An atom is electrically neutral because the number of protons present in it is equal to the number of electrons present in it.

A neutron has mass of 1 u and no charge. It is a neutral particle.

Chlorine has 7 electrons in its valence shell and the valency is 1.  Both magnesium and oxygen have valency 2. The valency of aluminium is 3.

The relative mass of a proton is 1 u and the relative charge is +1.

If K-shell and L-shell of an atom are full and the other shells are empty, then the atomic number of the atom will be 10.

Number of neutrons = 23 - 11 = 12

Oxygen has atomic number 8. Electronic configuration is 2, 6. The valence shell in the oxygen atom is the L- shell and the maximum number of electrons that can be accommodated in it is 8. However, the outermost orbit of oxygen has 6 electrons. 

Option (2) is correct. Dalton stated that atoms are indivisible. He did not propose the existence of sub-atomic particles.

Electrons are not present in the atomic nucleus. They revolve around the nucleus in their respective shells.

Mass number (A) = Number of protons + number of neutrons = 8 + 9 = 17