Astrontium atom differs from a strontium ion in that the atom has a greater 1. number of electrons 2. number of protons 3. atomic number 4. atomic mass

A strontium atom differ from a strontium ion in that the atom has a greater NUMBER OF PROTONS.
Every atom is made up of three particles, which are proton, neutron and electron. The proton and the neutrons are located inside the nucleus and they do not take part in chemical reactions, it is only electron that takes part in bond formation and chemical reaction.
Strontium has atomic number of 38, that means, it has 38 electrons, 38 protons and 38 neutrons.
Strontium has two electrons in its outermost shell, in order to attain octet form, strontium will donate these two electrons in a chemical reaction. By the time strontium donate two electrons, it will become a cation and has a charge of +2. At this state, strontium only has 36 electrons while the number of proton and neutron remain 38, thus, the atom has a greater number of protons.