Atoms

Your phone, the air you are breathing right now, even you yourself: everything is made of tiny particles. But what are those particles made of? The answer is atoms. They are so small that millions of them fit side by side into a single dot on this screen.

Nucleus and shell

An atom has two regions. In the middle sits the nucleus with the protons, which carry a positive charge, and the neutrons, which carry no charge. Around the nucleus lies the shell with the electrons, which carry a negative charge. The nucleus is tiny, yet it holds almost all of the atom's mass. A comparison shows how extreme that size difference is: if an atom were as big as a football pitch, the nucleus in the middle would be about the size of a pea. In between there is almost nothing but empty space, and that is where the electrons are.

The proton number makes the element

How can you tell which element you are looking at? By the number of protons alone. An atom with 1 proton is always hydrogen, one with 8 protons is always oxygen. If you add up protons and neutrons, you get the mass number. It tells you roughly how heavy the nucleus is.

Neutral or charged?

A neutral atom has exactly as many electrons as protons: the charges balance out. But atoms can give away or take up electrons. Then the balance is gone and a charged particle appears, an ion. If an atom gives away an electron it becomes positive, if it takes one up it becomes negative.