What Are Atoms Made Of? A Complete Guide to Atomic Structure

What Are Atoms Made Of?

Atoms are made of three primary subatomic particles: protons (positively charged), neutrons (neutral), and electrons (negatively charged). The protons and neutrons cluster together in a dense central nucleus, whilst electrons orbit in energy levels around the nucleus. These particles are themselves composed of even smaller fundamental particles called quarks and leptons, according to the Standard Model of particle physics.

Particle	Charge	Mass (kg)	Location	Role
Proton	+1	1.673 × 10⁻²⁷	Nucleus	Defines element identity
Neutron	0 (neutral)	1.675 × 10⁻²⁷	Nucleus	Stabilises nucleus
Electron	−1	9.109 × 10⁻³¹	Electron shells	Determines chemical behaviour

💡  Simple analogy: Think of the atom like our solar system. The nucleus (protons + neutrons) is the Sun at the centre. Electrons are the planets orbiting around it, except electrons don’t follow fixed paths. They exist as probability clouds.

The Three Main Subatomic Particles Inside an Atom

Atoms consist of three primary types of subatomic particles, each with distinct properties and roles. Understanding these building blocks is essential for grasping all of chemistry and much of physics. Every single substance you have ever encountered, from water to steel to the air you breathe, exists because of how these three particles are arranged.

Protons: The Positively Charged Particles

Protons are positively charged particles that reside in the atom’s nucleus. Each proton carries a charge of +1 and has a mass of approximately 1.673 × 10⁻²⁷ kilograms. The number of protons in an atom determines what element it is. This is the atom’s fundamental identity card, known as the atomic number.

All carbon atoms have exactly six protons. All oxygen atoms have eight. All gold atoms have 79. Change the number of protons even by one, and you create an entirely different element with completely different properties. This is exactly what happens in nuclear reactions: when uranium undergoes fission, it splits into lighter elements because the proton count in each resulting nucleus differs from the original uranium atom.

• Charge: +1 (positive)
• Mass: 1.673 × 10⁻²⁷ kg
• Location: Inside the nucleus
• Discovered by: Ernest Rutherford (1919)
• Key role: Defines the element. Change proton count = change element entirely
• In chemical reactions, the proton count never changes; only electrons move

Neutrons: The Neutral Particles

Neutrons are electrically neutral particles that also inhabit the nucleus alongside protons. They have slightly more mass than protons (approximately 1.675 × 10⁻²⁷ kilograms) but carry no electrical charge. Neutrons act as nuclear ‘glue’, they help stabilise the nucleus by offsetting the electromagnetic repulsion between positively charged protons.

Without neutrons, all nuclei except hydrogen would fly apart because of the mutual repulsion between their protons. According to CERN research, the strong nuclear force, which acts on both protons and neutrons, is approximately 100 times stronger than the electromagnetic force at nuclear distances.

The number of neutrons can vary within atoms of the same element, creating different isotopes with different masses but identical chemical properties. Carbon-12 has six neutrons, carbon-13 has seven, and carbon-14 has eight, yet all three behave chemically as carbon because they all have six protons.

• Charge: 0 (neutral)
• Mass: 1.675 × 10⁻²⁷ kg (slightly more than a proton)
• Location: Inside the nucleus alongside protons
• Discovered by: James Chadwick (1932)
• Key role: Stabilises the nucleus; creates isotopes when the count varies
• In chemical reactions, the neutron count never changes; only electrons move

Electrons: The Negatively Charged Particles

Electrons are negatively charged particles that orbit the nucleus in regions called electron shells or energy levels. Electrons are remarkably light; their mass is only 9.109 × 10⁻³¹ kilograms, roughly 1/1836th the mass of a proton. Each electron carries a charge of −1, which perfectly balances the positive charge of a proton in a neutral atom.

Despite their tiny mass, electrons determine an atom’s entire chemical behaviour. They are the particles that interact when atoms bond together to form molecules. Electron behaviour explains why sodium violently reacts with water, whilst gold sits completely inert. It explains why copper conducts electricity and rubber does not.

In a neutral atom, the number of protons always equals the number of electrons, creating overall electrical balance. However, atoms can gain or lose electrons to become ions, which carry a net positive or negative charge. This ability to form ions is crucial for chemical reactions, electrical conductivity, and biological processes such as the nerve impulses that carry signals through your body.

