The atom and its nucleus

1. Every atom has a tiny central nucleus surrounded by electrons.
2. The nucleus is extremely small compared with the whole atom, with a radius about $100000$ times smaller than the radius of the atom.
3. Almost all of the mass of an atom is concentrated in its nucleus.
4. The electrons are negatively charged and occupy the space around the nucleus, and their arrangement is studied separately from the nucleus.

Protons and neutrons

1. Atomic nuclei are made of two kinds of subatomic particle: protons and neutrons.
2. A proton carries a positive charge, with a relative charge of $+1$.
3. A neutron is neutral, so its relative charge is $0$.
4. Protons and neutrons are packed tightly together inside the nucleus and are held there by a very strong attraction that acts only over the tiny distances inside the nucleus.
5. Every nucleus contains at least one proton, and every nucleus except the simplest form of hydrogen also contains neutrons.
6. Protons and neutrons are much smaller than an atom, so they must not be confused with atoms or with molecules, which are groups of atoms joined together.

Nuclear charge

1. Because neutrons are uncharged, the only source of charge in a nucleus is its protons.
2. Each proton adds $+1$ to the relative charge, so the nuclear charge is equal to the number of protons in the nucleus.
3. A nucleus can therefore never be negatively charged or neutral, because it always contains at least one proton.
4. Adding or removing neutrons changes the mass of a nucleus but leaves its charge exactly the same.

Each element has its own charge

1. The number of protons in a nucleus is called the atomic number, $Z$.
2. Every atom of a particular element has the same number of protons in its nucleus.
3. This means that every nucleus of an element has the same positive charge, which is called its characteristic nuclear charge.
4. Every hydrogen nucleus has a charge of $+1$, every carbon nucleus has a charge of $+6$, every oxygen nucleus has a charge of $+8$ and every uranium nucleus has a charge of $+92$.
5. No two elements share the same nuclear charge, so the charge of a nucleus identifies which element it belongs to.
6. The elements in the periodic table are arranged in order of increasing atomic number, so their nuclear charge rises by $+1$ from one element to the next.
7. If the number of protons in a nucleus changes, the nuclear charge changes and the nucleus becomes a nucleus of a different element.
8. A neutral atom contains the same number of electrons as protons, so the negative charge of the electrons exactly balances the positive charge of the nucleus.