1) All atoms of the same element must contain the same: a) mass number b) protons c) neutrons d) compounds

all atoms of the same element mist contain the same number of. protons

Explanation:

For isotopes, only the number of neutrons has been changed. examples are carbon-12 carbon-13 and carbon-14, because none of these situations change the number or protons, the atomic number stays the same regardless

since the atomic number is the number of protons in the nucleus, as we move to the left on the periodic table, the electrostatic pull between the nucleus and the valence electron shell decreases. as we move down a group (vertical column), the valence shell energy level gets further and further from the nucleus.

"hydrogen bonding is the attraction of h atoms in the water molecule (slight + charge) to o atoms in a neighboring water molecule (slight - charge); it is a weak interatomic force. but it is still strong enough to the water molecules "stick" together."