8. Which of the following elements reacts like calcium (Ca) but with less atomic mass?
Potassium (K)
Magnesium (Mg)
Strontium (Sr)
Scandium (Sc)

Magnesium (Mg)

Explanation:

An element with a lower atomic mass than Calcium that has similar chemical property is the element Magnesium (Mg).

This because, elements in the same group or family on the periodic table has similar chemical properties.

Magnesium belongs to the same group as Calcium, other elements are:

     Be  Mg  Ca    Sr  Ba  Ra

The group is made of the alkaline earth metals. They have two valence electrons in their outer shell.

answer:

the answer is 6 valence electrons.

explanation:

valence is the # of electrons on the outer most shell of an atom.

oxygen has 6.

although, the valency of oxygen is 2, because it only needs to make 2 covalent bonds, or gain 2 electrons, to achieve the stable octet configuration of a noble gas.

that's it!

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~ trueboss72

there are two significant figures.