Describe the structure of an atom in terms of a positively charged nucleus and negatively charged electrons in orbit around the nucleus

5.1.1 The Atom

Learning objective

Describe the structure of an atom in terms of a positively‑charged nucleus and negatively‑charged electrons in orbit (or cloud) around the nucleus, and use this knowledge to explain ions, isotopes and the overall charge balance.

Key concepts required by the Cambridge IGCSE syllabus

• An atom is the smallest unit of an element that retains the element’s chemical properties.
• Each atom consists of a tiny, dense, positively‑charged nucleus surrounded by a cloud of negatively‑charged electrons.
• The nucleus contains protons (charge + e) and neutrons (neutral).
• Electrons occupy discrete electron shells (energy levels) labelled n = 1, 2, 3… .
• In the simple Bohr model electrons move in fixed circular orbits**; the modern quantum‑mechanical picture replaces these orbits with orbitals—three‑dimensional probability regions.
• Atoms are electrically neutral because the total negative charge of the electrons exactly balances the total positive charge of the protons.

Atomic notation (nuclide notation)

A nuclide is written as ^A_Z X, where:

Isotopes are atoms of the same element (same Z) that have different mass numbers A, i.e. different numbers of neutrons.

The nucleus

The nucleus occupies only about 10⁻⁵ of the atom’s volume but contains almost all of its mass.

• Protons: charge + e (e = 1.60 × 10⁻¹⁹ C), mass ≈ 1 u. The number of protons defines the element.
• Neutrons: charge 0, mass ≈ 1 u (slightly larger than a proton).

Electrons

Electrons have charge − e (e = 1.60 × 10⁻¹⁹ C) and a mass ≈ 1⁄1836 u.

• They are arranged in discrete electron shells (energy levels) labelled n = 1, 2, 3… .
• In the Bohr model each shell corresponds to a fixed circular orbit at a particular distance from the nucleus.
• Quantum mechanics describes electrons by orbitals** – regions where the probability of finding an electron is high. For the IGCSE syllabus only the idea of shells is required; orbitals are introduced later for deeper study.

Relative masses and charges of the sub‑atomic particles

*Relative to a proton, which is defined as 1.

Simple atomic model – the Bohr model

The Bohr model is useful for introductory work, especially for hydrogen‑like atoms. It makes three key assumptions:

1. Electrons travel in fixed circular orbits around the nucleus.
2. Only certain discrete orbits are allowed; each orbit corresponds to a specific energy level (n = 1, 2, 3…).
3. An electron can move between orbits by absorbing or emitting a photon. The photon energy is given by E = hν, where h is Planck’s constant and ν the frequency of the radiation.

Note: The Bohr model gives a good description of the hydrogen atom but fails for multi‑electron atoms. IGCSE questions never require quantitative Bohr calculations; the model is only needed to illustrate the idea of discrete energy levels.

Modern quantum‑mechanical picture

• Electrons are not confined to precise paths; they are described by wave functions.
• The result is an orbital – a region of space where the probability of finding an electron is high.
• Orbitals are grouped into shells (n = 1, 2, 3…) and subshells (s, p, d, f). This terminology appears later in the syllabus.

Ions – atoms that are not neutral

If an atom loses one or more electrons it becomes a positive ion (cation); if it gains electrons it becomes a negative ion (anion). Examples:

• Na → Na⁺ (loss of one electron)
• Cl + e⁻ → Cl⁻ (gain of one electron)

In a neutral atom the number of electrons equals the number of protons (Z). In an ion the numbers differ, giving the atom an overall charge.

Evidence for a tiny, dense nucleus

Rutherford’s gold‑foil experiment (1911) showed that most α‑particles passed straight through a thin foil, but a few were deflected at large angles. This could only be explained if the atom’s positive charge and most of its mass were concentrated in a very small central region – the nucleus.

Charge balance – why a neutral atom has zero net charge

The nucleus carries a total positive charge of +Ze (Z = number of protons). A neutral atom also contains Z electrons, each with charge − e. Therefore:

+ Z e + (‑ Z e) = 0

Thus a neutral atom has no overall electric charge.

Worked example – calculating Z, A and N

For the nuclide ¹²₆C (carbon‑12):

• Z = 6 → 6 protons (and, in a neutral atom, 6 electrons).
• A = 12 → total nucleons = 12.
• N = A − Z = 12 − 6 = 6 neutrons.

This illustrates how to read nuclide notation and determine the numbers of protons, neutrons and electrons.

Summary

• An atom consists of a dense, positively‑charged nucleus (protons + neutrons) surrounded by a cloud of negatively‑charged electrons.
• The nucleus contains almost all the mass; electrons occupy most of the volume.
• Atomic number Z defines the element; mass number A (= Z + N) gives the total number of nucleons.
• Electrons are arranged in shells (n = 1, 2, 3…) – the Bohr model shows them in fixed orbits, while the quantum‑mechanical model uses orbitals.
• Neutral atoms have zero net charge because the total negative charge of the electrons balances the total positive charge of the protons.
• Loss or gain of electrons produces ions; atoms of the same element with different numbers of neutrons are isotopes.
• Rutherford’s gold‑foil experiment provided the experimental evidence for a tiny, dense nucleus.