Observation and experimentation have led scientists to accept a theory about the atom. this theory is called the atomic theory. scientific evidence collected and observed by scientists over time suggests that the atom has a nucleus, containing protons and neutrons, and electrons that surround the nucleus. what makes this a scientific theory?

b.final

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.