Know how atoms may form positive ions by losing electrons or form negative ions by gaining electrons

5.1 The Atom

Objective

Know how atoms may form positive ions by losing electrons or form negative ions by gaining electrons, and understand the basic nuclear structure of an atom.

Core learning outcomes (Cambridge IGCSE Physics 0625)

• Identify the three sub‑atomic particles (protons, neutrons, electrons) and their relative charges (+1, 0, –1).
• State that protons determine the atomic number (Z) and that protons + neutrons give the mass number (A).
• Describe the atom as a positively‑charged nucleus surrounded by negatively‑charged electrons in discrete energy levels (shells).
• Write and interpret nuclide notation ⁽ᴬ⁾₍ᶻ₎X where Z = atomic number, A = mass number.
• Calculate the number of neutrons in an isotope (N = A – Z).
• Explain the concept of isotopes.
• Define ions, cations and anions, and write the ion‑formation equations.
• State the meaning of ionisation energy and electron affinity and the sign of the energy change.
• Recognise the general trends: metals → cations, non‑metals → anions.

1. Nuclear model of the atom

• Nucleus – a tiny, dense centre containing:

  • Protons (p⁺) – charge +1, mass ≈ 1 u.
  • Neutrons (n⁰) – charge 0, mass ≈ 1 u.

• Electrons (e⁻) – charge –1, mass ≈ 1/1836 u, occupy discrete energy levels (shells) surrounding the nucleus.

Experimental evidence

Nuclide notation

The standard way to denote a specific isotope is

⁽ᴬ⁾₍ᶻ₎X

where:

• Z = atomic number = number of protons (defines the element).
• A = mass number = protons + neutrons.
• X = chemical symbol.

Worked example – sodium‑23

⁽²³⁾₍₁₁₎Na

• Protons, Z = 11
• Mass number, A = 23
• Neutrons, N = A – Z = 23 – 11 = 12

Isotopes

Atoms of the same element (same Z) that have different numbers of neutrons (different A) are called isotopes. They have identical chemical behaviour but different nuclear properties.

Activity – neutrons in calcium

Calculate the number of neutrons in the most abundant isotope of calcium, ⁴⁰Ca (⁴⁰₂₀Ca).

Solution: N = A – Z = 40 – 20 = 20 neutrons.

2. Ions – charged atoms

An atom is electrically neutral when the number of protons equals the number of electrons. If this balance is disturbed, the atom carries a net charge and is called an ion.

• Fewer electrons than protons → positive ion (cation).
• More electrons than protons → negative ion (anion).

Formation of positive ions (cations)

Atoms lose one or more electrons. The removal requires energy called ionisation energy (IE), which is always positive (energy absorbed).

X → Xⁿ⁺ + n e⁻

where n is the number of electrons removed.

Formation of negative ions (anions)

Atoms gain one or more electrons. The addition releases energy called electron affinity (EA), which is negative (energy released) for most non‑metals.

X + n e⁻ → X^n‑

Energy‑level perspective

Electrons occupy discrete energy levels. Removing an electron moves it from a bound level to a free state (requires IE). Adding an electron places it into the lowest available vacant level, releasing EA.

3. Examples of common ions

4. Key points to remember

1. Atoms are neutral when the number of protons equals the number of electrons.
2. Loss of electrons → cation; gain of electrons → anion.
3. The magnitude of the ionic charge equals the number of electrons lost or gained.
4. Ionisation energy (IE) is the energy absorbed to remove an electron; it is always positive.
5. Electron affinity (EA) is the energy released when an electron is added; it is usually negative.
6. Metals (e.g., Na, Ca, Al) tend to lose electrons and form cations.
7. Non‑metals (e.g., Cl, O, S) tend to gain electrons and form anions.
8. Isotopes have the same Z but different A; they share chemical behaviour but differ in nuclear properties.

5. Radioactivity (new subsection – 5.2)

Radioactivity is the spontaneous emission of particles or electromagnetic radiation from an unstable nucleus. It is a core part of the IGCSE syllabus.

Types of radiation

• α‑particles – helium nuclei (2p + 2n), +2 charge, high ionising power, low penetration (stopped by a sheet of paper).
• β‑particles – high‑speed electrons (or positrons), –1 charge, moderate ionising power, moderate penetration (stopped by a few mm of aluminium).
• γ‑rays – high‑energy photons, no charge, low ionising power, high penetration (requires dense shielding such as lead or several cm of concrete).

Detection

A Geiger‑Müller (GM) counter measures the number of ionising events per second (counts s⁻¹). It is the most common detector used in school labs to demonstrate background radiation and the intensity of different sources.

Simple decay equation

Example – α‑decay of uranium‑238:

⁽²³⁸⁾₉₂U → ⁽⁴⁾₂He + ⁽²³⁴⁾₉₀Th

The mass number decreases by 4 and the atomic number by 2, reflecting the loss of an α‑particle.

Half‑life

The half‑life (t½) is the time required for half of a given number of radioactive nuclei to decay. If a sample contains N₀ nuclei, after one half‑life it contains N₀/2, after two half‑lives N₀/4, etc.

Sample calculation: A 20 g sample of a radionuclide with t½ = 5 days decays to 5 g after how many half‑lives?

Solution: 20 g → 10 g (1 t½) → 5 g (2 t½). Therefore, 2 × 5 days = 10 days.

Safety principles (time, distance, shielding)

• Time – minimise exposure time.
• Distance – increase distance from the source (inverse‑square law).
• Shielding – use appropriate material (paper for α, aluminium for β, lead or concrete for γ).

Applications (supplementary)

• Medical imaging (γ‑rays in PET scans, X‑rays).
• Radiotherapy – using high‑energy γ‑rays or β‑emitters to destroy cancer cells.
• Industrial radiography – γ‑rays to inspect welds and metal thickness.
• Carbon‑14 dating – measuring the remaining β‑activity to determine the age of archaeological samples.

6. Practice questions

1. Write the ionic symbol for an aluminium atom that has lost three electrons.
2. What charge will a sulphur atom have after gaining two electrons?
3. Explain why sodium readily forms a +1 ion while chlorine readily forms a –1 ion, referring to electron configuration and ionisation energy/electron affinity.
4. Using nuclide notation, represent the most common isotope of calcium and calculate the number of neutrons it contains.
5. State the sign (positive or negative) of ionisation energy and electron affinity and give a brief reason for each.
6. Identify the type of radiation emitted in the decay ²³⁸₉₂U → ⁴₂He + ^{234₉₀Th and describe one practical shielding material for it.}
7. A sample of a radionuclide has a half‑life of 12 h. If the initial activity is 800 cps, what will be the activity after 36 h?