Structure of the Atom

2. The Nuclear Atom

Inside the Atom

All ordinary matter is made of atoms. Atoms are extremely small, but they have an internal structure made from even smaller particles.

The three main subatomic particles are:

• Protons
• Neutrons
• Electrons

Protons and neutrons are found together in the nucleus at the centre of the atom. Electrons occupy the region surrounding the nucleus.

Although diagrams often make the nucleus look quite large, the nucleus is actually tiny compared with the overall size of an atom. Most of an atom's volume is the region occupied by its electrons.

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The Nucleus

The nucleus is the small, dense central region of an atom.

It contains:

• Protons
• Neutrons

Together, protons and neutrons are sometimes called nucleons because they are particles found in the nucleus.

Almost all of the atom's mass is concentrated in the nucleus.

The nucleus also has an overall positive charge because protons are positively charged while neutrons have no electric charge.

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Comparing the Three Main Subatomic Particles

The three main subatomic particles have different charges, masses, and locations.

A proton and neutron have approximately the same mass.

An electron has much less mass. For many basic atomic calculations, the mass of an electron is treated as negligible compared with the masses of protons and neutrons.

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Protons

A proton has:

• A positive charge
• A relative charge of +1
• A relative mass of approximately 1
• A location inside the nucleus

The number of protons is particularly important because it determines the identity of the element.

For example:

• An atom with 1 proton is hydrogen.
• An atom with 6 protons is carbon.
• An atom with 8 protons is oxygen.
• An atom with 11 protons is sodium.

Changing the number of protons changes the element itself.

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Neutrons

A neutron has:

• No electric charge
• A relative charge of 0
• A relative mass of approximately 1
• A location inside the nucleus

Neutrons contribute to the mass of the atom but do not determine which element it is.

Atoms of the same element can contain different numbers of neutrons. These different forms are called isotopes.

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Electrons

An electron has:

• A negative charge
• A relative charge of −1
• A very small mass
• A location outside the nucleus

Electrons are involved in many important processes, particularly:

• Chemical bonding
• Ion formation
• Electricity
• Chemical reactions

In a neutral atom, the number of electrons equals the number of protons.

For example, a neutral atom containing 8 protons also contains 8 electrons.

The positive and negative charges balance, giving an overall charge of zero.

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Atomic Number

Every element has an atomic number.

The atomic number tells us the number of protons in the nucleus.

Atomic number = number of protons

The symbol Z is often used for atomic number.

For example, carbon has an atomic number of 6.

Therefore, every carbon atom has:

6 protons

A neutral carbon atom also has:

6 electrons

The atomic number is what defines an element.

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Nucleon Number

The nucleon number, also commonly called the mass number, is the total number of protons and neutrons in the nucleus.

Nucleon number = number of protons + number of neutrons

The symbol A is commonly used for nucleon number.

For example, suppose an atom contains:

• 6 protons
• 8 neutrons

Its nucleon number is:

6 + 8 = 14

Therefore:

Nucleon number = 14

Notice that electrons are not included in the nucleon number.

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Finding the Number of Neutrons

If you know the atomic number and nucleon number, you can determine the number of neutrons.

Use:

Number of neutrons = nucleon number − atomic number

For example, an atom has:

• Atomic number = 11
• Nucleon number = 23

Therefore:

Number of neutrons = 23 − 11 = 12

The atom contains:

• 11 protons
• 12 neutrons
• 11 electrons if it is neutral

Because it has 11 protons, the element is sodium.

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Nuclear Notation

Scientists use nuclear notation to show important information about an atom.

A common format is:

ᴬ_Z X

where:

• X = chemical symbol
• A = nucleon number
• Z = atomic number

For example, carbon-12 can be represented as:

¹²₆C

This tells us:

• Nucleon number = 12
• Atomic number = 6
• Protons = 6
• Neutrons = 12 − 6 = 6

If the atom is neutral:

• Electrons = 6

This notation allows us to determine the composition of an atom very quickly.

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Worked Example: Sodium-23

Consider:

²³₁₁Na

The lower number is the atomic number.

Therefore:

Protons = 11

The upper number is the nucleon number.