Importantly, electrons do not orbit the nucleus like planets around the Sun, a common misconception from early atomic models. According to quantum mechanics, electrons exist in probability clouds described by wave functions. We can never know both an electron’s exact position and momentum simultaneously, a fundamental limit expressed in Heisenberg’s Uncertainty Principle: Δx · Δp ≥ ℏ/2.

• Charge: −1 (negative)
• Mass: 9.109 × 10⁻³¹ kg (approximately 1/1836 of a proton)
• Location: Electron shells/orbitals around the nucleus
• Discovered by: J.J. Thomson (1897)
• Key role: Controls ALL chemical bonding and chemical behaviour
• In chemical reactions, Electrons are gained, lost, or shared. This is chemistry

Comparison Table: Protons vs Neutrons vs Electrons

A vast space between the nucleus and shells	Proton	Neutron	Electron	Why It Matters
Charge	+1	0	−1	Opposite charges attract holds atom together
Mass (kg)	1.673×10⁻²⁷	1.675×10⁻²⁷	9.109×10⁻³¹	Nucleus holds 99.9% of atom’s mass
Location	Nucleus	Nucleus	Shells/orbitals	Vast empty space between nucleus and shells
Discovered	Rutherford 1919	Chadwick 1932	Thomson 1897	Electron found first atoms are NOT indivisible
Made of	Quarks (uud)	Quarks (udd)	Fundamental (lepton)	Quarks are the deepest layer we know
Can change?	Only nuclear rxn	Only nuclear rxn	Yes, chemical rxn	Electrons are the basis of all chemistry

To understand how atoms combine to form molecules, read our guide on the difference between atoms and molecules

The Atomic Nucleus: Structure and Composition

The nucleus is the dense, compact centre of an atom. It occupies an incredibly small space whilst containing more than 99.9% of the atom’s total mass. If you could somehow compress all of humanity’s mass into nuclear density, the entire human race would fit inside a sugar cube, a thought experiment that perfectly illustrates just how extraordinarily dense the nucleus is.

The nucleus contains protons and neutrons, collectively called nucleons. The number of protons (atomic number, Z) defines which element the atom is. The total number of protons plus neutrons gives the mass number (A). For example, the most common carbon isotope has 6 protons and 6 neutrons mass number of 12, written as carbon-12 or ¹²C.

What Holds the Nucleus Together? (Strong Nuclear Force)

This is one of the most fascinating questions in all of physics. Protons are all positively charged, and like charges repel. So why don’t all nuclei simply explode from the electromagnetic repulsion between their protons?

The answer is the strong nuclear force, one of the four fundamental forces of nature alongside gravity, electromagnetism, and the weak nuclear force. The strong force is the most powerful force in the universe. Within its range, it is approximately 100 times stronger than the electromagnetic force, completely overwhelming proton repulsion and binding nucleons together.

However, the strong force only operates over extremely short distances about one femtometre (10⁻¹⁵ metres), roughly the diameter of a proton. Outside this range, it drops off sharply. Research published in Physical Review Letters has shown that the strong force has a fascinating property: it increases with distance up to about one femtometre, much like a rubber band that becomes harder to stretch. This is why free quarks are never observed in nature.

Force	Carrier	Relative Strength	Range	Acts On
Strong Nuclear	Gluons	1 (strongest)	10⁻¹⁵ m	Quarks, nucleons
Electromagnetic	Photons	10⁻²	Infinite	Charged particles
Weak Nuclear	W & Z bosons	10⁻⁶	10⁻¹⁸ m	All particles
Gravity	Gravitons (hypothetical)	10⁻³⁹ (weakest)	Infinite	All particles with mass

Nuclear stability depends on the balance between the strong nuclear force (attracting all nucleons) and the electromagnetic force (pushing protons apart). Neutrons play a critical role; they add an attractive, strong force without adding electromagnetic repulsion. This is why heavier elements need proportionally more neutrons than protons to remain stable.

The ‘magic numbers’ of nuclear physics 2, 8, 20, 28, 50, 82, and 126 represent particularly stable configurations of nucleons. Nuclei with these magic numbers of protons or neutrons are exceptionally stable, analogous to the way complete electron shells create stable noble gases.

