What is inside an atom?
An atom is made of three kinds of even tinier particles: protons, neutrons and electrons. Protons and neutrons are packed into a small, heavy center called the nucleus, and electrons move around it. Protons are positive, electrons are negative and neutrons have no charge.
Tinier than tiny
Atoms are incredibly small. A few million atoms lined up side by side would only just stretch across the period at the end of this sentence.
The word atom comes from the Greek word atomos, which means “uncuttable.” For a long time, people thought atoms were the smallest things there could be. Then, starting in 1897, scientists discovered that atoms are built from even smaller parts.
The nucleus: protons and neutrons
At the center of every atom is the nucleus. It holds two kinds of particles: protons, which carry a positive charge, and neutrons, which carry no charge at all.
The nucleus is tiny compared with the whole atom. If an atom were as big as a sports stadium, its nucleus would be about the size of a pea in the middle. Yet the nucleus holds more than 99.9% of the atom’s mass, because protons and neutrons are much heavier than electrons.
Like charges repel, so the protons in a nucleus push against each other. A very strong force that acts only over tiny distances holds the protons and neutrons together anyway.
Electrons: on the outside
Electrons are much lighter than protons. It takes about 1,836 electrons to match the mass of a single proton. Each electron carries a negative charge.
Opposite charges attract, so the positive nucleus pulls on the negative electrons. That pull keeps the electrons close to the nucleus.
Electrons are arranged in layers called shells. The first shell holds up to 2 electrons and the second holds up to 8. Diagrams often draw electrons as dots on circles, but real electrons do not follow neat paths like planets. They spread out in a fuzzy cloud, and each shell is really an energy level.
Charges and masses at a glance
A proton has a charge of +1 and a mass of about 1 atomic mass unit (u), a very small unit made for weighing atoms.
A neutron has no charge (0) and a mass of about 1 u, almost exactly the same as a proton.
An electron has a charge of −1 and a mass of only about 0.0005 u. That is so small that it is often treated as zero.
Why atoms have no overall charge
In an atom, the number of electrons equals the number of protons. The positive and negative charges cancel out, so the atom as a whole is neutral.
A carbon atom is a good example. It has 6 protons and 6 electrons, so its charges balance. Most carbon atoms also have 6 neutrons. The 6 electrons sit in two shells: 2 in the first shell and 4 in the second.
Ordinary chemical reactions never change the nucleus. Atoms only gain, lose or share electrons. An atom that gains or loses electrons becomes a charged particle called an ion. A sodium atom that loses one electron becomes Na⁺, and a chlorine atom that gains one becomes Cl⁻.
Key points
- Atoms are made of three kinds of particles: protons, neutrons and electrons.
- Protons (+1) and neutrons (0) sit in the nucleus, which holds almost all of the atom’s mass.
- Electrons (−1) are very light and move around the nucleus in shells.
- A neutral atom has the same number of electrons as protons, so its charges cancel out.
- Chemical reactions only move electrons; an atom that gains or loses electrons becomes an ion.
Check yourself
Three quick questions on this lesson.
1. Which particle has no electric charge?
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Neutron — Neutrons are neutral, while protons are positive and electrons are negative.2. Where is almost all of an atom’s mass found?
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In the nucleus — Protons and neutrons are far heavier than electrons, and both are packed into the nucleus.3. A neutral atom has 8 protons. How many electrons does it have?