Atom Builder
Add or remove protons, neutrons, and electrons to build any atom — and instantly see the element, isotope, ion status, and electron configuration update in real time.
- Atomic Number
- 1
- Mass Number
- 1
- Protons
- 1
- Neutrons
- 0
- Electrons
- 1
- Charge
- 0
- Period
- 1
- Group
- 1
Isotope
Protium (Hydrogen-1)
11H
Neutral Atom
Charge = Protons − Electrons = 1 − 1 = 0
View on Periodic Table
Click any element to load its neutral atom. The highlighted cell tracks your current element.
Understanding Atomic Structure
Use these quick references alongside the simulator above to connect what you’re building with the underlying chemistry concepts.
What is an atom?
An atom is the smallest unit of an element that keeps that element's chemical identity. It's built from a dense central nucleus — containing protons and neutrons — surrounded by electrons moving through shells around it.
What are protons?
Protons are positively charged particles in the nucleus. The number of protons, called the atomic number, uniquely identifies an element — change it, and you get a completely different element.
What are neutrons?
Neutrons are uncharged particles in the nucleus, roughly the same mass as protons. Changing the neutron count doesn't change the element, but it does create a different isotope of that element.
What are electrons?
Electrons are negatively charged particles that occupy shells around the nucleus. Their count determines the atom's electric charge — equal to the proton count means neutral, different means an ion.
What is atomic number?
The atomic number (Z) is simply the number of protons in the nucleus. It defines the element and its position on the periodic table, and never changes for a given element.
What is mass number?
The mass number (A) is the total count of protons plus neutrons in the nucleus: A = Z + N. It's specific to an individual atom, unlike the periodic table's standard atomic weight, which is an average across all naturally occurring isotopes.
What are isotopes?
Isotopes are atoms of the same element (same proton count) with different numbers of neutrons, and therefore different mass numbers. Carbon-12, Carbon-13, and Carbon-14 are all carbon — just with 6, 7, and 8 neutrons respectively.
What are ions?
Ions are atoms that have gained or lost electrons, giving them a net electric charge. Losing electrons creates a positively charged cation (like Na⁺); gaining electrons creates a negatively charged anion (like Cl⁻).