Which of the following particles is the smallest in radius

Its helium

Explanation: Because teh effective nuclear charge holds the valence ectrons clos to the nuclues. the Atomic radius would decrease as you move from the led=ft and right of the perodic table

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.