neutral atom has no overall electrical charge.
How to use atomic number and mass number to find protons, neutrons and electrons.
How to read atom notation like $1123Na{}^{23}_{11}\text{Na}$ .
What isotopes are, and how atoms become positive ions.

Starting point: what is inside an atom?

An atom is the smallest particle of an element that can still be identified as that element.

Atoms are made from three smaller particles: protons, neutrons and electrons. Protons and neutrons are found in the tiny central nucleus. Electrons are found around the nucleus in shells.

Definition

Subatomic particles

A proton is a positively charged particle found in the nucleus.
A neutron is an uncharged particle found in the nucleus.
An electron is a negatively charged particle found around the nucleus.

For this topic, the most important idea is charge. A proton has a positive charge, an electron has an equal-sized negative charge, and a neutron has no charge.

Neutral atoms: protons balance electrons

In an atom, the number of electrons is equal to the number of protons in the nucleus.

So the positive charge from the protons cancels the negative charge from the electrons. This means atoms have no overall electrical charge.

Key Idea

Neutral atom

A neutral atom has equal numbers of protons and electrons, so its total charge is zero.

For example, if an atom has 6 protons, it also has 6 electrons. The atom is neutral because the positive and negative charges balance.

Common Mistake

Forgetting electrons affect charge

Neutrons do not affect the charge of an atom. Only protons and electrons matter when deciding whether the atom is neutral or charged.

Atomic number: the number of protons

All atoms of the same element have the same number of protons.

For example, every carbon atom has 6 protons. If an atom has 6 protons, it is carbon. If it has 7 protons, it is nitrogen instead.

Definition

Atomic number

The atomic number is the number of protons in an atom of an element.

The atomic number identifies the element. Changing the number of protons changes the element.

Key Idea

Atomic number identifies the element

If two atoms have the same atomic number, they are atoms of the same element.

Mass number: protons plus neutrons

The mass number tells you the total number of particles in the nucleus.

That means:

neutrons\text{mass number} = \text{number of protons} + \text{number of neutrons}

Electrons are not included in the mass number because their mass is tiny compared with the mass of protons and neutrons.

Definition

Mass number

The mass number is the total number of protons and neutrons in an atom.

You can rearrange the idea to find the number of neutrons:

number\text{number of neutrons} = \text{mass number} - \text{atomic number}

Tip

Quick neutron method

To find neutrons, subtract the smaller number from the bigger number: mass number minus atomic number.

Atom notation

Atoms can be represented using their element symbol, with the mass number at the top left and the atomic number at the bottom left.

For sodium, the notation is:

1123Na{}^{23}_{11}\text{Na}

This tells you that sodium has mass number 23 and atomic number 11.

Diagram showing sodium atom notation and particle numbers

The top number is the mass number. The bottom number is the atomic number.

Common Mistake

Mixing up the two numbers

The mass number goes at the top because it is usually the bigger number. The atomic number goes at the bottom and tells you the number of protons.

Example

Finding protons, neutrons and electrons

An atom is written as $1123Na{}^{23}_{11}\text{Na}$ . Find the number of protons, neutrons and electrons.

Use the atomic number to find protons. The atomic number is 11, so the atom has 11 protons.
Use the mass number to find neutrons:
$neutrons=23−11=12\text{neutrons} = 23 - 11 = 12$
Use the fact that this is an atom, not an ion. A neutral atom has the same number of electrons as protons, so it has 11 electrons.

Isotopes

Atoms of the same element always have the same number of protons. However, they can have different numbers of neutrons.

These different versions of the same element are called isotopes.

Definition

Isotopes

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.

Because isotopes have the same number of protons, they have the same atomic number. Because they have different numbers of neutrons, they have different mass numbers.

For example, these are both chlorine atoms:

1735Cl1737Cl{}^{35}_{17}\text{Cl} \qquad {}^{37}_{17}\text{Cl}

They both have atomic number 17, so they both have 17 protons. They are both chlorine. But their mass numbers are different, so they have different numbers of neutrons.

Example

Checking whether atoms are isotopes

Decide whether $1735Cl{}^{35}_{17}\text{Cl}$ and $1737Cl{}^{37}_{17}\text{Cl}$ are isotopes of the same element.

Compare the atomic numbers. Both atoms have atomic number 17, so both have 17 protons and are the same element.
Compare the mass numbers. One atom has mass number 35 and the other has mass number 37, so the total number of protons and neutrons is different.
Work out what must have changed. Since the number of protons is the same but the mass number is different, the number of neutrons must be different. Therefore, they are isotopes.

