What must differ between the atom of two different elements?

1. protons in the atom 2. electrons in the atom 3. neutrons in the atom 4. protons and neutrons in the nucleus

1. protons in the atom

Explanation:

Elements in the periodic table are classified according to their number of protons: this means that each element has a different number of protons with respect to another element. There cannot be two elements having the same proton number.

The number of electrons in a neutral atom is equal to the number of protons, however an atom can loose or gain electrons (becoming an ion), therefore two elements can have the same number of electrons if one of them is an ion.

Moreover, an atom of an element can have different isotopes, which means atoms with same proton number but different number of neutrons: therefore, two elements can have the same number of neutrons in the nucleus.

Therefore, the only correct choice is

1. protons in the atom

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