Periods are the rows of the periodic table. Whilst they are not based on the chemical reactivity they do share an interesting feature: electron shells. Each period is linked by the fact that the elements in that period have the same number of shells of electrons.
Elements in Period 2, for example, all have 2 shells of electrons but are very different in their properties and reactivity. Lithium and Fluorine, whilst in the same group are very different elements but have the same number of shells of electrons. The key difference is the Fluorine has more electrons in this shell than Lithium.
Periods are the rows of the periodic table. As you go along periods the number of protons and neutrons increases and the number of electron shells remains the same.
The electron is the smallest sub atomic particle that make up the atom. Has a negative charge and is located in shells that orbit the nucleus
The shell is the path that electrons follow outside the nucleus. Shells can be considered as energy levels and the further away from the nucleus the higher in energy.
How an element or compound will easily gain or lose electrons when interacting with other elements. Reactivity can also be a measure of how violent a reaction is giving our heat or light.
Elements are atoms with the same number of protons in the nucleus