📌  Elements heavier than uranium (92 protons) have no stable isotopes at all. Scientists have created elements as heavy as oganesson (118 protons), but these superheavy elements exist for only fractions of a second before decaying.

What Is the Nuclear Radius?

The nucleus is extraordinarily small, even relative to the atom itself. Here is a perspective that makes this real:

• Average atom diameter: 0.1 to 0.5 nanometres (1 nm = one billionth of a metre)
• Nuclear diameter: approximately 10⁻⁵ nm, which is 100,000 times smaller than an atom
• A single human hair is about one million atoms wide
• If you lined up atoms side by side, you would need approximately five trillion to stretch across the full stop at the end of this sentence

💡  Football Stadium Analogy: If you magnified an atom to the size of a football stadium, its nucleus would be roughly the size of a marble placed at the centre of the pitch. The electrons would be near the outer walls. Everything in between 99.9999% of the atom is completely empty space.

Yet despite atoms being almost entirely empty space, matter feels solid. Why? The electromagnetic force between the electron clouds of neighbouring atoms creates a repulsive barrier that prevents them from passing through each other. The solidity you feel when you touch a table is not atoms touching it, but electromagnetic forces between electron clouds.

Isotopes — Same Element, Different Mass:

Atoms of the same element can have different numbers of neutrons, creating isotopes. They have identical chemical properties (same protons, same electrons) but different masses.

Isotope	Protons	Neutrons	Mass Number	Stability	Common Use
Carbon-12	6	6	12	Stable	Definition of atomic mass unit
Carbon-13	6	7	13	Stable	NMR spectroscopy, research
Carbon-14	6	8	14	Radioactive	Radiocarbon dating (5,730 yr half-life)
Uranium-235	92	143	235	Radioactive	Nuclear fuel, weapons
Uranium-238	92	146	238	Radioactive	Nuclear fuel, depleted uranium

Electron Shells and Energy Levels

Contrary to what many textbook diagrams show, electrons do not orbit the nucleus in neat circular paths like planets around the Sun. This planetary model was proposed by Niels Bohr in 1913, and whilst it was a huge step forward at the time, modern quantum mechanics has replaced it with a far more accurate and far more fascinating picture.

In reality, electrons exist in a ‘cloud’ of probability regions around the nucleus where they are most likely to be found at any given moment. These regions are called electron shells or energy levels. The shells are designated by numbers (1, 2, 3…) or letters (K, L, M…), with the first shell closest to the nucleus having the lowest energy.

What Are Electron Orbitals?

Each main energy level (shell) contains one or more subshells, designated by the letters s, p, d, and f. These letters come from old spectroscopy terminology: sharp, principal, diffuse, and fundamental. Each subshell contains orbitals, specific three-dimensional regions where an electron is most likely to be found.

Each type of orbital has a characteristic shape:

• s orbitals: Spherical one orientation per subshell
• p orbitals: Dumbbell-shaped, three orientations (px, py, pz)
• d orbitals: Complex cloverleaf shapes, five orientations
• f orbitals: Even more complex shapes, seven orientations

Each orbital can hold a maximum of two electrons (with opposite spins), following the Pauli Exclusion Principle no two electrons in the same atom can have identical quantum states.

Shell (n)	Letter	Subshells	Max Electrons	Elements
1	K	1s	2	H, He
2	L	2s, 2p	8	Li through Ne
3	M	3s, 3p, 3d	18	Na through Ar
4	N	4s, 4p, 4d, 4f	32	K through Kr

Rules for Electron Filling:

• Aufbau Principle: Electrons fill orbitals starting with the lowest energy level first (innermost shells fill before outer shells)
• Hund’s Rule: Electrons occupy empty orbitals singly before pairing up like passengers preferring an empty seat on a bus before sitting next to someone
• Pauli Exclusion Principle: No two electrons in the same atom can have identical quantum states. Each orbital holds a maximum of two electrons with opposite spins

💡  Hotel Room Analogy: Think of orbitals as hotel rooms. Electrons prefer their own room before sharing. When they do share a room, they must sleep at opposite ends. This is what ‘opposite spin’ means in practice.

What Are Valence Electrons and Why Do They Matter?

