One of physicists’ foundational assumptions is that atoms aren’t unique—if two atoms have the same number of protons, neutrons and electrons, they will look and act exactly alike. This belief is ...

Atoms are very small. They have a radius of around 1 × 10 10 metres. This symbol shows that chlorine has 35 particles in the nucleus (protons and neutrons), 17 of which are protons. It also tells us ...

A helium atom has two electrons in an energy level outside the nucleus. The atom is neutral as it has two positive protons and two negative electrons. A helium atom that has lost or gained an electron ...