ChemistryDifficulty 1-3

Atoms

An atom is the smallest unit of an element that keeps that element's chemical identity. Every atom has a tiny, dense nucleus of protons and neutrons, which holds nearly all of its mass, surrounded by electrons that fill almost all of its volume. The number of protons fixes which element the atom is; changing the number of electrons makes an ion, and changing the number of neutrons makes a different isotope.

Scales, masses and charge

With an atomic diameter of order 10⁻¹⁰ m and a nuclear diameter of order 10⁻¹⁵ m, the nucleus occupies roughly (10⁻¹⁵ ÷ 10⁻¹⁰)³ = 10⁻¹⁵ of the atom's volume (an order-of-magnitude estimate, treating both as spheres). Yet the proton–electron mass ratio of about 1836 means essentially all the mass resides there.

\frac{V_{\text{nucleus}}}{V_{\text{atom}}} \approx \left(\frac{10^{-15}}{10^{-10}}\right)^{3} = 10^{-15}

Order-of-magnitude volume fraction (ScienceVerse calculation).

From scattering to structure

Rare large-angle deflections of alpha particles require a concentrated positive charge: a diffuse 'plum pudding' of charge could not reverse a fast, heavy alpha particle. The rarity of such events bounds the nucleus to a tiny fraction of the atom's cross-section.

Electrons as wavefunctions

The quantum-mechanical model describes an electron by a wavefunction ψ, a solution of the Schrödinger equation from which the probability of finding the electron in each region of space is obtained. Orbitals are regions where the electron is most probably found, not trajectories. A shell n contains n² orbitals of at most two electrons each, hence the 2n² capacity.

Common misconception: Shell diagrams with electrons on circles are bookkeeping devices for counting electrons, not pictures of motion. ScienceVerse's 3D atom scene shows an on-screen note saying its rings are a simplified model for this reason.

Full explanation — the complete reference version every reading depth is based on

What an atom is made of

Atoms are built from three kinds of particle. Protons carry a positive charge and neutrons carry no charge; both sit together in the nucleus at the centre. Electrons carry a negative charge and spread through the space around the nucleus. The nucleus is extraordinarily small: an atom is on the order of 10⁻¹⁰ m across, while its nucleus is roughly 100,000 times smaller.

Proton: positive charge (+1 in units of the elementary charge), found in the nucleus.
Neutron: no charge, found in the nucleus, with about the same mass as a proton.
Electron: negative charge (−1), found outside the nucleus; a proton is about 1836 times heavier.

Counting particles: Z, A and charge

Two whole numbers describe an atom's nucleus. The atomic number Z is the number of protons, and it alone decides which element the atom is. The mass number A counts protons plus neutrons. The overall charge depends on how many electrons the atom has compared with its protons.

A = Z + N

Mass number = protons (Z) + neutrons (N).

\text{charge} = (\text{protons} - \text{electrons})\,e

Net charge in units of the elementary charge e; zero for a neutral atom.

Worked example (as the Atom Builder calculates it)

Build an atom with 11 protons, 12 neutrons and 10 electrons. Eleven protons means the element is sodium. The mass number is 11 + 12 = 23, so the isotope is sodium-23. The charge is 11 − 10 = +1: there is one more proton than electron, so this is a cation, written Na⁺. (Arithmetic by ScienceVerse, using the definitions above.)

Electron shells: a useful model

Electrons are arranged in shells around the nucleus. The shell numbered n can hold at most 2n² electrons — 2 in the first shell and 8 in the second. For the first 20 elements a simple rule works: fill 2, then 8, then 8. Potassium (Z = 19) shows why the third shell stops at 8 here: its nineteenth electron goes into the 4s subshell of the fourth shell rather than the 3d subshell, so its electrons are arranged 2, 8, 8, 1 — one outer electron, like lithium and sodium (arrangement worked out by ScienceVerse from those rules; it is the arrangement the Atom Builder draws).

Energy levels also explain why atoms give out radiation. When an electron drops from a higher energy level to a lower one, the atom releases the energy as electromagnetic radiation — such as the X-rays Henry Moseley measured in 1913 — and the more protons in the nucleus, the more energy is given out.

How we know

John Dalton's atomic theory, in the early nineteenth century, treated matter as made of atoms and gave methods for calculating atomic weights. Later, in gold-foil experiments by Ernest Rutherford and his colleagues Hans Geiger and Ernest Marsden, a beam of positively charged alpha particles was fired at very thin gold foil. Most passed straight through, but a very few bounced almost straight back. The only way to explain this was a tiny, dense, positively charged nucleus inside mostly empty space. In 1932 James Chadwick reported in Nature the missing neutral particle in the nucleus: the neutron.

