Which of the following statements is/are correct regarding the structure of an atom?
1) The number of protons usually = the number of electrons in an atom.
2) If an atom changes its number of protons, it changes identity, such as 40K decaying to Ar.
3) Different isotopes of the same element may have different masses.
4) An atom’s identity (whether it’s hydrogen, helium, or something else) is determined by its atomic mass.