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Chemistry · Grades 7–10

Atoms, isotopes and ions

Track protons, neutrons and electrons separately.

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Learn the idea

Identity

Atomic number Z counts protons and identifies an element.

Mass number

For one isotope, A = protons + neutrons. This is not a weighted periodic-table atomic mass.

Charge

For these simple ions, charge number = protons − electrons.

Isotopes

Atoms of one element can have different neutron counts while retaining the same proton count.

Worked examples

An isotope has atomic number 8 and mass number 18. How many neutrons does it have?

Neutrons = 18 − 8 = 10.

10

A neutral atom has 11 protons. How many electrons does it have?

Neutrality requires equal positive and negative charge counts.

11

Check your understanding

  1. An ion has 12 protons and 10 electrons. What is its signed charge number?

  2. What must two isotopes of one element share?

    1. Their proton count.
    2. Their mass number.
    3. Their neutron count.

  3. Which change produces a positive ion in an ordinary ion-formation model?

    1. Losing electrons.
    2. Gaining protons by chemical bonding.
    3. Losing all neutrons.

Answers and marking notes

Answers and marking notes
  1. 2

    12 − 10 = +2.

  2. Their proton count.

    The proton count defines the element.

  3. Losing electrons.

    A chemical ion changes electron count, not nuclear identity.

Further reading and review references

These are reference links, not copied exercises, institutional endorsement or proof of independent review.

Independent educational review required. Grade bands are approximate; these are not officially aligned curricula.