When the first periodic table of the elements was published, the elements were arranged by atomic mass. although several elements seemed out of order, later discoveries showed the order to be correct. which event most likely the development of the periodic table?

New information changed the understanding of the atom’s structure.

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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.