Two atoms of a single element that differ in number of neutrons are said to represent two distinct of that element.

Two atoms of a single element that differ in a number of neutrons are said to represent two distinct isotopes of that element.
Isotopes is an any of two or more forms of a chemical element with the same atomic number, but different numbers of neuron in a nucleus.

The independent variable is the type of music.the dependent variable is the pulse rate.the intervening variable is the age of the subject.if the mother does not like rock, her pulse is likely to not increase.the same might be true for the sister.the friend may like rap and gets excited.to establish cause/effect relationship, all of the subjects must be consistent.hope i .