Tip

Isotope test

Same protons but different neutrons means isotopes. In notation, that usually means the bottom numbers match but the top numbers differ.

Finding neutrons in isotopes

Isotopes are often tested using simple subtraction.

For example:

1735Cl{}^{35}_{17}\text{Cl}

has:

\text{ neutrons}

But:

1737Cl{}^{37}_{17}\text{Cl}

has:

\text{ neutrons}

So chlorine-35 and chlorine-37 are isotopes because they both have 17 protons, but one has 18 neutrons and the other has 20 neutrons.

Example

Comparing isotope particle numbers

Compare the number of protons, neutrons and electrons in $612C{}^{12}_{6}\text{C}$ and $614C{}^{14}_{6}\text{C}$ .

Use the atomic number to find protons. Both atoms have atomic number 6, so both have 6 protons.
Use the mass number minus atomic number to find neutrons:
$neutrons{}^{12}_{6}\text{C}: 12 - 6 = 6 \text{ neutrons}$ $neutrons{}^{14}_{6}\text{C}: 14 - 6 = 8 \text{ neutrons}$
Treat both as neutral atoms. Each has the same number of electrons as protons, so each has 6 electrons.

Positive ions

So far, we have talked about neutral atoms. An atom can become an ion if it gains or loses electrons.

In this specification section, you need to know what happens when atoms lose electrons.

Definition

Positive ion

A positive ion is formed when an atom loses one or more outer electrons.

If an atom loses electrons, it has fewer negative charges than positive charges. The protons have not changed, so the atom is now overall positive.

For example, a sodium atom has 11 protons and 11 electrons. If it loses one outer electron, it still has 11 protons but now has only 10 electrons. This gives it an overall charge of 1+.

Key Idea

Losing electrons makes positive ions

Atoms become positive ions when they lose outer electrons, because the number of protons becomes greater than the number of electrons.

Example

Working out the charge of a positive ion

A magnesium atom has 12 protons and 12 electrons. It loses two outer electrons. Work out the charge of the ion formed.

Keep the number of protons the same. Losing electrons does not affect the nucleus, so magnesium still has 12 protons.
Subtract the lost electrons from the original number of electrons:
$\text{ electrons}$
Compare positive and negative charges. There are 12 positive charges and 10 negative charges, so the ion has 2 more positive charges than negative charges. Its charge is 2+.

Common Mistake

Changing protons when ions form

Atoms form ions by gaining or losing electrons, not by changing the number of protons. If the number of protons changed, it would become a different element.

The full technique

When you see atom notation, use this order:

Atomic number gives the number of protons.
If it is a neutral atom, the number of electrons equals the number of protons.
Mass number minus atomic number gives the number of neutrons.
To check for isotopes, compare protons first, then neutrons.

Example

Using the full technique

An atom is represented as $1840Ar{}^{40}_{18}\text{Ar}$ . Work out the number of protons, neutrons and electrons, and explain whether $1838Ar{}^{38}_{18}\text{Ar}$ is an isotope of it.

Use the atomic number. Both atoms have atomic number 18, so each atom has 18 protons and both are argon.
Find neutrons in $1840Ar{}^{40}_{18}\text{Ar}$ :
$\text{ neutrons}$
Find neutrons in $1838Ar{}^{38}_{18}\text{Ar}$ :
$\text{ neutrons}$
Decide whether they are isotopes. They have the same number of protons but different numbers of neutrons, so they are isotopes.
For the neutral $1840Ar{}^{40}_{18}\text{Ar}$ atom, electrons equal protons, so it has 18 electrons.

Exam technique

In the exam

Read atom notation carefully: the top number is mass number, and the bottom number is atomic number.
Use atomic number for protons first, then use mass number minus atomic number for neutrons.
If the question says atom, assume it is neutral, so electrons equal protons; if it says ion, adjust the electron number.

Self review

Check yourself

What do the top and bottom numbers mean in $1123Na{}^{23}_{11}\text{Na}$ ?
Why do isotopes of an element have the same atomic number but different mass numbers?
Why does losing electrons make an atom into a positive ion?

Mass number, atomic number and isotopes

What you'll learn

Why a neutral atom has no overall electrical charge.
How to use atomic number and mass number to find protons, neutrons and electrons.
How to read atom notation like $1123Na{}^{23}_{11}\text{Na}$ .
What isotopes are, and how atoms become positive ions.

Starting point: what is inside an atom?

An atom is the smallest particle of an element that can still be identified as that element.

Definition

Subatomic particles

A proton is a positively charged particle found in the nucleus.
A neutron is an uncharged particle found in the nucleus.
An electron is a negatively charged particle found around the nucleus.