The electrons in the outermost shell are called valence electrons, and they are the most important electrons in all of chemistry. Valence electrons determine how atoms interact with each other, form chemical bonds, and participate in reactions. Everything from the rusting of iron to the folding of proteins comes down to valence electrons.

Elements in the same column (group) of the periodic table have the same number of valence electrons, which is why they share similar chemical properties:

• Group 1 (Li, Na, K…): 1 valence electron → highly reactive metals that readily lose that electron to form +1 ions
• Group 2 (Be, Mg, Ca…): 2 valence electrons → reactive metals that form +2 ions
• Group 17 (F, Cl, Br…): 7 valence electrons → highly reactive non-metals that readily gain one electron to form −1 ions
• Group 18 (He, Ne, Ar…): Complete outer shells → chemically inert noble gases that rarely form compounds

Electron Configuration:- the complete arrangement of electrons in an atom. For example:

Hydrogen:  1s¹             (1 electron, 1 shell)

Oxygen:    1s² 2s² 2p⁴    (8 electrons, 2 shells, 6 valence e⁻)

Sodium:    1s² 2s² 2p⁶ 3s¹ (11 electrons, 3 shells, 1 valence e⁻)

Oxygen’s electron configuration explains why it forms exactly two bonds in water (H₂O). It has six valence electrons and needs two more to complete its outer shell. This drive to complete the outer shell is the engine behind virtually all of chemistry.

Electrons also exhibit wave-particle duality; they behave as both particles and waves simultaneously. We can never know both an electron’s exact position and momentum at the same time, a fundamental limit of nature, not just of our instruments, known as Heisenberg’s Uncertainty Principle.

‘To understand exactly how valence electrons create chemical bonds, read our complete guide: How Are Atoms Held Together?’

What Are Subatomic Particles Made Of? (Quarks and Leptons)

Protons, neutrons, and electrons are the building blocks of atoms, but they are not the end of the story. Modern particle physics has revealed that protons and neutrons themselves are composite particles, made of even smaller fundamental particles. This is where atomic structure becomes truly extraordinary.

What Are Quarks?

Protons and neutrons are each composed of three fundamental particles called quarks, held together by particles called gluons through the strong nuclear force. There are six types (‘flavours’) of quarks: up, down, charm, strange, top, and bottom, but only two types appear in ordinary matter:

Proton  =  2 up quarks  +  1 down quark   (uud)   Charge: 2(+²⁄₃) + 1(−¹⁄₃) = +1 ✓

Neutron =  1 up quark   +  2 down quarks  (udd)   Charge: 1(+²⁄₃) + 2(−¹⁄₃) = 0  ✓

This slight difference in quark composition, two up quarks vs two down quarks, accounts for the mass difference between protons and neutrons, and explains why protons are charged whilst neutrons are neutral.

Quarks are never observed in isolation due to a property called colour confinement. The strong force between quarks actually increases with distance, like a rubber band that becomes harder to stretch. If you tried to pull quarks apart, you would create new quark-antiquark pairs rather than separating the originals, a bizarre quantum effect confirmed by decades of experiments at CERN and other particle physics facilities.

Quark Type	Charge	Approx. Mass	Found In
Up	+²⁄₃	~2.3 MeV/c²	Protons and neutrons (ordinary matter)
Down	−¹⁄₃	~4.8 MeV/c²	Protons and neutrons (ordinary matter)
Charm	+²⁄₃	~1.3 GeV/c²	Exotic/unstable particles only
Strange	−¹⁄₃	~95 MeV/c²	Exotic/unstable particles only
Top	+²⁄₃	~173 GeV/c²	High-energy particle collisions only
Bottom	−¹⁄₃	~4.2 GeV/c²	High-energy particle collisions only

The Standard Model of Particle Physics (Simplified)

The Standard Model is our current best theory of fundamental particles and the forces between them. It describes 17 fundamental particles, developed throughout the 20th century and validated by countless experiments. The discovery of the Higgs boson at CERN in 2012 was the final confirmation of the Standard Model, completing a prediction made nearly 50 years earlier.

