MrChemCoach

Structure of the Atom · Build an Atom

IGCSE 0620 · CBSE Class 9-11 · 3D lab
All labs
Nucleus
Electron shells
Build mode Drag from the trays, or tap + and −
Protons 0
Neutrons 0
Electrons 0
The atom you have built Empty
- ?
Nothing yet
Add a proton and the atom becomes an element.

How a chemist writes it

-- ?
Top: mass number, protons + neutrons Bottom: proton number, which element it is Right: net charge, protons − electrons Middle: the element symbol
Inside the atomRead it off

1 · What you have put in

ProtonsIN THE NUCLEUS · CHARGE +1 EACH 0-
NeutronsIN THE NUCLEUS · NO CHARGE 0-
ElectronsIN SHELLS · CHARGE −1 EACH 0-
Mass number  A = p + n0
Net charge  p − e0

2 · Electron arrangement

-

Shells fill from the inside out: two in the first, then eight, then eight. The number in the outer shell is the group number, and the number of shells in use is the period.

3 · Show

4 · Go further

5 · The numbers behind it

Build an atom - game 0 / 10 Question 1 of 5

10 / 10
Isotopes -

An isotope is an atom of the same element with a different number of neutrons. The proton number does not change, so the chemistry does not change either - only the mass. That is why relative atomic mass is usually not a whole number: it is the average over an element’s isotopes, weighted by how common each one is.

What this model tells you
Protons decide the element Change the proton number and you have a different element altogether. Nothing else on this page changes which element it is - not the neutrons, not the electrons.
Neutrons decide the isotope Same element, different mass. Carbon-12 and carbon-13 both behave as carbon, because chemistry is decided by electrons and there are six either way.
Electrons decide the charge A neutral atom has as many electrons as protons. Take one away and you have a positive ion; add one and you have a negative ion.
The outer shell decides the reactions Two in the first shell, then eight, then eight. The number left in the outer shell is the group number, and it is what the element does its chemistry with.
Quick check 0 / 6

6 / 6

What this experiment shows

An atom is a nucleus of protons and neutrons with electrons in shells around it, and almost all of it is empty space. Three counts describe it completely. The number of protons - the proton number - decides which element it is, and nothing else does. The protons and neutrons together give the mass number, because a proton and a neutron each weigh about one unit while an electron weighs about one eighteen-hundredth of that. The electrons decide the charge: as many as there are protons and the atom is neutral, one fewer and it is a positive ion. On this page you build the atom yourself, one particle at a time, and every readout follows from arithmetic on those three counts rather than from a lookup table.

mass number A = protons + neutrons  ·  charge = protons − electrons

What you need

  • No apparatus - the atom is built on screen from three trays of particles
  • A tray of protons, a tray of neutrons and a tray of electrons
  • A periodic table, for checking proton numbers and relative atomic masses

How it is done

  1. Drag protons into the middle, or use the plus button. The element name appears as soon as the first one lands.
  2. Add neutrons. The element does not change, but the mass number climbs and the panel says whether that isotope is a stable one.
  3. Add electrons. They fill the shells from the inside out, two then eight then eight.
  4. Match the electrons to the protons for a neutral atom, or leave them unmatched for an ion.
  5. Read the nuclear symbol off the panel, then switch to the shell diagram and check you could draw it yourself.

What you should see

  • Protons and electrons are equal in number in a neutral atom.
  • Adding a neutron changes the mass number but not the element.
  • Removing an electron leaves a positive ion; adding one gives a negative ion.
  • The number of electrons in the outer shell is the group number, and the number of shells in use is the period.
  • Switch on true scale and the nucleus shrinks to a dot: it is about one twenty-thousandth of the width of the atom, and a million-millionth of its volume.

Where marks are lost

  • Confusing mass number with relative atomic mass. One is a whole number for one atom; the other is an average over the isotopes, and is rarely whole.
  • Saying an ion forms by gaining or losing protons. Ions form by moving electrons. Change the protons and you have changed the element.
  • Calling two atoms isotopes when they have different proton numbers. Isotopes share the proton number and differ in neutrons.
  • Writing the electron arrangement of sodium as 2, 9 or 11. Shells fill in order and the first holds only two: it is 2, 8, 1.
  • Forgetting that the mass number ignores the electrons, because they are far too light to matter.

Questions students ask

What decides which element an atom is?
The number of protons in its nucleus, and nothing else. That number is the proton number, and it is what the periodic table is ordered by. Six protons is carbon whatever the neutrons and electrons are doing.
What is the difference between an isotope and an ion?
An isotope has a different number of neutrons, so a different mass number but the same chemistry. An ion has a different number of electrons, so a charge. Carbon-13 is an isotope of carbon; a sodium ion, Na+, is sodium that has lost one electron.
How do I work out an electron arrangement?
Take the number of electrons and fill the shells outwards: two in the first, eight in the second, eight in the third. Chlorine, with 17 electrons, is 2, 8, 7 - and those seven outer electrons are why it is in group 17.
Why is relative atomic mass not a whole number?
Because it is the average mass of an element’s atoms, weighted by how common each isotope is. Chlorine is about three quarters chlorine-35 and one quarter chlorine-37, which averages to roughly 35.5.
Where is the mass of an atom?
Almost all of it is in the nucleus. A proton and a neutron each weigh about 1 u; an electron weighs about 1/1836 of that, so the electrons contribute almost nothing to the mass even though they take up nearly all the room.

Try these next

Written for Cambridge IGCSE Chemistry 0620 and CBSE Class 9-12 by Ajay Shekhawat, founder of MrChemCoach. Run the simulator above, then check yourself against the questions.

1