#sigma#-bonds are the first bond between atoms. #pi#-bonds are those built up on top. A double bond is a #sigma# and a #pi#; a triple bond is a #sigma# and 2 #pi# bonds.
#C_2H_4# is also known as ethene and has a double bond equivalent (DBE) of #1#.
DBE is calculated using the formula
#1/2 (A - X)#,
where #A# is the potential number of hydrogen and #X# is the actual number. #A# is calculated using
#2n + 2#
where #n# is the number of carbons in the molecule.
Therefore, when #n = 2#,
#1/2 (A - X) = 1/2 (6 - 4) = 1/2 * 2 = 1#
Although double bond equivalents are not necessarily double bonds, as they could be triple bonds or cyclic structures, in a molecule this small there is no opportunity for those.
Since double bonds have one #sigma# and one #pi#, this is the total number of #pi#-bonds: #1#.
