Build an atom and see how it is put together

How to use this simulation

1. Move the Protons slider: each extra proton turns the atom into a different element (sodium into magnesium), and a new element arrives in its commonest neutral form.
2. Now change only the Neutrons: the name stays, the mass number changes. That is an isotope, and the readings tell you whether it is stable or radioactive.
3. Take away or add one Electron: the nucleus is untouched but the atom becomes an ion. Press the Na⁺ and Cl⁻ presets to compare.
4. Watch the shells: K holds at most 2 electrons, L holds 8, and for the first 20 elements M holds 8. The electrons in the outermost shell are the valence electrons.
5. Slow it to 0.25× to follow the electrons. Remember this is the Bohr picture; real electrons do not travel fixed tracks like planets but spread out as orbital "clouds".

From the pen in your hand to the stars: everything is atoms

Pick up a grain of sugar. Break it, break it again, keep going. How far can you go? About 2,500 years ago the Greek thinker Democritus guessed you would eventually reach a piece that cannot be cut any further. He called it atomos, "uncuttable", and that is where our word atom comes from.

Your body, water, air, your phone screen and the stars are all made of atoms. Just 118 kinds of atom, the elements, combine in different ways to make every substance there is, the way a few letters of the alphabet spell millions of words.

The twist is that atoms are not uncuttable after all. Inside are three smaller particles: protons, neutrons and electrons. How many of each an atom has, and how they are arranged, decides whether it is gold or oxygen and whether it bonds with others or keeps to itself. In the simulation above you can add and remove these particles and build atoms yourself.

Starting from zero: what exactly is an atom?

Definition: an atom is the smallest particle of an element that can take part in a chemical reaction. Every atom has two parts: a nucleus at the centre and electrons in shells around it.

The nucleus is unbelievably small. An atom has a radius of roughly 1 × 10⁻¹⁰ m, but its nucleus only about 1 × 10⁻¹⁵ m, so the atom is around 100,000 times wider. If the nucleus were a marble 1 cm across, the atom would be about 1 km wide: a marble in the middle of a huge field, with a few electrons buzzing around in almost empty space.

Then why does a table feel solid? Because electrons are negatively charged, and the electrons of the atoms in your hand strongly repel the electrons of the atoms in the table. You never really "touch" anything; what you feel as solid is that repulsion.

And where is nearly all the mass? In the nucleus. Protons and neutrons are each almost two thousand times heavier than an electron, so the mass of an atom is essentially the mass of its nucleus.

The story of atomic models: from Dalton to the modern model

Nobody has looked inside an atom with their eyes. Scientists ran experiments, pictured a model that explained the results, and replaced it when a new experiment showed where it failed. That chain of improvements is how science works, and exams ask about every link in it.

The three particles: proton, neutron and electron

Here are the three fundamental particles of the atom in one table. Masses are given in kilograms and, beside them, in atomic mass units (u). Notice that protons and neutrons weigh almost the same, while the electron is tiny by comparison.

A proton is about 1,836 times heavier than an electron. That is why electrons are ignored when we count the mass number.

Atomic number and mass number

The number of protons in the nucleus of an atom is its atomic number (Z). It is the element's ID number: 11 protons means sodium, anywhere in the universe, in any state. Add one proton and it is magnesium. Try it with the Protons slider.

The total number of protons and neutrons in the nucleus is the mass number (A). Protons and neutrons together are called nucleons, so the mass number is simply the count of nucleons.

In a neutral atom the number of electrons equals the number of protons. A proton's positive charge and an electron's negative charge are equal in size and opposite in sign, so equal numbers cancel to a total charge of zero. That is the answer to "why is an atom electrically neutral?"

Isotopes, isobars and isotones

Atoms of the same element, with the same number of protons but different numbers of neutrons and therefore different mass numbers, are called isotopes. Isotopes have almost identical chemical properties (their electrons are arranged the same way) but can differ in physical properties such as mass, density and radioactivity.

Hydrogen's three isotopes even have their own names: protium (¹H, no neutron), deuterium (²H, one neutron) and tritium (³H, two neutrons, radioactive). Choose hydrogen in the simulation and set the neutrons to 0, 1 and 2 to make all three.

Atoms of different elements with the same mass number are isobars; atoms with the same number of neutrons are isotones. Compare the examples in the table.

Rules for writing electron configuration

Electron configuration is the way an atom's electrons are arranged in its shells. Knowing it tells you whether an element is a metal or a non-metal, how many bonds it makes and where it sits in the periodic table, which is why electron configuration is called the basis of the periodic table.

Electron configuration of the first 20 elements (table)

This table is generated by the same rule the simulation uses, so it matches exactly what you see as you move the Protons slider. The configurations of the first 20 elements are asked almost every year, so write the table out once yourself.

