Why do you think space is good environment to demonstrate Newton's third law of motion?

Yes

Explanation:

Outer space is an excellent environment to demonstrate the law of Action-Reaction, since there is no friction with boundaries, and also one is away from the Earth gravitational pull, then such force can be neglected.

Their charge is —1

Explanation:

Halogens form ionic compounds by receiving 1 electron from a metallic atom to obtain their octet configuration. This is clearly seen when sodium(Na) react with chlorine(Cl) to form sodium chloride (NaCl) as shown below:

Na —> Na+ + e-

Cl + e- —> Cl-

Combining both equation together, we have:

Na + Cl + e- —> Na+ + Cl- + e-

Eliminate the e-

Na + Cl —> Na+Cl-

From the ionic equation we can see that the charge of the Halogen (Cl) is —1