How do you call when a substance goes from a solid to a liquid?

Melting point as a chemistry concept

The solid begins to go from a solid state to a liquid state — a process called melting. The temperature at which melting occurs is the melting point (mp) of the substance.

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.