Hi can anyone answer these two distance and time questions

In the 1800s, one of the statements in john dalton's atomic theory was that atoms are indivisible. later experimental evidence led to the discovery of subatomic particles such as neutrons, electrons, and protons. what happened to the indivisible atom part of dalton's atomic theory, and why?

Why does his teacher ask him to balance the equation by including the correct coefficient

The valence of aluminum is +3, and the valence of the chlorine is -1. the formula fir the aluminum chloride is correctly written as