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Science · Grades 8–11

Atoms, ions and particle bookkeeping

Atomic number is the proton count.

Make a worksheet

Understand the idea

Identity depends on protons

Atomic number is the proton count.

Charge compares particles

For these examples, net charge comes from unequal proton and electron counts.

Mass number counts nucleons

Add protons and neutrons; do not confuse mass number with average atomic mass.

Worked examples

An atom has 13 protons and 14 neutrons. What is its mass number?

13 + 14 = 27.

27

A particle has 9 protons and 10 electrons. Give its net charge in elementary-charge units.

9 positive charges minus 10 negative charges gives −1.

-1

Try it yourself

  1. Two atoms of the same element must have the same number of

    1. protons
    2. electrons in every ion
    3. neutrons
    4. total nucleons

  2. Isotopes of one element differ in their number of

    1. nuclear charge
    2. neutrons
    3. protons
    4. element names

  3. An ion has 20 protons and charge +2. How many electrons does it have?

  4. Explain why losing an electron does not by itself change an atom into a different element.

Answers and marking notes

Answers and marking notes
  1. protons

    Proton count defines the element.

  2. neutrons

    They retain the same proton count.

  3. 18

    20 − 18 = +2.

  4. Its proton count remains unchanged, so its elemental identity stays the same; its charge changes.

    Distinguish ion formation from changes to the nucleus.

Further reading and review references

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Preview material · independent educational review required. Grade bands are approximate, not official curriculum alignment.