Common misconception: A common picture shows electrons circling the nucleus like planets around the Sun. That is a drawing convention, not what atoms are like. In the quantum-mechanical model an electron has no fixed path; the theory gives the probability of finding it in each region of space, which is why diagrams often show a fuzzy cloud.

Info: Atoms connect to almost everything else in chemistry: the proton count organises the periodic table, the outer electrons decide how atoms bond, and rearranging atoms is what a chemical reaction does.

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Sources and methodology

An atom consists of a tiny, dense central nucleus of positively charged protons and uncharged neutrons that holds nearly all of the atom's mass, surrounded by negatively charged electrons that occupy most of its volume. (awaiting scientific review)
- 1.3: Atoms, Elements and the Mole (Chemistry: Physical Principles), CC BY 4.0 — Textbook
An atom's diameter is on the order of 10⁻¹⁰ m, while the diameter of its nucleus is roughly 10⁻¹⁵ m, about 100,000 times smaller. (awaiting scientific review)
- 1.3: Atoms, Elements and the Mole (Chemistry: Physical Principles), CC BY 4.0 — Textbook
An atom with equal numbers of protons and electrons is electrically neutral; an ion with more protons than electrons is a positively charged cation, and one with more electrons than protons is a negatively charged anion. (awaiting scientific review)
- 1.3: Atoms, Elements and the Mole (Chemistry: Physical Principles), CC BY 4.0 — Textbook
The atomic number Z is the number of protons in an atom, the mass number A is the total number of protons and neutrons, and so the number of neutrons equals A − Z. (awaiting scientific review)
- 1.3: Atoms, Elements and the Mole (Chemistry: Physical Principles), CC BY 4.0 — Textbook
The elementary charge e has the exact SI value 1.602 176 634 × 10⁻¹⁹ coulombs (CODATA 2022). (awaiting scientific review)
- CODATA value: elementary charge e — Government or standards body
A proton is about 1836 times as massive as an electron: the CODATA 2022 proton–electron mass ratio is 1836.152 673 426. (awaiting scientific review)
- CODATA 2022 recommended value: proton-electron mass ratio — Government or standards body
In the gold-foil experiments of Ernest Rutherford and his colleagues Hans Geiger and Ernest Marsden, most alpha particles passed straight through the foil and only a very small number were deflected almost straight back, showing that an atom is mostly empty space with a small, dense, positively charged nucleus. (awaiting scientific review)
- 2.2: Evolution of Atomic Theory (Chemistry 2e, OpenStax, on LibreTexts), CC BY 4.0 — Textbook
In 1932 James Chadwick showed that the penetrating radiation produced when alpha particles strike beryllium consists of a neutral particle with about the same mass as a proton — the neutron — for which he received the 1935 Nobel Prize in Physics. (awaiting scientific review)
- The Nobel Prize in Physics 1935: James Chadwick — Facts — Other (unclassified)
The quantum-mechanical model replaces the Bohr picture of an electron as a particle moving in a fixed circular orbit: it describes the electron by a wavefunction that gives the probability of finding it in each part of the three-dimensional space around the nucleus. (awaiting scientific review)
- 6.4: Development of Quantum Theory (Chemistry 2e, OpenStax, on LibreTexts), CC BY 4.0 — Textbook
The shell with principal quantum number n contains n² orbitals, and each orbital holds at most two electrons, so a shell can hold at most 2n² electrons: 2 in the first shell, 8 in the second. (awaiting scientific review)
- 6.4: Development of Quantum Theory (Chemistry 2e, OpenStax, on LibreTexts), CC BY 4.0 — Textbook
In potassium (atomic number 19) the nineteenth electron is not added to the 3d subshell but to the 4s subshell, so potassium resembles lithium and sodium. (awaiting scientific review)
- 6.5: Electronic Structure of Atoms (Electron Configurations) (Chemistry 2e, OpenStax, on LibreTexts), CC BY 4.0 — Textbook
When an electron in an atom falls from a higher energy level to a lower one, the energy is released as electromagnetic radiation; the more protons in the nucleus, the more strongly the electrons are attracted and the more energy is given out. (awaiting scientific review)
- Development of the periodic table — Science media
James Chadwick's report 'Possible Existence of a Neutron' was published in Nature on 27 February 1932. (awaiting scientific review)
- Possible Existence of a Neutron — Peer-reviewed paper
John Dalton is best known for his pioneering theory of atomism, and he also developed methods to calculate atomic weights. (awaiting scientific review)
- John Dalton (Scientific Biographies) — Science media

Claims marked “awaiting scientific review” cite the sources listed but have not yet been signed off by a scientific reviewer.

Content status: published 1 October 2026.

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