Magnesium is an important component of chlorophyll. If there is no magnesium in the soil, plants will not be able to make chlorophyll, they will be yellow rather than green and will not be able to carry out photosynthesis so they will not grow properly.
Calcium is necessary for our teeth, bones, muscles and nerves. Calcium is absorbed by plants from the soil so most of our food contains some calcium. Young mammals need lots of calcium for their growing teeth and bones, so there is a lot of calcium in milk. There is also a lot of calcium in your drinking water if you live in a “hard water” district.
Strontium can also get out of the soil and into our bones through the food chain. Unlike calcium, our bones cannot remove strontium: this is only a problem with radioactive Strontium 90.
Barium sulphate can be used as a “Barium Meal” to make our intestines visible to X-rays; radiographers can use this to find ulcers. Soluble Barium is very toxic, but since Barium Sulphate is very insoluble it does not do us any harm.
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 animation shows electron pairs: since electrons have negative charges, they repel each other so they would normally be at opposite ends of their orbit.