Have you tightened the chain?
Otherwise it looks like the pitch of the chain is different to the sprocket, if this is the chain and sprocket that have always been on the bike the chain and sprockets will need replacing as over time cheap chains will stretch killing the sprockets pitch, I have seen a few gold china chains labeled did( def not real) that have had over 3 links of stretch and still went smooth but you couldn't push the bike backwards. I good chain should set you back at least 40 bills sprockets don't matter too much if the chain is good quality.