Therefore:

Protons + neutrons = 23

Calculate the neutrons:

Neutrons = 23 − 11 = 12

Because no ionic charge is shown, we treat the atom as neutral:

Electrons = 11

So sodium-23 contains:

• 11 protons
• 12 neutrons
• 11 electrons

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Worked Example: Chlorine-37

Consider:

³⁷₁₇Cl

Atomic number = 17

Therefore:

Protons = 17

Nucleon number = 37

Therefore:

Neutrons = 37 − 17 = 20

For a neutral atom:

Electrons = 17

So chlorine-37 contains:

• 17 protons
• 20 neutrons
• 17 electrons

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What Are Isotopes?

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

Because they have the same number of protons, they have the same atomic number and are the same element.

Because they have different numbers of neutrons, they have different nucleon numbers.

For example, carbon has several isotopes.

Carbon-12:

• 6 protons
• 6 neutrons
• Nucleon number = 12

Carbon-13:

• 6 protons
• 7 neutrons
• Nucleon number = 13

Carbon-14:

• 6 protons
• 8 neutrons
• Nucleon number = 14

All three are carbon because all three contain 6 protons.

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Comparing Isotopes

Consider these two atoms:

³⁵₁₇Cl

and

³⁷₁₇Cl

Both have atomic number 17.

Therefore, both contain:

17 protons

This means both are chlorine.

For chlorine-35:

Neutrons = 35 − 17 = 18

For chlorine-37:

Neutrons = 37 − 17 = 20

The two atoms are isotopes because they have:

• The same number of protons
• Different numbers of neutrons

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What Changes an Element's Identity?

It is important to distinguish between changes in protons, neutrons, and electrons.

Changing the Number of Protons

Changing the number of protons changes the element.

For example:

6 protons → carbon

7 protons → nitrogen

Changing the Number of Neutrons

Changing the number of neutrons changes the isotope, but not the element.

For example:

Carbon-12 → 6 protons and 6 neutrons

Carbon-14 → 6 protons and 8 neutrons

Both are still carbon.

Changing the Number of Electrons

Changing the number of electrons changes the electrical charge of the atom.

The atom becomes an ion, but it remains the same element.

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Neutral Atoms and Ions

In a neutral atom:

Number of protons = number of electrons

If an atom loses electrons, it has more protons than electrons and becomes positively charged.

If an atom gains electrons, it has more electrons than protons and becomes negatively charged.

For example, sodium contains 11 protons.

Neutral sodium:

• 11 protons
• 11 electrons
• Net charge = 0

A sodium ion, Na⁺:

• 11 protons
• 10 electrons
• Net charge = +1

The number of protons has not changed, so it is still sodium.

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Atomic Number and the Periodic Table

Elements on the periodic table are arranged according to atomic number.

As you move from one element to the next, the atomic number increases.

For example:

• Hydrogen: atomic number 1
• Helium: atomic number 2
• Lithium: atomic number 3
• Beryllium: atomic number 4
• Boron: atomic number 5
• Carbon: atomic number 6

This means that the number of protons increases by one as you move through these elements.

The periodic table is therefore fundamentally organized around the structure of the atomic nucleus.

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Nucleon Number vs Relative Atomic Mass

The nucleon number of a particular atom is always a whole number because it counts individual protons and neutrons.

For example:

Carbon-12 has a nucleon number of 12.

However, the periodic table often shows carbon's relative atomic mass as approximately 12.01.

These are not the same thing.

The decimal value on the periodic table reflects the relative abundances and masses of the naturally occurring isotopes of an element.

Therefore:

Nucleon number → describes one particular isotope

Relative atomic mass → represents an average based on naturally occurring isotopes

This is why many relative atomic masses on the periodic table are not whole numbers.

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Why Isotopes Matter

Different isotopes of an element have very similar chemical behaviour because they have the same number of protons and, for neutral atoms, the same number and arrangement of electrons.

However, their nuclear properties can be different.

Some isotopes are stable, while others are radioactive.

Radioactive isotopes have many applications, including:

• Medical diagnosis
• Medical treatment
• Scientific research
• Archaeological dating
• Industrial monitoring

Carbon-14, for example, is radioactive and can be used in dating once-living materials.

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A Quick Method for Atomic Composition Problems

When given nuclear notation, work through the information systematically.

