Howard Sacknovitz, age 14, of Philadelphia, Penna., for his question:
What is an isotope?
Excuse me, but are you Tim or Jim? These look‑alike, act‑alike., talk alike$ dress‑alike identical twins can be confusing even to their friends. There are also Tim and Jim twins among the atoms. There are also triplets, quadruplets and even bigger groups of atomic copycats. We call them isotopes, a word which means in the same place. We might stretch the meaning and say that isotopes play each others roles and fill each other's shoes.
Since atoms are such bitsy thirds, isotopes are far harder to separate than Tim and Jim. If all the people in North America shrank to atom size and stood in a row, they would measure less than an inch. Tim and ,Tim may look exactly alike, but each has his own set of fingerprints and there are many other differences. There are also subtle differences between the isotope atomic twins.
The difference in isotopes lies in the number of tiny particles of which an atom is made. If an atom grew to the size of a football stadium, its particles still would be no bigger than toy balloons. No wonder the first isotope was not tagged until thirty years ago or so. Since then we have discovered isotopes among the atoms of most elements.
An atom is the smallest particle of an element. It has been compared to a miniature Solar System. The role of the sun is played by the nucleus, a tight bundle usually made of assorted particles. The planets would be the electrons, those busy particles which provide our electric current. Atoms come in different sizes, and it is the size which gives them character. Size depends upon the number of particles. The smallest atom is hydrogen. It has but one proton particle in the nucleus and but one planet electron. Next in size is lithium with two protons. The atoms are numbered according to the number of protons in the nucleus. Hydrogen is one, carbon six, oxygen eight, gold 79 and so on all through the basic elements.
Proton particles carry charges of positive electricity and electron particles carry charges of negative electricity. A normal atom has an equal number of protons and electrons and as a unit is electrically neutral. Odd electrons may wander among atoms, but the number of protons in each atom remains the same. Should a hydrogen atom gain an extra proton it would be an atom of lithium, a different element altogether.
Yet delicate measuring showed that hydrogen in bulk was slightly heavier than it should be. Some of the atoms must have extra particles of some sort And so they had. A small amount of this heavy hydrogen was at last isolated. The atoms had a neutron, a neutral particle with neither negative nor positive charge. These atoms were isotopes, atoms weighing a trifle more but acting like and identical with normal hydrogen atoms in every other respect.
The larger atoms with 10, 20, 50 or more protons normally carry a quota of neutrons in the nucleus. But here again we find isotopes with more than the usual number of neutrons. Isotopes, like twins, it seems, can be expected every so often.