Question

how do you draw the lewis structure for C2H2

Answer 1

Well there's a triple bond in between. Carbon has 4 electrons each to begin with. The triple bond takes up 6 of the 8 electrons. That leaves 1 electron on either side...ok maybe describing in words isn't best.

Draw 2 c's
6 dots in the middle (:::)
Then put a H on either side of the C's.
Then draw 2 dots between the H's and the C's.

It should look like

H:C:::C:H

Since the H has one electron to begin with.
And the C's have four each.

Answer 2

H:C:::C:H

or

H-C---C-H

(that's intended to be a triple bond between the C's)