Brainlies ! explain your answers. : )

POTASSIUM and RUBIDIUM 
Potassium and Rubidium are both Alkali Metals, as such is Sodium. They are alike in many ways, as is why they are categorized in the same group (Alkali Metals). 
The properties of Alkali Metals are as follows:
they have low melting and boiling points compared to most other metals.they are very soft and can be cut easily with a knife.they have low densities (lithium, sodium and potassium will float on water)

The two are potassium and rubidium. Why? Na is an alkali metal. As you go down the metals become more reactive. 
Calcium has 20 protons. Why? The atomic # is 20 (the one at the top).
It has 20 neutrons. Why? The atomic mass (the number on the bottom) is ~40, ignoring electron mass. 40- 20 protons = 20 neutrons. 
It has 20 electrons. 
Why? The # of protons and electrons is the same. Ions aren't on the periodic table.
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