An atom is made of a nucleus and an electron shell: the tiny nucleus holds protons and neutrons, while electrons move around it. Knowing just two numbers from the periodic table, you can "build" any atom: count all its particles and place electrons on their levels. This page covers a visual atom model, the Z and A formulas, isotopes, worked examples, and an interactive quiz for practice.
What Is an Atom Made Of
An atom is the smallest particle of a chemical element that keeps its properties. It's simpler than it looks — just two "parts":
- The nucleus is the center of the atom. It holds two kinds of particles: positive protons and neutral neutrons. The nucleus is unimaginably tiny — about 100,000 times smaller than the whole atom — yet it holds more than 99.9% of the atom's mass.
- The electron shell is the rest of the atom's space, where light, negative electrons move around the nucleus.
If you scaled an atom up to the size of a stadium, the nucleus would be a pea at the center of the field — everything else is the "almost empty" electron shell.
Protons, Neutrons, and Electrons: The Atom's Three Particles
Every atom in the universe is built from just three kinds of particles — only their numbers differ. It's easiest to compare them by three traits: charge, mass, and "where they live."
A proton and a neutron have almost the same mass — about 1 amu (atomic mass unit) each. An electron is about 1,836 times lighter, so its mass is ignored in calculations. But the charges of a proton and an electron are equal in size and opposite in sign: as long as an atom has as many protons as electrons, it's electrically neutral.
Atomic Formulas: Z, A, and the Number of Neutrons
The composition of any atom is described by two numbers:
- Atomic number Z — the number of protons in the nucleus. This is the element's position in the periodic table: #1 is hydrogen (1 proton), #8 is oxygen (8 protons), #79 is gold (79 protons). That's why in the planetary model of the atom, the number of protons in the nucleus equals the number of electrons: the atom is neutral, with exactly one electron per proton.
- Mass number A — the total number of particles in the nucleus: \(A = Z + N\), where N is the number of neutrons. This gives the main working formula for school problems: \(N = A - Z\).
An atom's composition is written as \(^{A}_{Z}\text{X}\). For example, \(^{35}_{17}\text{Cl}\) is chlorine, with 17 protons and 35 − 17 = 18 neutrons. The cheat sheet below sums up the whole method in three lines.
The Electron Shell: Levels of 2, 8, 18
Electrons don't fly around the nucleus randomly: they sit on energy levels (shells). The farther a level is from the nucleus, the more electrons it can hold — the maximum is given by \(2n^2\):
- Level 1 — no more than 2 electrons;
- Level 2 — no more than 8;
- Level 3 — no more than 18;
- Level 4 — no more than 32.
Levels fill in order, from the innermost outward. For example, oxygen (Z = 8) has electrons arranged as 2, 6; sodium (Z = 11) as 2, 8, 1; chlorine (Z = 17) as 2, 8, 7. Electrons on the outer level are called valence electrons — they take part in chemical reactions and determine the element's properties.
The number of levels is also shown by the periodic table: it equals the period number the element is in. In higher grades, levels are further split into s, p, d, f sublevels, written as electron configurations — for example, oxygen is 1s²2s²2p⁴.
Isotopes: One Element, Different Atoms
An element's number of protons can't change: add a proton to the nucleus and you get a different element. But the number of neutrons can vary. Isotopes are atoms of the same element with the same Z but a different number of neutrons — and therefore a different mass number A.
The classic example is hydrogen and its three isotopes: protium \(^{1}_{1}\text{H}\) (no neutrons at all), deuterium \(^{2}_{1}\text{H}\) (1 neutron), and tritium \(^{3}_{1}\text{H}\) (2 neutrons). Chemically, isotopes are almost indistinguishable: electrons play the main role in reactions, and isotopes have exactly the same number of them.
It's because of isotopes that atomic masses in the periodic table are fractional: each is a weighted average over all the element's natural isotopes. Chlorine, for instance, is 35.45 because natural chlorine is a mix of chlorine-35 and chlorine-37.
Models of the Atom: From "Indivisible Particle" to the Planetary Model
The word "atom" comes from the Greek atomos — "indivisible": that's how Democritus pictured it, and in the early 19th century John Dalton built the first scientific theory on the idea of an indivisible atom. But discoveries at the turn of the 19th–20th centuries showed that the atom does have internal structure.
- 1897 — Thomson's model (the "plum pudding" model). J. J. Thomson discovered the electron and suggested that electrons are embedded in a positively charged "medium," like plums in a pudding.
- 1911 — Rutherford's planetary model. By firing alpha particles at gold foil, Ernest Rutherford found that almost all the mass and all the positive charge are packed into a tiny nucleus, with electrons orbiting it like planets around the Sun.
- 1913 — Bohr's model. Niels Bohr refined this: electrons move not on any path, but on strictly defined orbits with fixed energy — this is how energy levels came about.
- 1920s — the quantum-mechanical model. Modern physics describes the electron not as a ball on an orbit, but as an electron cloud — a region of space where the electron can be found with a certain probability.
