2. Atomic Number and Position

Atomic Number and Position

Every element has its own unique atomic number. The atomic number is one of the most important pieces of information about an element because it tells us the number of protons in its nucleus and determines where the element appears on the periodic table.

What Is Atomic Number?

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

It is usually represented by the symbol Z.

For example:

Therefore:

Atomic number = Number of protons

So an oxygen atom has atomic number 8 because every oxygen atom contains 8 protons.

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Atomic Number Identifies the Element

The number of protons determines the identity of an element.

Every atom with 6 protons is carbon.

Every atom with 8 protons is oxygen.

Every atom with 17 protons is chlorine.

If the number of protons changes, the atom becomes a different element.

For example:

11 protons → Sodium (Na)

12 protons → Magnesium (Mg)

13 protons → Aluminium (Al)

This means that no two different elements can have the same atomic number.

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Atomic Number on the Periodic Table

Each element's atomic number is shown in its box on the periodic table.

A typical element box might look like this:

11
Na
Sodium
22.99

Here:

• 11 is the atomic number.
• Na is the chemical symbol.
• Sodium is the element name.
• 22.99 is the relative atomic mass.

Be careful not to confuse atomic number with relative atomic mass.

Suggested placement: Place an enlarged element square here so students can practise identifying the atomic number, symbol, element name and relative atomic mass.

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How Is the Periodic Table Arranged?

The modern periodic table is arranged in order of increasing atomic number.

Starting from hydrogen:

H → He → Li → Be → B → C → N → O → F → Ne

Their atomic numbers increase:

1 → 2 → 3 → 4 → 5 → 6 → 7 → 8 → 9 → 10

The pattern continues throughout the periodic table.

This means that as you move through the periodic table from one element to the next, the atomic number normally increases by one.

For example:

Atomic number therefore gives every element a specific position in the periodic table.

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Why Is the Periodic Table Arranged This Way?

Arranging elements by atomic number creates repeating patterns in their electron arrangements and chemical properties.

When one period is completed, a new electron shell begins to fill and a new period starts.

This produces the repeating pattern that gives the periodic table its name.

Elements with similar outer-electron arrangements appear in the same groups, which is why elements in the same group often have similar chemical properties.

Atomic number therefore does more than simply number the elements. It provides the foundation for the entire organisation of the modern periodic table.

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Atomic Number and Protons

The relationship between atomic number and protons is always:

Atomic number = Number of protons

Consider chlorine.

Chlorine has atomic number 17.

Therefore:

Atomic number = 17

Number of protons = 17

This is true for every chlorine atom. An atom cannot be chlorine unless it contains 17 protons.

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Atomic Number and Electrons

Atoms also contain negatively charged electrons.

A proton has a charge of +1, while an electron has a charge of −1.

In a neutral atom, the positive and negative charges balance.

Therefore:

Number of electrons = Number of protons

For a neutral atom:

Atomic number = Protons = Electrons

For example, magnesium has atomic number 12.

A neutral magnesium atom therefore contains:

• 12 protons
• 12 electrons

The positive charge from the 12 protons is balanced by the negative charge from the 12 electrons.

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Using Atomic Number to Predict Atomic Structure

If we know the atomic number, we can immediately determine the number of protons and, for a neutral atom, the number of electrons.

Example 1: Carbon

Carbon has atomic number 6.

Therefore:

• Protons = 6
• Electrons = 6

Example 2: Aluminium

Aluminium has atomic number 13.

Therefore:

• Protons = 13
• Electrons = 13

Example 3: Calcium

Calcium has atomic number 20.

Therefore:

• Protons = 20
• Electrons = 20

This gives us a useful rule:

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From Atomic Number to Position

Because the periodic table follows atomic number, we can also use atomic number to locate an element.

Suppose an unknown element has atomic number 16.

We can find atomic number 16 on the periodic table and identify the element as sulfur (S).

Its position tells us that sulfur is:

• in Period 3
• in Group 16
• a non-metal

Similarly, atomic number 19 identifies potassium (K), which is found in Period 4 and Group 1.

The atomic number therefore connects an element's identity, atomic structure and position on the periodic table.

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What About Neutrons?

Atomic number tells us the number of protons, but it does not directly tell us the number of neutrons.

For example, carbon atoms always have 6 protons, but different carbon atoms can contain different numbers of neutrons.

These different forms of the same element are called isotopes.

The number of protons remains the same because changing the number of protons would change the identity of the element.

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Did You Know?

The earliest periodic tables were not arranged using atomic number because scientists did not yet understand the structure of the atom.

In 1913, physicist Henry Moseley showed that elements could be organised according to their nuclear charge, corresponding to their atomic number. This helped establish the organisation used by the modern periodic table.

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Key Terms

Atomic number – The number of protons in the nucleus of an atom.

Proton – A positively charged particle found in the nucleus.

Electron – A negatively charged particle found outside the nucleus.

Neutral atom – An atom containing equal numbers of protons and electrons.

Nucleus – The central region of an atom containing protons and neutrons.

Element – A substance made of atoms that all contain the same number of protons.

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Key Takeaways

• The atomic number is equal to the number of protons in an atom.
• Every element has a unique atomic number.
• The number of protons determines the identity of an element.
• The modern periodic table is arranged in order of increasing atomic number.
• Atomic number determines where an element appears on the periodic table.
• In a neutral atom, the number of electrons equals the number of protons.
• Therefore, for a neutral atom: Atomic number = Number of protons = Number of electrons
• Atomic number can be used to identify an element and predict basic information about its atomic structure.