Save guides, add subjects and pick up where you left off with your BBC account. An element's atomic number defines it. An element with 17 protons will always be chlorine. However an element's mass ...

The valency of an element is linked to how many electrons an atom must gain or lose to achieve the same electronic structure as a noble gas i.e., have a full outer shell. For example an element in ...

Everyone at that time imagined the atom as a "plum pudding ... emitted by the radioactive element radium) at a sheet of gold foil only 1/3000 of an inch thick, and tracing the particles' paths.