Why are compounds a solid, liquid, or gas?

Compounds are either a solid, liquid, or gas at room temperature (standard temperature and pressure) because of the bonding between particles. For each compound, the bonds between particles will hold different amounts of energy that have to be overcome when heating. Thus the more energy a bond holds, the more likely it is for the substance to be a solid.

Why do elements react differently?

Elements react differently due to their electronic configuration. The closer the element is to the first and seventh (A-Level: seventeenth) groups (excluding group eight - or A-Level eighteen - which are extremely unreactive) the more reactive it is. This is beacause it is easier to lose or gain only one electron, giving an element its reactivity.

The reactivity also depends on the reactants, as different elements reacting together behave differently. However, this can also be explained using electron configuration - with extreme reactions occuring with elements that only gain or lose one electron (such as sodium with chlorine).

What makes elements radioactive?

Radioactivity arises because each element wants to acheive stability. To do this, an element normally forms an ion in a compound; however, there are two forms of stability: a full outer electron shell and - the one that causes radioactive decay - equal numbers of nucleons (protons and neutrons) in the nucleus.

Radioactive decay occurs due a difference in the number of nucleons (protons and neutons) in the nucleus. Because the atom wants to acheive stability, its only option with a mismatched amount of neucleons is to eject some of its component particles (protons, neutrons, and electrons) and become a (hopefully stable) daughter isotope.

Radium is the most radioactive element and has the largest gap in neucleon numbers.

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