How do atoms join to form molecules?

In one line

The rule

Atoms combine to form molecules by making chemical bonds, which are connections formed by sharing or transferring electrons between atoms.

There are two main types of chemical bonds:

• Covalent bond: Atoms share electrons.
• Ionic bond: Atoms transfer electrons (one gives, one takes).

Let's understand it simply

Imagine you and your friend each have one cricket bat, but you need two bats to play a proper game. You both decide to share your bats, so now you can play together. Atoms do something similar. They "want" to be stable, like having a full set of bats (electrons). So, they either share their bats (electrons) or give/take bats from each other to become stable. When they do this, they stick together and form a molecule.

How it works

1. Atoms have electrons: Electrons move around the centre (nucleus) of an atom.
2. Atoms want to be stable: Most atoms are stable when they have 8 electrons in their outer shell (this is called the "octet rule").
3. Not all atoms are stable: Many atoms have less than 8 electrons in their outer shell.
4. Atoms combine: To become stable, atoms join with other atoms.
5. Two main ways to join:

• Sharing electrons (Covalent bond): Both atoms share some electrons. Example: In water (H₂O), each hydrogen shares one electron with oxygen.
• Transferring electrons (Ionic bond): One atom gives electrons, the other takes. Example: In salt (NaCl), sodium gives one electron to chlorine.

1. A bond forms: This connection is called a chemical bond. The joined atoms are now a molecule.

The story behind it

The idea of atoms joining to form molecules grew over many years. In the 1800s, John Dalton said all matter is made of atoms. Later, scientists like Gilbert Lewis (early 1900s) explained how atoms use electrons to bond. Lewis drew simple dot diagrams to show how atoms share or transfer electrons. His work helped us understand why water is H₂O and salt is NaCl. One fun fact: Lewis loved puzzles, and he saw chemistry as a puzzle of how atoms fit together.

Where you see it in real life

• Water (H₂O): Two hydrogen atoms and one oxygen atom join by sharing electrons. This makes the water you drink.
• Salt (NaCl): Sodium and chlorine atoms join by transferring electrons. This makes table salt in your kitchen.
• Oxygen gas (O₂): Two oxygen atoms share electrons to form the air you breathe.
• Sugar (C₆H₁₂O₆): Many atoms join by sharing electrons to make sugar for your chai.
• Cooking gas (LPG, mainly C₃H₈): Atoms of carbon and hydrogen join to make the gas you use for cooking.
• Plastic bottles (polyethylene): Thousands of carbon and hydrogen atoms join in long chains by sharing electrons.

Why it matters

• Everything is made of molecules: Food, water, air, clothes—all are made of atoms joined together.
• Understanding reactions: Knowing how atoms join helps you understand chemical reactions (like cooking or rusting).
• Medicine: Medicines are molecules designed by joining atoms in special ways.
• Technology: Batteries, plastics, and even your phone screen work because of how atoms join.

Worked example

Let's see how sodium (Na) and chlorine (Cl) join to make salt (NaCl):

Given: Sodium (Na) has 1 electron in its outer shell. Chlorine (Cl) has 7 electrons in its outer shell.
Step 1: Sodium wants to lose 1 electron to become stable (8 electrons in the next shell).
Step 2: Chlorine wants to gain 1 electron to have 8 electrons in its outer shell.
Step 3: Sodium gives its 1 electron to chlorine.
Step 4: Sodium becomes Na⁺ (positive ion), chlorine becomes Cl⁻ (negative ion).
Step 5: Opposite charges attract, so Na⁺ and Cl⁻ stick together to form NaCl (salt).

Common mistakes

• Thinking all bonds are the same: Some students think atoms always share electrons, but sometimes they transfer electrons.
• Mixing up atoms and molecules: An atom is one unit; a molecule is two or more atoms joined.
• Forgetting about stability: Atoms join to become stable, not just for fun.
• Believing molecules can be made from any atoms: Only certain atoms can join in certain ways (not all combinations work).

Remember it

Atoms join to become stable—by sharing or giving/taking electrons—to make molecules.