4. Elements in the same group on the periodic table have the same number of valence electrons.
5. Properties of elements within a period on the periodic table change in a predictable way from one side of the table to the other.
6. The number of protons in an atom determines the identity of the element.
Answer:

<h2>10.5 gram(cm)^-3 is the right answer.</h2>
A possibily I’m not 100% sure
Hydrazine has a higher boiling point than water because hydrazine exhibits a stronger intermolecular forces than water. Although both compounds exhibit hydrogen bonding which is a strong intermolecular force resulting to a high boiling point, hydrogen bonding is stronger in hydrazine. This is because in a hydrazine molecule, there are two atoms of nitrogen available for hydrogen bonding as compared to water which has only 1 oxygen atom for hydrogen bonding.