Which statement best explains why compounds have different properties than the elements that form them?
a. when elements form a compound, the bonds and structure of the elements change.
b. when elements form a compound, the masses of the elements increase.
c. when elements form a compound, the masses of the elements decrease.
d. when elements form a compound, the elements become neutral.

The correct answer is A.

When elements are joined together , the atoms lose their individual properties and have different properties from the elements they are composed of because the bonds and the structure of the elements change.

An example are the elements chorine gas and sodium metal.  Chlorine is a poisonous gas and sodium is a toxic, explosive metal. When chlorine gas reacts with sodium, the resulting compound is sodium chloride which is neither toxic nor poisonous.  In fact sodium chloride or table salt is such a vital compound that without it, man cannot live.

a bird that nests only in a particular tree species.