The diagram shows the first twenty elements in their positions in the periodic table - Junior Cycle Science - Question (b) - 2008

Isotopes are variants of the same chemical element that have the same number of protons (and thus the same atomic number), but different numbers of neutrons. This results in different mass numbers for isotopes of the same element. For example, lithium has two naturally occurring isotopes: lithium-6 and lithium-7, which differ in their neutron counts.

Group two metals are known as alkaline earth metals. They are characterized by their shiny appearance and low density.

Noble gases, such as helium, neon, and argon, are very chemically unreactive due to their stable electronic configuration. They have a full outer orbit (shell) with eight electrons, which makes them energetically stable and less likely to form chemical bonds with other elements.