Why do the properties change when a compound forms?

The properties of a compound depend not only on which atoms the compound contains, but also on how the atoms are arranged. Atoms of carbon and hydrogen, for example, can combine to form many thousands of different compounds. Sugar is a compound composed of atoms of carbon, hydrogen, andoxygen.

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.

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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.