Build an atom: protons, neutrons and electrons, shells, isotopes, ions and valency

What it does

Every atom is made of three kinds of particle. Protons and neutrons sit together in the tiny nucleus in the middle; electrons move around it in shells. This page lets you build an atom one particle at a time and see what you have made.

• The number of protons decides the element. One proton is hydrogen, six is carbon, eleven is sodium. Change it and the name changes.
• Neutrons add mass but no charge. Atoms of the same element with different numbers of neutrons are isotopes, such as carbon-12 and carbon-14.
• Electrons carry a negative charge. When there are as many electrons as protons the atom is neutral; fewer electrons make a positive ion, more make a negative ion.

The picture shows the protons (filled, with a +) and neutrons (open circles) in the nucleus, and the electrons filling the shells K, L, M and N. Below it you get the element, the atomic number Z, the mass number A, the charge, the shell numbers written as 2,8,1 and so on, the valence electrons, the valency and whether that isotope is found in nature. The formulas underneath show the sums with your numbers filled in.

It follows the topics of Class 9 Science (structure of the atom: protons, neutrons and electrons, Bohr’s shells, valency, atomic number, mass number, isotopes and isobars) and gets you ready for Class 11 Chemistry. Next, see where each element sits in the interactive periodic table, fill orbitals one electron at a time in the electron configuration filler filler, or combine ions in the formula writer.

How to use it

1. Tap + and − next to Protons, Neutrons and Electrons. The buttons are large so they work well on a phone.
2. Or pick an element from Start from an element: it sets the protons and electrons to the atomic number and the neutrons to the most common isotope.
3. Make it neutral sets the electrons equal to the protons.
4. Read the results and the formulas, then try the questions in Check yourself. Tap New questions for a fresh set with different numbers.

The page covers the first 20 elements, hydrogen to calcium, because that is where the simple 2, 8, 8, 2 shell rule works. From element 21 the shells fill in a more complicated order, which Class 11 explains with orbitals.

Worked example: sodium and the sodium ion

Choose sodium from the list. The page shows 11 protons, 12 neutrons and 11 electrons and reads ²³Na Sodium.

• Mass number A = protons + neutrons = 11 + 12 = 23.
• Charge = protons − electrons = 11 − 11 = 0, so it is a neutral atom.
• The electrons fill the shells as K 2, L 8, M 1, written 2,8,1. There is one valence electron, so the valency of sodium is 1.
• Sodium-23 is the only natural isotope of sodium (100 %).

Now tap − next to Electrons once. The single electron in the M shell disappears, the shells read 2,8 and the charge becomes 11 − 10 = +1. The page now shows ²³Na⁺, a sodium ion, a cation. This is what happens when sodium reacts with chlorine to make common salt.

Valency in one line

Atoms combine so that their outer shell ends up full (8 electrons, or 2 for the first shell). An atom with 1 to 4 outer electrons usually loses or shares them, so its valency equals the number of outer electrons. An atom with 5 to 7 outer electrons usually gains or shares enough to reach 8, so its valency is 8 minus that number. Helium, neon and argon already have full shells and a valency of zero.

Tips

• Isotopes have the same number of protons but different numbers of neutrons. Isobars are atoms of different elements with the same mass number, such as argon-40 and calcium-40: build both and compare.
• Mass number is a whole number count of particles. The atomic mass on the periodic table (such as 35.45 for chlorine) is an average over the natural isotopes.
• If you add many neutrons the page says that nucleus is not found in nature. Real nuclei need a balance of protons and neutrons to hold together.

FAQ

Why do electrons not count in the mass number? An electron has about 1/1836 of the mass of a proton, so it hardly changes the mass.

Why does the M shell hold only 8 here when it can hold 18? For the first 20 elements the fourth shell starts filling after 8 electrons in the third. The extra 10 places in the M shell fill later, from scandium on.

Is anything saved? No. Everything happens in your browser and nothing is stored or sent. Questions are made fresh each time.

Is this an official textbook tool? No. It is an independent practice page that follows the same school topics.

Last updated 2026-11-09.