For this topic, the most important idea is charge. A proton has a positive charge, an electron has an equal-sized negative charge, and a neutron has no charge.

Neutral atoms: protons balance electrons

In an atom, the number of electrons is equal to the number of protons in the nucleus.

So the positive charge from the protons cancels the negative charge from the electrons. This means atoms have no overall electrical charge.

Key Idea

Neutral atom

A neutral atom has equal numbers of protons and electrons, so its total charge is zero.

For example, if an atom has 6 protons, it also has 6 electrons. The atom is neutral because the positive and negative charges balance.

Common Mistake

Forgetting electrons affect charge

Neutrons do not affect the charge of an atom. Only protons and electrons matter when deciding whether the atom is neutral or charged.

Atomic number: the number of protons

All atoms of the same element have the same number of protons.

For example, every carbon atom has 6 protons. If an atom has 6 protons, it is carbon. If it has 7 protons, it is nitrogen instead.

Definition

Atomic number

The atomic number is the number of protons in an atom of an element.

The atomic number identifies the element. Changing the number of protons changes the element.

Key Idea

Atomic number identifies the element

If two atoms have the same atomic number, they are atoms of the same element.

Mass number: protons plus neutrons

The mass number tells you the total number of particles in the nucleus.

That means:

neutrons\text{mass number} = \text{number of protons} + \text{number of neutrons}

Electrons are not included in the mass number because their mass is tiny compared with the mass of protons and neutrons.

Definition

Mass number

The mass number is the total number of protons and neutrons in an atom.

You can rearrange the idea to find the number of neutrons:

number\text{number of neutrons} = \text{mass number} - \text{atomic number}

Tip

Quick neutron method

To find neutrons, subtract the smaller number from the bigger number: mass number minus atomic number.

Atom notation

Atoms can be represented using their element symbol, with the mass number at the top left and the atomic number at the bottom left.

For sodium, the notation is:

1123Na{}^{23}_{11}\text{Na}

This tells you that sodium has mass number 23 and atomic number 11.

Diagram showing sodium atom notation and particle numbers

The top number is the mass number. The bottom number is the atomic number.

Common Mistake

Mixing up the two numbers

The mass number goes at the top because it is usually the bigger number. The atomic number goes at the bottom and tells you the number of protons.

Example

Finding protons, neutrons and electrons

An atom is written as $1123Na{}^{23}_{11}\text{Na}$ . Find the number of protons, neutrons and electrons.

Use the atomic number to find protons. The atomic number is 11, so the atom has 11 protons.
Use the mass number to find neutrons:
$neutrons=23−11=12\text{neutrons} = 23 - 11 = 12$
Use the fact that this is an atom, not an ion. A neutral atom has the same number of electrons as protons, so it has 11 electrons.

Isotopes

Atoms of the same element always have the same number of protons. However, they can have different numbers of neutrons.

These different versions of the same element are called isotopes.

Definition

Isotopes

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.

Because isotopes have the same number of protons, they have the same atomic number. Because they have different numbers of neutrons, they have different mass numbers.

For example, these are both chlorine atoms:

1735Cl1737Cl{}^{35}_{17}\text{Cl} \qquad {}^{37}_{17}\text{Cl}

They both have atomic number 17, so they both have 17 protons. They are both chlorine. But their mass numbers are different, so they have different numbers of neutrons.

Example

Checking whether atoms are isotopes

Decide whether $1735Cl{}^{35}_{17}\text{Cl}$ and $1737Cl{}^{37}_{17}\text{Cl}$ are isotopes of the same element.

Compare the atomic numbers. Both atoms have atomic number 17, so both have 17 protons and are the same element.
Compare the mass numbers. One atom has mass number 35 and the other has mass number 37, so the total number of protons and neutrons is different.
Work out what must have changed. Since the number of protons is the same but the mass number is different, the number of neutrons must be different. Therefore, they are isotopes.

Tip

Isotope test

Same protons but different neutrons means isotopes. In notation, that usually means the bottom numbers match but the top numbers differ.

Finding neutrons in isotopes

Isotopes are often tested using simple subtraction.

For example:

1735Cl{}^{35}_{17}\text{Cl}

has:

\text{ neutrons}

But:

1737Cl{}^{37}_{17}\text{Cl}

has:

\text{ neutrons}

So chlorine-35 and chlorine-37 are isotopes because they both have 17 protons, but one has 18 neutrons and the other has 20 neutrons.

Example

Comparing isotope particle numbers

Compare the number of protons, neutrons and electrons in $612C{}^{12}_{6}\text{C}$ and $614C{}^{14}_{6}\text{C}$ .

