In the formula xso4, the symbol x could represent the element

The answer is: Mg.

Sulfate anion (SO₄²⁻) has negative charge, so element X must be element with positive charge +2.

Magnesium (Mg) is metal from 2. group of Periodic table of elements, it lost two electrons to form magnesium cation (Mg²⁺).

Magnesium has atomic number 12, which means it has 12 protons and 12 electrons.

It lost two electrons to form magnesium cation (Mg²⁺) with stable electron configuration like closest noble gas neon (Ne) with 10 electrons.

Electron configuration of magnesium atom: ₁₂Mg 1s² 2s² 2p⁶ 3s².  

Electron configuration of magnesium ion: ₁₂Mg²⁺ 1s² 2s² 2p⁶.  

c is the answer because when something is left after something evaporates that shows the chemical change.

explanation:

let the different atoms be x and y. each with four protons and same mass will be represented as follows.

mass number is number of protons + number of neutrons.

            and  

since, it is given that one has a negative charge while the other is neutral. therefore, there will be increase in number of protons and the atom will be represented as follows.

according to periodic table, mass number of beryllium is 9 and it has atomic number 4 so, the atoms will be and .  

the protons and neutrons will be located inside the nucleus whereas the electrons will be revolving around the nucleus.