Ions and electron counting

Your goal is to deduce electron counts and ion charges while keeping proton number—and therefore element identity—fixed.

1. Retrieve what you know

For a neutral atom, how are proton and electron counts related? Which count fixes the element’s identity?

2. Atom-and-ion model

Ion formation changes electrons, not the nucleus. Compare proton and electron counts to decide whether one species is neutral or charged; compare proton and neutron counts between species to decide whether they are isotopes.

A positive ion has lost electrons; a negative ion has gained electrons. Check every result using:

charge = protons-electrons

“Isotope” and “ion” answer different questions: neutron count establishes the isotope, while electron imbalance establishes the charge. A species can be both a particular isotope and an ion.

3. Modelled example: aluminium ion

Modelled example 1

Count particles in an aluminium ion

Core

Problem

Find the proton, neutron and electron counts in ²⁷₁₃Al³⁺.

Study the worked solution

1. Read the nucleus

   Method

   Use Z = 13 and calculate A-Z.

   Reason

   Ion formation does not change the nucleus.

   Working

   Protons = 13; neutrons = 27-13 = 14.

2. Apply the charge

   Method

   Remove three electrons from the neutral count.

   Reason

   A 3 + ion has three fewer electrons than protons.

   Working

   Electrons = 13-3 = 10; check: 13-10 = 3 +.

4. Guided attempt: chloride ion

Guided practice 2

Decode a chloride ion

About 4 min

Problem

Find the counts in ³⁵₁₇Cl⁻.

Try this before viewing the solution

Protons

Neutrons

Electrons

Check my attempt

Hints

Hint 1: nucleus first

Use proton and nucleon numbers before applying the charge.

Hint 2: negative charge

A negative ion has more electrons than protons.

View solution step by step

1. Keep the nucleus fixed

   Method

   Use 17 protons and 18 neutrons.

   Reason

   The charge does not alter proton or neutron count.

   Working

   35-17 = 18 neutrons.

2. Gain one electron

   Method

   Use 18 electrons.

   Reason

   One extra electron produces charge 1-.

   Working

   17-18 = -1.

5. Misconception contrast

Ion formation never changes proton number. A positive ion has lost, not gained, electrons. Two particles can have the same electron count and still be different elements because proton number—not electron number—fixes identity.

6. Changed-context transfer

Two particles each have 10 electrons. Particle P has 11 protons and particle Q has 9 protons. Identify each charge and decide whether they are the same element.

7. Independent check

A particle has 12 protons, 13 neutrons and 10 electrons. Write its charge, explain whether electron loss or gain formed it, and state which count must remain unchanged for it to remain the same element.