What is an example of two elements with similar properties and an explanation of how you know?
Sodium (Na) & Potassium (K)
Both are alkali metals that are highly reactive.
Both are soft metals and have a similar 'shiny' appearance.
The reason for the above similarities is due to the fact that both elements are in Group 1 of the periodic table and are separated only by one period ('row'). Group 1 means that they have 1 valence electron and thus (very) readily form a positive ion to fulfil the octet rule and have a full, outer shell, of electrons.