Valence electrons, the octet rule and ions

The electrons in the outermost shell are the valence electrons. Almost all of chemistry happens here: bonding, forming ions, reacting. The noble gases (helium, neon, argon) have full outer shells, which is why they hardly react with anything.

Other elements try to reach 8 electrons in the outer shell like a noble gas (2 in the first shell, for hydrogen and helium); this is the octet rule. An atom with 1, 2 or 3 outer electrons tends to give them away (metals); one with 5, 6 or 7 tends to gain more (non-metals).

Losing electrons leaves more protons than electrons, so the atom becomes a positive ion, a cation, such as Na⁺. Gaining electrons makes a negative ion, an anion, such as Cl⁻. The nucleus does not change at all, so the element stays the same element.

Valency is the number of electrons an atom gives, takes or shares. Magnesium has 2 outer electrons, so its valency is 2. Chlorine has 7 and needs 1 more to complete its octet, so its valency is 1. Oxygen's valency is 2.

Try these experiments in the simulation

Before each experiment, predict what will happen; then move the sliders and check. A wrong prediction is the one you remember best.

• Experiment 1: raise the protons one by one from 1 to 20. At which elements does a new shell start? (Lithium, sodium and potassium.)
• Experiment 2: with carbon, set the neutrons to 6, 7 and 8. The first two are stable; the third is radioactive carbon-14, used to date ancient objects.
• Experiment 3: remove one electron from sodium (2, 8, 1). The configuration becomes 2, 8, exactly like neon. That is why sodium forms Na⁺ so easily.
• Experiment 4: add one electron to chlorine (2, 8, 7). Now it is 2, 8, 8, a full octet like argon.
• Experiment 5: build neon (Z = 10). It has 8 valence electrons and a full outer shell, which is why neon is inert.
• Experiment 6: give an element far too many or too few neutrons. The readings say "not stable in nature", because a nucleus needs a balance of protons and neutrons to hold together.

Solved examples, step by step

Every answer below is calculated by this page's code, so it agrees with the simulation. Try each one with pen and paper first.

Mistakes almost everyone makes

These are the errors examiners see most often. Read them once and they will not cost you marks.

• Adding electrons to the mass number. Mass number = protons + neutrons only.
• Defining atomic number as the number of electrons. They are equal in a neutral atom but not in an ion, so always write "number of protons".
• Writing potassium as 2, 8, 9. The outer shell never holds more than 8; the right answer is 2, 8, 8, 1.
• Thinking an ion is a different element. Only electrons change in an ion, never protons; Na⁺ is still sodium.
• Thinking isotopes are different elements. Isotopes have the same number of protons, so they are the same element.
• Treating Rutherford's model as final. Its big gap (why electrons do not fall into the nucleus) was fixed by Bohr.

Atomic structure in real life

This is not just exam theory; knowing how atoms are built saves lives and powers cities every day.

• Carbon-14 dating: archaeologists find the age of ancient bones, wood and cloth.
• Medicine: iodine-131 treats thyroid disease, cobalt-60 is used in cancer radiotherapy and technetium-99m in body scans.
• Electricity: nuclear power plants split uranium-235 nuclei to produce heat and electricity.
• Agriculture: radioactive isotopes help develop better crop varieties and test how well fertilisers work.
• Fireworks and neon signs: light is given out when electrons jump between shells, and each element has its own colour.
• Smoke detectors: alpha particles from americium-241 ionise the air, and smoke interrupts that current.

Exam corner

In Class 9 (CBSE, "Structure of the Atom") and SSC chemistry, short questions usually ask for definitions (isotope, atomic number, valency), the reason an atom is neutral, the observations and conclusions of the gold foil experiment and the limitations of each model.

Longer questions usually give the atomic numbers of a few elements and ask you to write electron configurations, show ion formation or calculate an average atomic mass. Examples 2 and 4 above are exactly this kind.

In Class 11 (HSC and NEET/JEE) the chapter adds quantum numbers, orbital shapes, the Aufbau, Pauli and Hund rules and the hydrogen spectrum from Bohr's energy levels, a steady source of MCQs.

Quick revision

Read just this list the night before the exam.

• Atom = nucleus (protons + neutrons) + electrons in shells.
• Z = number of protons; A = protons + neutrons; N = A − Z.
• Neutral atom: protons = electrons; fewer electrons makes a cation, more makes an anion.
• Isotopes share Z; isobars share A; isotones share N.
• A shell holds at most 2n² electrons; the outermost holds no more than 8.
• Order of models: Dalton → Thomson → Rutherford → Bohr → quantum model.

Frequently asked questions