Atomic Number
Atomic number is the number of protons present in the nucleus of an atom.
In one line: Atomic number is the element's proton ID card.
Practice This ConceptUnderstand it
Atomic number is represented by Z. It identifies the element because each element has a fixed number of protons. In a neutral atom, the atomic number also equals the number of electrons, but the main identity of the element depends on protons.
Example
Rule to remember
Z = number of protons; for a neutral atom, Z = number of electrons.
See it worked out
Follow the example line by line, then use the same method on new questions.
Worked example
- 1If an atom has 8 protons, its atomic number is 8 and the element is oxygen.
- 2A neutral oxygen atom has 8 electrons.
Method you can reuse
Sketch it in your answer
Show the nucleus with labelled protons and connect proton count to the element name.
Check yourself
Answer the question in your head before opening the answer.
Try it yourself
Why does atomic number identify an element?
Think first, then show answerHide answer
Common wrong answer
Writing Z = protons + neutrons.
Why it loses marks: That sum is mass number.
Don't mix this up
Exam tip
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
- 1
Enough for 1 mark
Atomic number is the number of protons present in the nucleus of an atom.
- 2
Add for 2 marks
Example: Carbon has atomic number 6, so every carbon atom has 6 protons.
- 3
Add for 3 marks
Atomic number is represented by Z.
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