Atomic Theory
Atomic Structure
Nucleus, electrons, and the stadium of empty space
Atoms are the fundamental building blocks of all matter, consisting of a dense central nucleus surrounded by electrons in precise shells, yet they are more than 99.9999999999% empty space. To visualize this scale: if an atom's nucleus were the size of a marble (1 cm), the entire atom would span the width of a professional football stadium. First quantitatively modeled by Niels Bohr in 1913, atomic structure defines how every element in the universe interacts, bonds, and forms the physical world we experience.
- Empty Space> 99.9999999999%
- Nucleus Size~10⁻¹⁵ meters
- Atom Size~10⁻¹⁰ meters (100,000× larger)
- First ModelBohr 1913
Interactive visualization
Press play, or step through manually. The visualization is yours to drive — try it before reading on.
Watch the 60-second explainer
A condensed visual walkthrough — narrated, captioned, under a minute.
How it works
An atom consists of three subatomic particles: Protons (positive charge) and Neutrons (neutral charge) packed into the nucleus, and Electrons (negative charge) orbiting in specific energy levels or 'shells.' The number of protons (the Atomic Number) determines which element the atom is. For example, every Hydrogen atom has 1 proton, while every Gold atom has 79.
The Scale of the Void
The nucleus contains nearly all of the atom's mass (over 99.9%), yet it occupies a tiny fraction of its volume. The rest of the atom is defined by the probability-clouds of electrons. This means that when you touch a solid object, you aren't feeling 'matter' touching 'matter' in the traditional sense; you are feeling the intense electromagnetic repulsion between the electron clouds of your hand and the object.
Common pitfalls
- The 'Solar System' Myth: While the Bohr model helps us understand energy levels, electrons don't actually fly in neat circles; they exist in 'orbitals'—fuzzy regions of probability defined by quantum mechanics.
- Forgetting Neutrons: While protons define the element, neutrons act as the 'nuclear glue' that overcomes the repulsion between positive protons. Without neutrons, the nucleus would fly apart.
- Assuming Atoms are 'Solid': Because of the vast empty space, high-energy particles (like alpha particles) can pass straight through thousands of atoms without hitting anything.
| Particle | Charge | Relative Mass | Location |
|---|---|---|---|
| Proton | +1 | 1 | Nucleus |
| Neutron | 0 | 1 | Nucleus |
| Electron | -1 | 1/1836 (Negligible) | Shells / Orbitals |
Frequently asked questions
What keeps the nucleus together?
The Strong Nuclear Force. It is much stronger than the electromagnetic force that tries to push the positive protons apart, but it only works over extremely short distances.
Why don't electrons spiral into the nucleus?
According to quantum mechanics, electrons can only exist in specific, discrete energy states. They cannot exist 'between' shells, which prevents them from losing energy and crashing into the center.
What is an isotope?
It is a version of an element with the same number of protons but a different number of neutrons (e.g., Carbon-12 and Carbon-14).
How many shells can an atom have?
Theoretically any number, but in the ground state (stable), known elements use up to 7 main energy levels (shells).
What is the 'Valence Shell'?
The outermost shell of an atom. The number of electrons in this shell determines how the atom will bond with others to form molecules.