Bonding

The atoms H, C, and N in Hydrogen Cyanide are held together by covalent bonds.  Between the H and the C there is a single covalent bond and between the C and the N there is a triple covalent bond.  If the bonds were to be represented by dashes they molecule would look like this. 

H-C=N

Structure of Hydrogen Cyanide

Percent Composition by Mass

Hydrogen:  Mass = 1

Total mass = 27.02

Percent Hydrogen by mass = 1/27.02 * 100 = 3.7%

Carbon: Mass = 12

Total Mass = 27.02

Percent Carbon by Mass = 12/27.02 *100 = 44.41%

Nitrogen: Mass = 14

Total Mass = 27.02

Percent Nitrogen by mass = 14/27.02 * 100 = 51.82%

Percent Composition by Number

Hydrogen: atoms per molecule = 1

Total Atoms per molecule = 3

Percent Hydrogen by Number = 1/3 * 100 = 33.3%

Carbon: atoms per molecule = 1

Total Atoms per molecule = 3

Percent Carbon by Number = 1/3 *100 = 33.3%

Nitrogen: atoms per molecule = 1

Total atoms per molecule = 3

Percent Nitrogen by number = 1/3 * 100 = 33.3%