Build an atom online

To build an atom, put protons and neutrons in the nucleus and electrons around it. The number of protons decides which element it is, protons plus neutrons give the mass number, and protons minus electrons give the charge. In our Atom Builder you add and take away each particle with a button, and it shows the element, the isotope, the charge and the electron shells as you go.

It opens on a carbon-12 atom: 6 protons, 6 neutrons and 6 electrons. Every element up to oganesson, element 118, can be built, and a periodic table lets you jump straight to one.

Try it

Atom Builder

Build atoms and ions from protons, neutrons and electrons

Open the simulatorGo to its page

How to use the Atom Builder

1. Use the + and − buttons under Protons, Neutrons and Electrons to add or take away one particle at a time.
2. Watch the name in This element change: for example "Carbon atom", or "Oxide ion, O²⁻" once you add extra electrons.
3. Read the facts above the picture: the mass number, the charge, whether the nucleus is stable, the electrons in each shell and the isotope name.
4. To jump to an element, press Open the periodic table. It loads a neutral atom of the element's usual isotope.
5. Under Stable isotopes, press one, such as C-13, to change the neutrons. Make it neutral sets the electrons equal to the protons.

The picture is a Bohr model, with the nucleus in the middle and the electrons in shells around it. Electrons orbit under Display starts or stops them moving. For practice, Practice and help asks you to count the particles from an isotope's notation and to name an element from its protons.

The three particles in an atom

Almost all of an atom's mass is in its nucleus, because protons and neutrons are each about 1,800 times heavier than an electron. In a neutral atom the electrons match the protons one for one, so the charges cancel out.

Atomic number, mass number and charge

Change the protons and the element changes. Change the neutrons and you get a different isotope of the same element. Change the electrons and you get an ion.

Example 1: count the particles from the notation

Fluorine-19 is written with the mass number 19 at the top left and the atomic number 9 at the bottom left of the symbol F. So it has:

• 9 protons, the atomic number.
• 10 neutrons, the mass number minus the atomic number: 19 − 9.
• 9 electrons, since the atom carries no charge.

To build it, open the periodic table, choose fluorine, which loads fluorine-19, its only stable isotope. The Atom Builder shows a mass number of 19, a charge of 0 and shells of 2 and 7. Now count the particles in the course question, then press Mark:

Building an ion

An ion is an atom that has gained or lost electrons, so it has a charge. Losing electrons gives a positive ion, or cation. Gaining electrons gives a negative ion, or anion.

Example 2: a sodium ion, Na⁺

Start with sodium: 11 protons, 12 neutrons and 11 electrons. Take away one electron. Now there are 11 protons and 10 electrons, so the charge is 11 − 10 = +1. The Atom Builder names it "Sodium cation, Na⁺" and the shells change from 2, 8, 1 to 2, 8: the ion has a full outer shell.

Example 3: an oxide ion, O²⁻

Start with oxygen-16: 8 protons, 8 neutrons and 8 electrons. Add two electrons. The charge is 8 − 10 = −2, and the Atom Builder names it "Oxide ion, O²⁻". The shells go from 2, 6 to 2, 8. Metals tend to lose electrons and non-metals tend to gain them, until the outer shell is full.

Building isotopes

Isotopes are versions of one element that differ only in their neutron count. They have the same atomic number but different mass numbers, and they behave the same way in chemical reactions because their electrons are the same.

Example 4: carbon-12 and carbon-14

Carbon-12 has 6 protons and 6 neutrons. Add two neutrons and you have carbon-14: still carbon, because it still has 6 protons, but with a mass number of 14. The Atom Builder marks carbon-12 as stable and carbon-14 as unstable, and says carbon-14 is likely to undergo beta minus decay because it has too many neutrons.

Stable means the nuclide is in the table of the stable nuclides. For an unstable one, the likely decay is a rule of thumb from how far it sits from the line of stability, and the Atom Builder says so. Heavy atoms such as uranium-238 have no stable isotopes at all. Try spotting an isotope:

Electron shells in the Bohr model

Electrons fill shells from the inside out. For the first 20 elements, the first shell holds up to 2 electrons, and the second and third hold up to 8 each. So chlorine, with 17 electrons, has shells 2, 8, 7, and a chloride ion, with 18, has 2, 8, 8. The Atom Builder shows the shell numbers for every element, including heavy ones where the pattern gets more complicated. For the full subshell configuration, such as 1s² 2s² 2p⁶ 3s² 3p⁵, use the Electron Configuration Calculator.

Common mistakes

• Thinking neutrons decide the element. Only the number of protons does. Adding a neutron makes a different isotope, not a different element.
• Adding electrons to change the mass number. Electrons have almost no mass, so they are not counted in the mass number.
• Getting the ion charge backwards. Losing electrons makes the charge positive; gaining them makes it negative.
• Subtracting the wrong way for neutrons. Neutrons = mass number − atomic number, never the other way round.
• Mixing up mass number and relative atomic mass. The mass number is a whole number for one isotope. The relative atomic mass, such as 35.45 for chlorine, is an average over all its isotopes.

Practise atomic structure in a short timed paper

Atomic structure comes up in High School Chemistry and Grade 8 Physical Science: Chemistry practice, and in GCSE Chemistry Higher in the UK. Each daily paper is a short timed paper of 8 questions worth 25 to 35 marks, and the papers mix topics. The Atomic Structure course builds up from the gold foil experiment to electron shells and ions.

Read next

Chemistry · Moles and quantitiesAtom economy formula: how to calculate it, with examplesThe atom economy formula, how to calculate percentage atom economy from a balanced equation, worked examples, atom economy versus yield, and practice.