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Chemistry Lab · Free lesson · 20 minutes

Count particles without changing the element

Use atomic and mass numbers to count protons, neutrons and electrons, and explain why making an ion does not change element identity.

SS1 · atomic structure

01 · Think first

Start with a question.

A neutral sodium atom becomes a positive sodium ion. Something has changed, but it is still sodium. Which particle count defines the element?

What you already know
  • Subtract whole numbers
  • Understand positive and negative charge
  • Distinguish an element from a compound
A neutral atom gains two electrons. What happens to its element identity?

02 · See it

An atom is a model with distinct counts

Select a sodium atom or ion. The schematic is not to scale, and electrons do not follow the circular orbits drawn in simple school diagrams.

11 p12 n11 electronscharge 0

11 − 11 = 0: neutral sodium atom.

Atomic number Z
protons
Mass number A
protons + neutrons
Neutral atom
electrons = protons
Positive ion
fewer electrons than protons

Use the justified worked steps below to explore this idea.

03 · Show steps · explain why

One justified step at a time.

  1. 1

    For sodium-23: Z = 11 and A = 23

    The atomic number tells us there are 11 protons. A mass number identifies one nuclide; it is not the periodic table's abundance-weighted relative atomic mass.

04 · Mistake clinic

Find the first wrong idea.

Tempting wrong line

Mg²⁺ is made by adding two protons to magnesium.

Compare the correction and its reason

A neutral magnesium atom loses two electrons to form Mg²⁺.

Magnesium keeps Z = 12. Changing the proton number from 12 to 14 would make a different element, not a magnesium ion.

Check: Calculate net charge as proton count minus electron count. Twelve minus ten gives +2.

A mistake is evidence about the next idea to repair. It is not a label for the learner.

05 · Your turn

Try first. Then use a hint.

1. An aluminium-27 atom has atomic number 13. How many neutrons does it have?guided practice
2. Calcium has atomic number 20. How many electrons does Ca²⁺ have?independent practice
3. Two particles both have 8 protons. One has 8 neutrons and the other 10. What can you conclude?far practice

06 · Check & reflect → next step

Understanding includes explaining.

Which count fixes identity, which changes an isotope, and which changes ion charge?

Choose your next learning need

Use ions and charge balance to explain an ionic compound's formula and its conductivity.

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Source and review status

JEM-CH-C02-001 · Phase 5A Chemistry Foundations Matter Atomic Structure Periodicity Bonding · Lab 05 · printed pp. 24–25

This published free lesson contains selected explanations and authored practice. Complete canonical volumes and tutor solutions remain in the protected Content Vault.