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medium difficulty5 min

How Atoms Combine

Atoms combine with other atoms by losing, gaining, or sharing electrons so that they can become more stable.

In one line: Outer shell decides bonding behaviour.

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Understand it

The outermost shell of an atom controls its combining behaviour. Atoms with incomplete outer shells tend to combine in a way that gives them a stable electronic arrangement, often similar to noble gases.

Example

Sodium loses one electron and chlorine gains one electron to form sodium chloride, while two hydrogen atoms share electrons with oxygen to form water.

Rule to remember

Atoms generally combine to achieve stable outer-shell arrangements through electron transfer or electron sharing.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

  1. 1A chlorine atom has seven valence electrons.
  2. 2It can gain one electron from sodium to complete its outer shell and form chloride ion.

Method you can reuse

  1. 1Find valence electrons.
  2. 2Check how many electrons are needed for a stable outer shell.
  3. 3Decide whether transfer or sharing is more suitable.
  4. 4Write the resulting bond type.

Visual tip

Bohr shell diagrams can show valence electrons before and after combination.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

What is the main reason atoms combine with other atoms?

Think first, then show answer
Atoms combine mainly to achieve a more stable outermost electronic arrangement. They may do this by losing, gaining, or sharing electrons.

Common wrong answer

Writing that atoms combine only to increase mass.

Why it loses marks: Chemical combination depends mainly on electron arrangement and stability.

Don't mix this up

Students often say atoms combine simply because they want to become bigger. The better exam reason is stability of the outermost shell.

Exam tip

Mention outermost-shell stability and the method of combination: loss, gain, or sharing of electrons.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

Asked as a reason why atoms form bonds, or to identify whether a given case shows transfer or sharing of electrons.
  1. 1

    Enough for 1 mark

    Atoms combine with other atoms by losing, gaining, or sharing electrons so that they can become more stable.

  2. 2

    Add for 2 marks

    Example: Sodium loses one electron and chlorine gains one electron to form sodium chloride, while two hydrogen atoms share electrons with oxygen to form water.

  3. 3

    Add for 3 marks

    The outermost shell of an atom controls its combining behaviour.

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