The planetary model with levels (as in the diagram above) is still used in school chemistry today: its accuracy is quite enough for counting particles and arranging electrons.
Worked Examples: Breaking Down an Atom Using the Periodic Table
Example 1. Full breakdown of a chlorine atom
Problem. Find the composition of a chlorine atom and arrange its electrons on levels. In the periodic table: chlorine Cl is element #17, atomic mass 35.45.
Step 1. Atomic number Z = 17 → the nucleus has 17 protons.
Step 2. The atom is neutral → the same number of electrons: 17 electrons.
Step 3. The mass number is the rounded atomic mass: A = 35. Neutrons: \(N = A - Z = 35 - 17 = 18\).
Step 4. Arrange 17 electrons on levels from innermost to outermost: 2, then 8, leaving 7 → 2, 8, 7. The outer level has 7 valence electrons — they determine chlorine's chemical properties.
Answer: 17 p⁺, 18 n⁰, 17 e⁻; electron levels: 2, 8, 7.
Example 2. How many neutrons does uranium-235 have
Problem. How many neutrons are in the nucleus of the uranium isotope \(^{235}_{92}\text{U}\)?
Step 1. Read the notation: the bottom number is the atomic number, Z = 92 (protons); the top number is the mass number, A = 235.
Step 2. Calculate: \(N = A - Z = 235 - 92 = 143\).
Answer: 143 neutrons. Note that natural uranium also has the isotope \(^{238}_{92}\text{U}\) — the same 92 protons, but 146 neutrons.
Example 3. Identify the element from its electrons
Problem. An atom's electrons are arranged on levels as 2, 8, 1. What element is this?
Step 1. Count the electrons: 2 + 8 + 1 = 11.
Step 2. In a neutral atom, the number of electrons equals the number of protons → Z = 11.
Step 3. Check the periodic table: element #11 is sodium (Na).
Answer: sodium. This also shows why it's so reactive: it has just one electron on its outer level, and sodium gives it up easily.
Common Mistakes in Atomic Structure Problems
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Confusing atomic mass with mass number: plugging the fractional mass from the table (e.g., 35.45 for chlorine) into N = A − Z.
The mass number A is always a whole number: it's the count of particles (protons and neutrons) in a specific atom. For problems, round the table's atomic mass to a whole number: for chlorine, A = 35, so N = 35 − 17 = 18.
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Assuming the element's atomic number is the total number of all particles in the atom.
The atomic number Z is only the number of protons (and electrons in a neutral atom). Neutrons aren't part of Z: they're found separately, using N = A − Z.
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Placing electrons inside the nucleus: "an atom consists of protons, neutrons, and electrons, all located in the nucleus."
Only protons and neutrons are in the nucleus. Electrons move around the nucleus in the electron shell, at distances tens of thousands of times larger than the nucleus itself.
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Counting electrons for an ion using the element's atomic number: saying Na⁺ has "11 electrons."
Z gives the number of electrons only for a neutral atom. An ion is charged: Na⁺ has lost one electron, leaving 10; Cl⁻ has gained one, giving it 18. The number of protons doesn't change.
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Filling levels equally or randomly: for chlorine, writing something like 8, 8, 1.
Levels fill strictly from the nucleus outward: first level 1 (max 2 electrons), then level 2 (8), then level 3. For chlorine's 17 electrons, the correct arrangement is 2, 8, 7.
Questions and Answers
What is an atom made of? In short
An atom is made of a nucleus and an electron shell. The nucleus is made of protons (charge +1) and neutrons (no charge); the shell is made of electrons (charge −1). The number of protons equals the element's atomic number in the periodic table.
Why is an atom called an indivisible particle if it can be divided?
The name comes from the ancient Greek atomos — "uncuttable": until the late 19th century, the atom was considered the simplest "building block" of matter. Then the electron and the nucleus were discovered. But in chemical reactions the atom truly isn't split apart — only the electron shells rearrange — so for chemistry it's still the smallest particle of an element.
Is it true that an atom consists of protons and neutrons?
Only half true: protons and neutrons make up the nucleus. The full composition of an atom is the nucleus (protons + neutrons) plus the electrons, which move around the nucleus and form the electron shell.
Why does the planetary model of the atom have the same number of protons in the nucleus as electrons?
Because an atom as a whole is electrically neutral: the nucleus's positive charge (+Z from the protons) must be fully balanced by the electrons' negative charge (−Z). If an atom loses or gains an electron, this balance is broken and you get a charged particle — an ion.
How do I find the number of protons, neutrons, and electrons from the periodic table?
Protons equal the element's atomic number Z. A neutral atom has the same number of electrons. For neutrons, round the atomic mass to a whole number (this is the mass number A) and subtract the atomic number: N = A − Z. For example, iron (#26, mass 56) has 26 protons, 26 electrons, and 56 − 26 = 30 neutrons.
Why is the atomic mass in the periodic table a fraction?
Because it's an average across all the element's natural isotopes, weighted by how common each one is. Any single atom has a whole-number mass number (35 or 37 for chlorine), but the average across the mix of isotopes comes out fractional — 35.45.
