Identify atoms that are isotopes in each of the following sets of four atoms.

Isotopes are elements with similar chemical properties meaning same atomic number, but varying atomic masses, meaning different number of neutrons in the element.  It can be easily determined knowing this and viewing the subscripts of the element symbol.  The lower subscript is the atomic number and the higher subscript is the atomic mass.  Simply look for similar atomic numbers and differing atomic masses in the sets.

a) 1, 2 are isotopes

b) 1, 3 are isotopes

c) 1, 2, 3 are isotopes

d) none are isotopes

a compound is a molecule made of atoms from different elements.

a. by the combination of an unlimited number of elements