Atomic Number, Isotopes & Ions: Class 9 Worked Examples

Build the particle accounting sheet

Protons have positive charge and neutrons have no charge; both are in the nucleus. Electrons carry negative charge and occupy the region around the nucleus. A simplified shell picture helps count them but is not a scale drawing of an atom.

Use one row each for protons, neutrons and electrons. Changing one row does not automatically change all three.

Atomic number identifies the element

The atomic number Z is the number of protons. Every carbon atom has 6 protons. If the proton number changed, it would no longer be carbon.

A neutral atom has equal proton and electron counts. This neutrality condition does not tell you the neutron count; you need its mass number or isotope information.

Mass number is a whole-particle count

Mass number A is protons plus neutrons. Therefore neutrons=A−Z. Carbon-14 has 14−6=8 neutrons. Carbon-12 has 6. Both have 6 protons.

Mass number is not the same as the relative atomic mass printed as a decimal in a periodic table. The latter reflects isotopic masses and abundances; do not round it and treat that rounded number as the mass number of every atom.

Isotopes share protons but differ in neutrons

Isotopes are atoms of the same element with different neutron counts and therefore different mass numbers. They have the same atomic number. Neutral isotopes of the same element also have the same electron count.

Isotopes can have different physical and nuclear properties. An isotope is not automatically radioactive, and a heavier isotope is not automatically a different element.

Ions change electron count

A positive ion has fewer electrons than protons; a negative ion has more. Sodium has Z=11. Na⁺ has 11 protons and 10 electrons. Chlorine has Z=17, so Cl⁻ has 17 protons and 18 electrons.

In ordinary chemical ion formation, the nucleus remains unchanged. Saying “sodium loses a proton to become Na⁺” would change the element and is not this process.

Interpret nuclear and charge notation

An isotope can be written with A above Z beside the element symbol, with charge at the upper right. For an ion with A=24, Z=12 and charge 2+, count 12 protons, 12 neutrons and 10 electrons.

Read the charge separately from A and Z. A negative charge does not mean a negative number of particles; it describes an excess of electrons relative to protons.

Distinguish isotope from ion comparisons

Compare carbon-12 and carbon-14: neutrons differ, so they are isotopes. Compare neutral carbon-12 and a carbon-12 ion: electrons differ, so they are different charge states of that isotope.

A species can be both a particular isotope and an ion. These labels answer different questions rather than competing for one category.

Finish with charge and mass checks

Add protons and neutrons to recover A. Subtract electrons from protons to recover the charge in elementary-charge units. Confirm that Z matches the named element.

This count-and-check approach is useful before studying bonding or nuclear topics. The exercises give the needed atomic numbers; no periodic-table memorisation is required to infer missing information.

See the reasoning

Worked examples and checks

Try, explain, check

12 questions with explained answers

Try each task before opening its explanation. These are original BananaLearn questions, not official exam or past-paper questions. Where several answers can work, the explanation gives the criteria.

1. An atom has 9 protons. What is Z?

   Check answer and reasoning

   9
   Atomic number counts protons.

2. A=19 and Z=9. Find neutrons.

   Check answer and reasoning

   10
   19−9=10.

3. The same atom is neutral. Find electrons.

   Check answer and reasoning

   9
   Neutrality requires p=e.

4. It forms a 1− ion. Find electrons.

   Check answer and reasoning

   10
   One extra electron gives charge −1.

5. A species has p=13,e=10. Find its charge.

   Check answer and reasoning

   3+
   13−10=+3.

6. A=27,Z=13. Does a 3+ charge change its neutron count?

   Check answer and reasoning

   No; it remains 14
   Ion formation changes electrons in this model.

7. Two atoms have the same Z but different A. What are they?

   Check answer and reasoning

   Isotopes
   Their neutron counts differ.

8. Do isotopes have to be radioactive?

   Check answer and reasoning

   No
   Radioactivity is not part of the isotope definition.

9. Do p=6,n=6 and p=7,n=5 name the same element because both total 12?

   Check answer and reasoning

   No
   Element identity depends on protons, which differ.

10. Can a negative ion have a negative electron count?

    Check answer and reasoning

    No
    Negative charge means more electrons than protons.

11. Is a periodic-table decimal atomic mass the mass number of every atom?

    Check answer and reasoning

    No
    It represents isotope-weighted relative atomic mass.

12. Give two checks on a completed particle record.

    Check answer and reasoning

    p+n=A and p−e=charge
    Also check p=Z for the named element.

A checking record to use in your notebook

Copy these headings and fill them for one worked task, then for a fresh attempt. Use your own evidence rather than copying the model answer.

Question or sentence

What am I being asked to explain?

Model or evidence

Which diagram, rule or passage detail supports my method?

Work

What steps connect the question to the answer?

Independent check

Can I substitute, recount, trace a path or check the text?

Next attempt

What changes in a fresh example?

Choose the next useful step

• Revisit atoms, elements and molecules
• Connect ion formation with material properties
• Keep atom counts consistent in reactions

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Sources, level and update

Original BananaLearn explanations, examples, illustrations and practice. Source facts checked 7 November 2026. Suggested levels are learning guides, not a claim that every board or textbook uses the same sequence.

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