Everything in the universe is made up of atoms . In atoms the number of protons and electrons are the same. However, the number of neutrons in atoms can vary. Different versions of atoms are called isotopes . They are different because they can have different numbers of neutrons crammed into the nucleus.
Isotopes always have the same numbers of electrons and protons and therefore react in the same way.
The elements are arranged in ascending order by their proton number. The proton number is also called the atomic number . The mass number for a particular element is found when you add together the two sub-atomic particles, which have mass, i.e. the protons and neutrons. The periodic Table is broken up into four different blocks given the letters s, d, p and f blocks . These four areas have been shown in different colours.