why is the first member of the homologous series in alkenes is ethene ????

Because in the structure of Alkenes, there is a double bond presnt. In compunds of Meth, there is only one carbon. And that carbon cannot make double bond with anyone not even with Hydrogen.
See the structure of Methane -

Where can be a double bond present in this? No where.

Now see the structure of Ethene -

There is a double bond present in between two Carbon Atoms and there are 4 more hydrogen atoms present.

In short, Alkenes start with Ethene, because there can be no double bond or triple bond present in a compund with single carbon atom.

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An Alkene is a Hydrocarbon which means that it is a molecule made up entirely of Carbon and Hydrogen atoms. The Alkene series is similar to theAlkane series except that each carbon chain has one, and only one, double bond in it. Since the element Carbon bonds four ways and the element Hydrogen bonds only one way we can easily construct the members of this series.

It is important to note that due to the properties of Carbon and Hydrogen, the double bond can ONLY be between two carbon atoms. Hydrogen is incapable of forming anything other than one single bond.

Alkenes are named in the same way as the alkanes except the "-ane" is replaced with "-ene" at the end.

Ethene: The Simplest Alkene

Since membership of this series requires one double bond somewhere in the Carbon chain, it is impossible to have a molecule with just one Carbon atom. Therefore the first member of the series is Ethene, which has the chemical formula C2H4. This is also known as Ethylene. The double bond is located between the two Carbon atoms, which leaves two unbonded electrons on each Carbon atom. Each of these unbonded electrons forms a single bond with one Hydrogen atom to give the final structure.

The diagrams below use the electron dot diagram for each of Carbon and Hydrogen. For convenience all structures are drawn flat, but they of course are really 3 dimensional and follow the vsepr rules for molecule structure.

ethene

Note that there is only this one possible location of the double bond in this molecule, and that no matter which way up or around you rotate it, the structure looks the same. This means that there is only this one structural isomer for Ethane. For an exploration of structural isomers, see the pageWhat is Octane?.

The Importance of Ethene

Ethene is a vital component of the plastics industry and is also used to mass produce commercial ethanol, detergents and a large range of other compounds. It is produced on a massive scale around the world and is manufactured from crude oil. Ethene from this source is essentially a limited resource.