A. Different atomic numbers B. The same mass number, but different atomic numbers C. The same number of protons and neutrons D. The same number of electrons, but a different number of neutrons E. The same atomic number, but a different mass number
The correct answer is answering E: the atoms have the same atomic number, but a different number of neutrons. This is key. The atomic number signifies how many protons an atom has. If it has three protons, the atomic number is three. When the electrons are greater or less than the protons, the atom has an electrical charge. If the protons and electrons are equal, then the atom has no charge.
Neutrons are special because they are not electrically charged. The only difference to anything that they can make is a difference in how heavy something is. So, one atom of uranium can have a certain amount of neurons and be radioactive, while another atom may have fewer and be completely stable.
Isotopes are two atoms of the same element that have the same atomic number but different mass numbers. The increase in mass number is due to the increase in the number of neutrons. This makes theatom heavier and makes them have different physical properties.