How can atoms of two elements that have a different number of protons have the same charge?

A. by also having a different number of neutrons
B. by also having a different number of electrons
C. by having the same mass number
D. by being the same size atoms

The charge of an atom is determined by the number of protons and electrons it has. Protons have a positive charge (+1), while electrons have a negative charge (-1). In an atom, the number of protons is equal to the number of electrons, so their charges cancel each other out, resulting in a neutral overall charge.

Now, let's look at the given options:

A. by also having a different number of neutrons: Neutrons are electrically neutral, meaning they do not have a charge. So, the number of neutrons does not affect the charge of the atom. This option is not correct.

B. by also having a different number of electrons: As mentioned earlier, the number of electrons determines the charge of an atom. If two elements have a different number of protons, they will also have a different number of electrons in order to maintain overall neutrality. This option is correct.

C. by having the same mass number: The mass number refers to the total number of protons and neutrons in an atom's nucleus. While the mass number is related to the overall mass of an atom, it does not directly determine its charge. This option is not correct.

D. by being the same size atoms: The size of an atom, often referred to as atomic radius, does not affect its charge. The charge depends on the number of protons and electrons. This option is not correct.

Based on the explanations above, the correct answer is B. by also having a different number of electrons.