Why is an alkyl group electron donating?
It is due to the difference between the electronegativities of carbon and hydrogen.
It is due to the difference in electronegativity between carbon and hydrogen. Because carbon is more electronegative, it pulls electron density slightly towards itself away from the hydrogen atoms. This gives the carbon a small negative charge and the hydrogens each get a small positive charge.
The additional electron density aquired by the carbon atom allows it to "donate" additional charge to other carbon atoms to which it is bonded.
The effect increases with carbon number, so that propyl groups are slightly more electron donating that ethyl groups, ethyl slightly more than methyl, and so on.