• 6 quarks: up, down, charm, strange, top, bottom
• 6 leptons: electron, muon, tau, plus three types of neutrino
• 4 force-carrying particles (bosons): photon (electromagnetic), gluon (strong force), W & Z bosons (weak force)
• 1 Higgs boson: gives other particles their mass

Leptons — Electrons belong to a family of fundamental particles called leptons. Unlike quarks, leptons are truly elementary; they have no internal structure and do not participate in the strong nuclear force. The electron is the lightest and most stable lepton. It has heavier cousins, the muon and the tau, both of which are unstable and decay rapidly.

Neutrinos — Each charged lepton has an associated neutrino. Neutrinos are almost massless and interact extremely weakly with matter. Trillions of solar neutrinos pass through your body every second without interacting with a single atom.

Antimatter — Every particle has an antimatter counterpart with opposite charge. The antiparticle of an electron is a positron. When matter and antimatter meet, they annihilate in a burst of pure energy (E = mc²). Antimatter is used in medical PET scans. Positrons from radioactive decay annihilate with electrons in your body to produce the gamma rays that create the image.

How Atomic Structure Determines Element Identity

Every atom of every element is defined by a precise combination of its three types of particles. Change any one of these, and you change the atom fundamentally:

• Change the proton count → you change the element entirely (requires a nuclear reaction, not possible in ordinary chemistry)
• Change the neutron count → you create an isotope of the same element (same chemical behaviour, different mass)
• Change the electron count → you create an ion of the same element (different charge and reactivity, but same element)

This is why the atomic number, the number of protons, is the single most important number in chemistry. It is an atom’s unique identity. Two atoms with the same atomic number are always the same element, regardless of their mass, charge, or environment.

What Changes	What Happens	Type of Change	Example
Proton count	New element created	Nuclear reaction	Uranium → Barium + Krypton (fission)
Neutron count	New isotope formed	Nuclear reaction	Carbon-12 → Carbon-14 (different mass)
Electron count	Ion formed	Chemical reaction	Na → Na⁺ (loses 1 electron)

This distinction has profound practical consequences. In chemistry, only electrons change, and protons and neutrons stay fixed. All of organic chemistry, all of biochemistry, all of materials science is built on electrons being rearranged between atoms. Nuclear reactions, by contrast, change proton counts and release millions of times more energy per event than any chemical reaction.

The periodic table is essentially a map of atomic structure. Each element’s position reflects its proton count, and the patterns in the table reflect how electron shells fill up across the elements. Atoms in the same column have the same valence electron configuration, which is why they behave so similarly in chemical reactions.

Atoms and Empty Space: How Solid Is Matter Really?

One of the most counterintuitive facts in all of science is this: solid matter is almost space.

An atom is approximately 99.9999% space. The nucleus, which contains over 99.9% of the atom’s mass, occupies only about one hundred-thousandth (10⁻⁵) of the atom’s total volume. If you could remove all the space from every atom in the human body, the remaining matter would fit inside a particle smaller than a grain of sand.

💡  The Football Stadium Analogy: Imagine enlarging a single atom until it fills a football stadium. The nucleus, containing almost all of the atom’s mass, would be a marble sitting at the centre of the pitch. The electron cloud would be near the outer walls of the stadium. Everything in between the vast terraces, the pitch, and the stands would be empty.

If matter is mostly space, why does a table feel solid? Why can you not push your hand straight through a wall? The answer is electromagnetism.

When two atoms approach each other, their electron clouds, both negatively charged, repel each other through the electromagnetic force. This repulsion is what prevents atoms from passing through each other. The solidity you feel is not atoms physically touching it is the electromagnetic force between electron clouds creating a repulsive barrier that your hand cannot overcome under normal conditions.

This also explains why nuclear density is so extraordinary. In a neutron star, the collapsed remnant of a massive exploded star, the electron clouds are stripped away, and the nuclei are packed directly together. A teaspoon of neutron star material would have a mass of approximately one billion tonnes.

Scale Visualisation (largest to smallest):

Human hair:  ~80,000 nm Cell:        ~10,000 nm Virus:       ~100 nm Protein:     ~10 nm Water mol.:  ~0.3 nm Atom:        ~0.1–0.5 nm Nucleus:     ~0.00001 nm Quarks:      < 0.0000001 nm  (current limit of detection)

People Also Ask

What are the 3 parts of an atom?

