I have one 20 amp circuit breaker, on approx. 20 outlets, instantly tripping. Curiously several outlets still maintain full power. Replaced breaker, which looked fine, same deal. Unplugged all items from dead outlets, same deal. Checked outer area of outlets, and listened in areas for pops, nothing, and same deal. Spark at breaker when trips, but no burning or scorching or smell. Loose connection between outlets? And why would part of the circuit work with a tripped breaker?

Need to check for voltage coming back on the wire leaving the panel from the breaker that is tripping, sounds like a miswire out in the circuit feeding power back to the breaker. Any chance this wire is part of a 3 wire cable?

Somewhere you have a short coming from 1 circuit that is getting into another circuit through a crossed wire. It shorts to that breaker but power continues on others since the other circuit is hot. The hot may be touching a neutral somewhere. BTW, 20 outlets on 1 20AMP is quite a potential load. Is all the wiring 12AWG? Pull the last outlet where no power exists and look at the wiring and then pull the outlet where the box is hot and look there. Look for a loose neutral or hot lead.

There is no code limitation on the quantity of general purpose outlets on a residential circuit.

A loose neutral or hot will not cause a circuit breaker to trip.

There can be a short or mis-wire between two circuits, either two in one cable or two in one junction box, that can backfeed from one circuit to another and cause a breaker to trip.

I should have been more clear about a "loose neutral or hot"... I was suggesting it was contacting another hot wire, not just be loose. IWhile there is no limitation on quantity of outlets, I used the term potential" since 20 outlets gives you a lot of places to plug things into and possibly overload a circuit.

Take an extension cord and a lamp and bring it back to the panel as you switch off breakers. Either you are wrong about which outlets are on the tripping breaker, or there is a incorrect connection somewhere in the line. Once you find the breaker, it's a matter or opening up the outlets and checking wiring.