Find the Protons

Protons = atomic number

Find the Neutrons

Neutrons = nucleon number − atomic number

Find the Electrons

For a neutral atom:

Electrons = protons

If the particle is an ion, adjust the electron number according to its charge.

Identify the Element

The number of protons determines the element.

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Worked Example

An atom has:

• Atomic number = 15
• Nucleon number = 31

Determine its composition.

First:

Protons = 15

Next:

Neutrons = 31 − 15 = 16

If the atom is neutral:

Electrons = 15

Atomic number 15 corresponds to phosphorus.

Therefore, the atom contains:

• 15 protons
• 16 neutrons
• 15 electrons

It is phosphorus-31.

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Common Mistakes

Confusing Atomic Number and Nucleon Number

Remember:

Atomic number = protons

Nucleon number = protons + neutrons

Including Electrons in the Nucleon Number

Electrons are not nucleons.

Only protons and neutrons are counted.

Assuming Isotopes Have Different Numbers of Protons

If the proton number changes, the element changes.

Isotopes have the same number of protons but different numbers of neutrons.

Assuming All Atoms Have Equal Numbers of Protons and Electrons

This is true only for neutral atoms.

Ions have unequal numbers of protons and electrons.

Using Relative Atomic Mass as the Nucleon Number

The decimal number on the periodic table is usually a relative atomic mass, not the nucleon number of one specific atom.

Thinking Electrons Contribute Most of the Atom's Mass

Almost all atomic mass is concentrated in the nucleus because electrons have very little mass compared with protons and neutrons.

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Check Your Understanding

1. Name the three main subatomic particles.

2. Which particles are found in the nucleus?

3. Compare the charges of protons, neutrons, and electrons.

4. Which particle determines the identity of an element?

5. What does atomic number tell us?

6. What does nucleon number tell us?

7. An atom has atomic number 12 and nucleon number 24. How many protons, neutrons, and electrons does a neutral atom contain?

8. Determine the numbers of protons, neutrons, and electrons in ²⁷₁₃Al.

9. Determine the numbers of protons and neutrons in ⁴⁰₂₀Ca.

10. Define an isotope.

11. Carbon-12 and carbon-14 are isotopes. What is the same about their nuclei, and what is different?

12. Why does changing the number of neutrons not change the identity of an element?

13. What happens to an atom if its number of electrons changes?

14. Explain why nucleon numbers are whole numbers while relative atomic masses are often decimals.

15. An atom contains 17 protons and 18 neutrons. What are its atomic number and nucleon number?

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Key Terms

• Atom – the smallest particle of an element that retains the properties of that element.
• Nucleus – the small, dense central region of an atom containing protons and neutrons.
• Proton – a positively charged subatomic particle found in the nucleus.
• Neutron – an electrically neutral subatomic particle found in the nucleus.
• Electron – a negatively charged subatomic particle found outside the nucleus.
• Nucleon – a proton or neutron found in the nucleus.
• Atomic number – the number of protons in an atom's nucleus.
• Nucleon number – the total number of protons and neutrons in a nucleus.
• Nuclear notation – a notation showing an element's chemical symbol, atomic number, and nucleon number.
• Isotope – an atom of an element with the same number of protons but a different number of neutrons.
• Ion – an atom or group of atoms with an overall electric charge.
• Relative atomic mass – the weighted average mass of an element's naturally occurring isotopes relative to a standard.

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Key Takeaways

• Atoms contain protons, neutrons, and electrons.
• Protons and neutrons are located in the nucleus.
• Electrons occupy the region surrounding the nucleus.
• Protons have charge +1, neutrons have charge 0, and electrons have charge −1.
• Protons and neutrons have much greater masses than electrons.
• The atomic number equals the number of protons.
• The number of protons determines the identity of the element.
• The nucleon number equals the number of protons plus neutrons.
• The number of neutrons can be calculated using: neutrons = nucleon number − atomic number.
• In a neutral atom, the number of electrons equals the number of protons.
• Isotopes have the same number of protons but different numbers of neutrons.
• Different isotopes therefore have the same atomic number but different nucleon numbers.
• Changing the number of electrons produces an ion; changing the number of neutrons produces a different isotope; changing the number of protons produces a different element.