The three main parts of an atom are protons, neutrons, and electrons. Protons (charge +1) and neutrons (charge 0) are located in the dense central nucleus and together make up over 99.9% of the atom’s mass. Electrons (charge 1) orbit the nucleus in energy levels called electron shells. In a neutral atom, the number of protons always equals the number of electrons, creating overall electrical balance.

Are protons and neutrons made of quarks?

Yes. Protons and neutrons are each made of three quarks held together by gluons through the strong nuclear force. A proton consists of two ‘up’ quarks and one ‘down’ quark (giving a net charge of +1). A neutron consists of one ‘up’ quark and two ‘down’ quarks (giving a net charge of 0). Electrons, however, are not made of quarks; they are fundamental leptons with no internal structure that we have detected.

What is the smallest particle inside an atom?

As far as current physics can determine, quarks and electrons (leptons) are the smallest particles inside or making up an atom. Quarks make up protons and neutrons. Electrons are fundamental; they appear to have no internal structure. However, quarks are never found in isolation due to colour confinement; they are always bound inside larger particles. The Standard Model describes 17 fundamental particles in total. Whether quarks and leptons themselves have internal structure remains an open question in physics.

How many electrons can an atom have?

Theoretically, there is no strict upper limit, but in practice, the heaviest confirmed element, oganesson (element 118) has 118 electrons. The maximum number of electrons per shell follows the formula 2n², where n is the shell number: shell 1 holds up to 2, shell 2 up to 8, shell 3 up to 18, and shell 4 up to 32. However, the actual arrangement of electrons in real atoms is more complex due to quantum mechanical rules about subshells and orbital filling.

What is an atom made of besides protons, neutrons, and electrons?

At the next level down, protons and neutrons are each made of three quarks (up and down quarks specifically) held together by gluons. Electrons are fundamental leptons; they are not made of anything smaller than we currently know of. At the deepest level described by the Standard Model, everything is made of 6 quarks, 6 leptons, 4 force-carrying bosons, and the Higgs boson, 17 fundamental particles in total.

What holds an atom together?

Three different forces hold different parts of an atom together. The strong nuclear force (carried by gluons) holds quarks inside protons and neutrons, and holds the nucleus itself together. The electromagnetic force holds electrons in orbit around the nucleus, due to the attraction between the negatively charged electrons and the positively charged nucleus. Without these two forces, atoms could not exist. For a complete explanation of how atoms are held together and how this relates to chemical bonding, see our dedicated article.

Can atoms be destroyed?

In ordinary chemistry, no. The Law of Conservation of Mass states that atoms cannot be created or destroyed in chemical reactions. They can only be rearranged. However, in nuclear reactions, the nucleus itself can be split (fission) or merged with another nucleus (fusion), which technically changes or ‘destroys’ the original atom and creates new elements. The total number of protons and neutrons (nucleons) is still conserved overall. Even in nuclear reactions, matter is not truly destroyed; a tiny fraction is converted into energy according to Einstein’s equation E = mc².

Frequently Asked Questions (FAQ)

Conclusion: What Are Atoms Made Of? Key Takeaways

Atoms are far more than the simple spheres depicted in early textbooks. They are complex, dynamic structures whose properties emerge from the interplay of three fundamental particles, and those particles are themselves built from even more fundamental constituents.

Key Concept	What You Need to Remember
Protons	Give the atom its identity (atomic number). Never change in chemistry.
Neutrons	Stabilise the nucleus. Varying neutron counts create isotopes.
Electrons	Control all chemical behaviour. Gain/lose/share electrons = chemistry.
Nucleus	>99.9% of mass, <0.001% of volume. Held together by the strong force.
Electron shells	Probability clouds, not fixed orbits. Valence electrons drive bonding.
Quarks	Protons = uud. Neutrons = udd. Never found in isolation.
Empty space	Atoms are 99.9999% empty. Solidity comes from electromagnetic repulsion.

From the hydrogen in the stars to the carbon in your DNA, from the silicon in your phone’s processor to the iron in your blood everything is built from these extraordinarily tiny, extraordinarily complex structures. Understanding atomic structure is not just chemistry it is understanding the foundation of everything that exists.

📌  Related Articles to Show at Bottom of Page: What Are Molecules Made Of? | Difference Between Atoms and Molecules | How Are Atoms Held Together? | Atomic Mass vs Atomic Number | John Dalton’s Atomic Theory | Subatomic Particles Explained