Tritium is an isotope of Hydrogen. Hydrogen, Deuterium and Tritium all have ONE proton and One electron: this means that they all have the same chemistry. Because they have different numbers of neutrons their physical behaviour is different. Tritium has TWO neutrons, this makes it three times as heavy as an ordinary hydrogen atom. It is unstable, i.e. it is radioactive. Tritium can be used as a radioactive tracer. So if tritiated water (water containing a bit of Tritium) is given to plants it can be followed through the plant using a suitable detector. So first the roots, then the stem and finally the leaves will become radiocative.
Electrons have a very tiny mass: they don’t have “Zero Mass” as it says in the animation. However, you don’t need to take account of their mass when you are calculating molecular weights: for your purposes, the atomic mass is calculated by adding the number of neutrons to the number of protons.
Since electrons do actually have some mass, they cannot travel at the speed of light. They would need an infinite amount of energy to travel this fast. However, they do buzz around the atomic nucleus at very fast speeds.
When an atom is excited, ie. it has too much energy, electrons can escape from their normal orbits into higher energy orbits or escape the atom completely; this would leave the atom in an ionised state.