Use the atomic number to find protons. Both atoms have atomic number 6, so both have 6 protons.
Use the mass number minus atomic number to find neutrons:
$neutrons{}^{12}_{6}\text{C}: 12 - 6 = 6 \text{ neutrons}$ $neutrons{}^{14}_{6}\text{C}: 14 - 6 = 8 \text{ neutrons}$
Treat both as neutral atoms. Each has the same number of electrons as protons, so each has 6 electrons.

Positive ions

So far, we have talked about neutral atoms. An atom can become an ion if it gains or loses electrons.

In this specification section, you need to know what happens when atoms lose electrons.

Definition

Positive ion

A positive ion is formed when an atom loses one or more outer electrons.

If an atom loses electrons, it has fewer negative charges than positive charges. The protons have not changed, so the atom is now overall positive.

For example, a sodium atom has 11 protons and 11 electrons. If it loses one outer electron, it still has 11 protons but now has only 10 electrons. This gives it an overall charge of 1+.

Key Idea

Losing electrons makes positive ions

Atoms become positive ions when they lose outer electrons, because the number of protons becomes greater than the number of electrons.

Example

Working out the charge of a positive ion

A magnesium atom has 12 protons and 12 electrons. It loses two outer electrons. Work out the charge of the ion formed.

Keep the number of protons the same. Losing electrons does not affect the nucleus, so magnesium still has 12 protons.
Subtract the lost electrons from the original number of electrons:
$\text{ electrons}$
Compare positive and negative charges. There are 12 positive charges and 10 negative charges, so the ion has 2 more positive charges than negative charges. Its charge is 2+.

Common Mistake

Changing protons when ions form

Atoms form ions by gaining or losing electrons, not by changing the number of protons. If the number of protons changed, it would become a different element.

The full technique

When you see atom notation, use this order:

Atomic number gives the number of protons.
If it is a neutral atom, the number of electrons equals the number of protons.
Mass number minus atomic number gives the number of neutrons.
To check for isotopes, compare protons first, then neutrons.

Example

Using the full technique

An atom is represented as $1840Ar{}^{40}_{18}\text{Ar}$ . Work out the number of protons, neutrons and electrons, and explain whether $1838Ar{}^{38}_{18}\text{Ar}$ is an isotope of it.

Use the atomic number. Both atoms have atomic number 18, so each atom has 18 protons and both are argon.
Find neutrons in $1840Ar{}^{40}_{18}\text{Ar}$ :
$\text{ neutrons}$
Find neutrons in $1838Ar{}^{38}_{18}\text{Ar}$ :
$\text{ neutrons}$
Decide whether they are isotopes. They have the same number of protons but different numbers of neutrons, so they are isotopes.
For the neutral $1840Ar{}^{40}_{18}\text{Ar}$ atom, electrons equal protons, so it has 18 electrons.

Exam technique

In the exam

Read atom notation carefully: the top number is mass number, and the bottom number is atomic number.
Use atomic number for protons first, then use mass number minus atomic number for neutrons.
If the question says atom, assume it is neutral, so electrons equal protons; if it says ion, adjust the electron number.

Self review

Check yourself

What do the top and bottom numbers mean in $1123Na{}^{23}_{11}\text{Na}$ ?
Why do isotopes of an element have the same atomic number but different mass numbers?
Why does losing electrons make an atom into a positive ion?

Mass number, atomic number and isotopes

What you'll learn

Starting point: what is inside an atom?

Neutral atoms: protons balance electrons

Atomic number: the number of protons

Mass number: protons plus neutrons

Atom notation

Isotopes

Finding neutrons in isotopes

Positive ions

The full technique

You've reached the end

Mass number, atomic number and isotopes

What you'll learn

Starting point: what is inside an atom?

Neutral atoms: protons balance electrons

Atomic number: the number of protons

Mass number: protons plus neutrons

Atom notation

Isotopes

Finding neutrons in isotopes

Positive ions

The full technique

You've reached the end

Mass number, atomic number and isotopes

What you'll learn

Starting point: what is inside an atom?

Neutral atoms: protons balance electrons

Atomic number: the number of protons

Mass number: protons plus neutrons

Atom notation

Isotopes

Finding neutrons in isotopes

Positive ions

The full technique

You've reached the end

Mass number, atomic number and isotopes Revision Guide

Mass number, atomic number and isotopes

What you'll learn

Starting point: what is inside an atom?

Neutral atoms: protons balance electrons

Atomic number: the number of protons

Mass number: protons plus neutrons

Atom notation

Isotopes

Finding neutrons in isotopes

Positive ions

The full technique

You've reached the end

Mass number, atomic number and isotopes Revision Guide