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Chemistry · 01 / 10

Atoms, ions and molecules

Middle school · Worked example and 4 exercises with solutions, at your pace.

UnderstandSee an exampleGuided exercise3 progressive challenges

The principle, simply

Nuclei contain positive protons and neutral neutrons; electrons are negative. Atomic number Z counts protons and identifies the element. Neutral atoms have Z electrons. Ions gain or lose electrons; changing protons changes the element. Molecules contain bonded atoms, whereas ionic solids are not simply isolated molecules.

THE PICTURE TO REMEMBERAtoms, ions and molecules
  1. 01Z and nucleus
  2. 02Neutrality
  3. 03Electron gain or loss
  4. 04Formula and charge

WORKED EXAMPLE

From principle to reasoning

Neutral sodium, Z=11, has 11 electrons; Na⁺ has 10. Cl⁻ gains an electron. H₂O contains two H and one O.

Solve a problem in this subject

  1. Identify species

    Write formulas and useful states. Distinguish atoms, ions, molecules and amount of substance. Total charge is conserved.

  2. Connect quantities

    Use m = nM and C = n/V with consistent units. V is the final solution volume.

  3. Use coefficients

    Balance before building an extent table. Compare initial amounts divided by coefficients to identify the limiting reactant.

  4. Check context

    Remaining amounts cannot be negative. Check conservation, units and assumptions: dilution, medium, completion or equilibrium.

YOUR TURN

01 · Try before checking

Z=12 becomes a 2+ ion. How many electrons?

One hint

Positive charge means fewer electrons than protons.

Reveal and understand the solution

It retains 12 protons and has 10 electrons.

Compare method, conditions and answer; identify the exact part to revisit.

IMPROVE THROUGH PRACTICE

Three additional challenges with solutions

Start with the first challenge. Write your method before opening a hint; use the solution to find the first step that needs work.

Reflect after practising
02 · Recognise and understandLink to this exercise

An atom has 11 protons and 11 electrons. What is its charge?

A hint to get started

Proton and electron charges oppose each other.

Step-by-step solution
  1. Positive and negative charge counts match.
  2. Total charge is zero.
  3. The atom is neutral.

Which step did you miss? Retry without help, then explain why the method works.

03 · Apply the methodLink to this exercise

The same atom loses one electron. What charge does the ion have?

A hint to get started

The proton count does not change.

Step-by-step solution
  1. It has 11 protons and 10 electrons.
  2. Net charge is +e.
  3. For sodium this is Na⁺; electron loss creates a cation.

Which step did you miss? Retry without help, then explain why the method works.

04 · Go furtherLink to this exercise

Two atoms have 6 protons but 6 and 8 neutrons. Are they the same element?

A hint to get started

Atomic number is proton count.

Step-by-step solution
  1. Both have Z = 6, so both are carbon.
  2. Their mass numbers are 12 and 14.
  3. They are isotopes, not ions: neutron counts differ.

Which step did you miss? Retry without help, then explain why the method works.

The trap to avoid

Charge does not mean protons were added. Formula subscripts differ from charge.

Without notes, can you…

  • Explain the principle in your own words?
  • Rebuild the diagram and its connections?
  • Solve the exercise and justify the method?

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