5. Atomic and Ionic Diagrams

Introduction

Atoms are far too small to see, even with the most powerful light microscopes. To understand their structure and how they form chemical bonds, scientists use a variety of atomic diagrams. These simplified models help us visualise the arrangement of electrons, predict how atoms behave, and explain how compounds are formed.

Three of the most common diagrams used in chemistry are Bohr diagrams, Lewis electron-dot diagrams, and dot-and-cross diagrams. Each diagram highlights different aspects of atomic structure and chemical bonding. Learning how to draw and interpret these diagrams provides a foundation for understanding ionic and covalent bonding.

Bohr Diagrams

A Bohr diagram is a simple model that shows:

• The nucleus
• Protons
• Neutrons
• Electrons arranged in shells (energy levels)

Electrons occupy shells around the nucleus.

For the first 20 elements, the maximum number of electrons in the first few shells is commonly represented as:

• First shell: 2 electrons
• Second shell: 8 electrons
• Third shell: 8 electrons (for introductory chemistry)
• Fourth shell: Remaining electrons

Bohr diagrams are useful for showing how electrons are arranged around the nucleus.

 

 

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Figure 1. Bohr diagrams show electrons arranged in shells around the nucleus.

Drawing a Bohr Diagram

To draw a Bohr diagram:

Step 1

Find the atomic number.

This tells you the number of:

• Protons
• Electrons (for a neutral atom)

Step 2

Draw the nucleus.

Label:

• Number of protons
• Number of neutrons (if known)

Step 3

Place electrons into shells.

Example:

Carbon (Atomic Number = 6)

• First shell = 2 electrons
• Second shell = 4 electrons

Electron arrangement:

2,4

More Examples

Electron Shells

Electrons occupy different energy levels, often called electron shells.

Important ideas:

• Electrons fill the inner shells first.
• Outer-shell electrons are called valence electrons.
• Valence electrons determine how atoms react chemically.

For example:

Atoms with full outer shells are generally very stable.

 

 

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Figure 2. The number of valence electrons determines how atoms react and form chemical bonds.

Forming Ions

Atoms become more stable by gaining or losing electrons.

When this happens, they form ions.

Positive Ions (Cations)

Metals usually:

• Lose electrons.
• Form positive ions.

Example:

Sodium

Na → Na⁺ + e⁻

Sodium loses one electron and becomes Na⁺.

Negative Ions (Anions)

Non-metals usually:

• Gain electrons.
• Form negative ions.

Example:

Cl + e⁻ → Cl⁻

Chlorine gains one electron and becomes Cl⁻.

The number of protons does not change—only the number of electrons changes.

 

 

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Figure 3. Metals lose electrons to form positive ions, while non-metals gain electrons to form negative ions.

Lewis Electron-Dot Diagrams

A Lewis electron-dot diagram shows:

• The element symbol.
• The valence electrons only.

The nucleus and inner electrons are not shown.

Examples:

Hydrogen

H•

Carbon

  •
• C •
  •

Oxygen

 ••
• O •
 ••

Chlorine

 ••
••Cl••
  •

Lewis diagrams help predict how atoms will bond.

 

 

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Figure 4. Lewis diagrams show only the valence electrons that participate in chemical bonding.

Lewis Diagrams for Ions

When atoms become ions, the Lewis diagram changes.

Sodium Ion

Na⁺

• No valence dots shown.
• Written inside brackets.

[Na]⁺

Chloride Ion

Chlorine gains one electron.

  ••
••Cl••
  ••

Written as:

[Cl]⁻

with eight electrons around the symbol.

This represents a complete outer shell.

Dot-and-Cross Diagrams

A dot-and-cross diagram shows:

• Which electrons belong to each atom.
• How electrons are transferred or shared.

Different symbols are used:

• Dots (•)
• Crosses (×)

This makes it easy to identify the origin of each electron.

Ionic Bond Example

Sodium chloride (NaCl)

Sodium transfers one electron to chlorine.

The diagram shows:

• Sodium losing one electron.
• Chlorine gaining that electron.
• Both ions achieving full outer shells.

Covalent Bond Example

Hydrogen molecule (H₂)

Each hydrogen contributes one electron.

The shared pair forms a covalent bond.

Dot-and-cross diagrams clearly show this sharing.

 

 

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Figure 5. Dot-and-cross diagrams show how electrons are transferred in ionic bonds and shared in covalent bonds.

Comparing Atomic Diagrams

Each diagram highlights different information about atoms and chemical bonding.

Why These Diagrams Are Important

Atomic diagrams help scientists:

• Understand electron arrangement.
• Predict chemical reactions.
• Explain ionic and covalent bonding.
• Understand the periodic table.
• Design new materials and compounds.

Although these diagrams are simplified models, they remain valuable tools for learning chemistry.

Worked Example

Question

Draw the Bohr electron arrangement and Lewis diagram for magnesium (Atomic Number = 12).

Solution

Bohr Diagram

Electron arrangement:

2,8,2

Magnesium has:

• 12 protons
• 12 electrons
• 2 valence electrons

Lewis Diagram

• Mg •

Two dots represent magnesium's two valence electrons.

Real-World Connection

Chemists use electron diagrams to understand how atoms bond together to form the substances we use every day. Whether designing stronger alloys, developing new medicines, producing batteries, or creating advanced electronic materials, understanding electron arrangements is essential. Engineers also rely on these models when developing semiconductors, solar cells, and rechargeable batteries.

Did You Know?

The Bohr model, proposed by Niels Bohr in 1913, was a major breakthrough in understanding atomic structure. Although modern quantum mechanics provides a more accurate description of electrons, Bohr diagrams are still widely used in schools because they clearly illustrate electron shells and help explain chemical bonding.

Key Terms

Anion – A negatively charged ion formed when an atom gains electrons.

Bohr diagram – A model showing electrons arranged in shells around the nucleus.

Cation – A positively charged ion formed when an atom loses electrons.

Dot-and-cross diagram – A diagram showing how electrons are transferred or shared during bonding using dots and crosses to distinguish electrons from different atoms.

Electron shell – An energy level around the nucleus where electrons are found.

Ion – A charged particle formed when an atom gains or loses electrons.

Lewis electron-dot diagram – A diagram showing an element's symbol surrounded by its valence electrons.

Valence electrons – The electrons in the outermost shell of an atom that are involved in chemical bonding.

Key Takeaways

• Bohr diagrams show the nucleus and the arrangement of electrons in shells.
• Electrons fill the inner shells first, and the outermost valence electrons determine how atoms react.
• Atoms become ions by gaining or losing electrons to achieve more stable electron arrangements.
• Lewis electron-dot diagrams show only the valence electrons involved in bonding.
• Dot-and-cross diagrams illustrate how electrons are transferred in ionic bonds or shared in covalent bonds.
• These diagrams are important tools for understanding atomic structure and predicting how elements form compounds.