Structure of Compound X:
Compound X is an unsaturated hydrocarbon that undergoes addition reaction. This means that it contains a double or triple bond between carbon atoms.

Structure of Compound Y:
Compound Y has the molecular formula C3H8, which suggests that it is an alkane. Alkanes are saturated hydrocarbons that contain only single bonds between carbon atoms.

Explanation:
When compound X reacts with hydrogen, it undergoes addition reaction, which means that the double or triple bond in compound X is broken and hydrogen atoms are added to the carbon atoms.

The molecular formula of compound Y is C3H8, which suggests that it contains three carbon atoms and eight hydrogen atoms. Since it is an alkane, it means that all the carbon atoms in compound Y are connected by single bonds.

To determine the structure of compound X, we need to consider the addition reaction. Since compound Y has three carbon atoms, compound X must also have three carbon atoms. Additionally, since compound Y is an alkane, it means that compound X must contain a double or triple bond between two of the carbon atoms.

Therefore, the structure of compound X can be represented as follows:

H H
| |
H - C = C - C - H
| |
H H

In this structure, the double bond between the two carbon atoms can be represented as a double line or a double bond symbol. The remaining bonds of the carbon atoms are filled with hydrogen atoms.

When compound X reacts with hydrogen, it undergoes addition reaction, resulting in the formation of compound Y. The hydrogen atoms add to the carbon atoms, breaking the double bond and forming single bonds.

Therefore, the structure of compound Y can be represented as follows:

H H H
| | |
H - C - C - C - H
| | |
H H H

In this structure, all the carbon-carbon bonds are single bonds, and each carbon atom is bonded to four hydrogen atoms, satisfying the valency requirements of all the atoms.

Overall, compound X is an unsaturated hydrocarbon with a double or triple bond between carbon atoms, while compound Y is an alkane with only single bonds between carbon atoms. The addition reaction of compound X with hydrogen leads to the formation of compound Y with the molecular formula C3H8.

The element x is pentene
the element y is pentane