Atoms are the smallest particles found in ordinary substances. They can usually join together to form molecules. Hydrogen has the smallest atoms, which consist of a single proton with a single electron orbiting around it. The largest atoms have over 100 protons and an even larger number of neutrons in their nuclei.
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.
The small number at the bottom left of each symbol ( H or He) is the Atomic Number for that element: One for Hydrogen, Deuterium and Tritium and Two for Helium. The number at the top left is the Atomic Mass: One for Hydrogen, Two for Deuterium, Three for Tritium and Four for Helium.