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 around the nucleus.

Key Concepts

• An atom is the smallest unit of an element that retains its chemical properties.
• The atom consists of a central nucleus surrounded by electrons.
• The nucleus contains positively charged protons and electrically neutral neutrons.
• Electrons carry a negative charge and occupy regions called electron shells or orbits.

The Nucleus

The nucleus is extremely small compared with the whole atom but contains almost all of the atom’s mass.

• Protons: each carries a charge of \$+e\$ (where \$e = 1.60 \times 10^{-19}\,\text{C}\$).
• Neutrons: electrically neutral, mass similar to a proton.

The number of protons (\$Z\$) defines the element, while the sum of protons and neutrons (\$A\$) gives the atomic mass number.

Electrons

Electrons are much lighter than protons or neutrons and carry a charge of \$-e\$.

• Electrons are arranged in discrete energy levels (shells) around the nucleus.
• In the Bohr model, electrons move in circular orbits at fixed distances from the nucleus.
• Modern quantum mechanics describes electrons as existing in probability clouds rather than fixed paths.

Relative Masses and Charges

The following table summarises the basic sub‑atomic particles.

Simple Atomic Model (Bohr Model)

For introductory purposes the Bohr model is useful. It assumes:

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.
3. An electron can move between orbits by absorbing or emitting a photon of energy \$E = h\nu\$.

Although superseded by quantum mechanics, the Bohr model helps illustrate the idea of quantised energy levels.

Why the Nucleus is Positive

The nucleus contains protons, each with a positive charge \$+e\$. Since neutrons have no charge, the net charge of the nucleus is \$+Ze\$, where \$Z\$ is the number of protons. The surrounding electrons, each with charge \$-e\$, balance this positive charge, making the overall atom electrically neutral.

Summary

• An atom consists of a dense, positively charged nucleus (protons + neutrons) and a cloud of negatively charged electrons.
• The nucleus holds almost all the mass; electrons contribute almost all of the atom’s volume.
• The number of protons determines the element; the arrangement of electrons determines chemical behaviour.