Jeff Scotty age 13, of Huntsville, Alabama, for his question:
Why is an isotope of the same element heavier?
Many science students think that an isotope is always heavier than its average brother atoms. But this is not necessarily true. In a weight judging contest among otherwise identical atoms of the same elements, some will be lighter and others heavier than average. But all the contenders, even the average ones, are really isotopes.
An isotope is an atom and every atom belongs to one and only one of the known chemical elements. As an atom, it is the smallest complete unit of an atomic element. The feature that makes an isotope different from other atoms o£ the same element is mass, or weight. And the differences in the weight of isotopes occur in precise units. A person can add 1 1/2 or 1 1/4 pound or ounce units to his weight. An isotope cannot. It varies from its atomic brethren by one or more complete mass units. Since the mass of an atomic particle does not vary, this should give us a hint about the overweight or underweight isotopes.
The mass of an atom is the total of its major nuclear particles. The masses of its orbiting electrons and of some assorted particles within its nucleus are so infinitesimal that our present system of measurements disregards them. The mass number of an atom comes down to the sum of its protons and neutrons. Carbon has been awarded the mass number of 12 and the scale of atomic mass numbers is based upon this. Compared with carbon, the atomic weight of chlorine is 34.453. Only a few elements have whole numbers for atomic weights.This fractional problem springs from neutron particles. We know that an atomic element has a constant number of protons. They give its atomic number in whole numbers. But not every atom in the same element has the same number of neutrons.
Every uranium atom has 92 protons and this element's atomic number is 92. Uranium 235 and uranium 238 both have 92 protons. U 235 has 143 neutrons and U 238 has three extra. The extra weight shows up in the mass number of U 238. You might class U238 as an isotope and expect U235 to be rated as a non isotope atom'. of uranium. But this is not so. Both atoms are isotopes of uranium. In most uranium samples, the heavier U238 atoms are more plentiful. In nature, lighter or heavier isotopes are present in samples of most elements. . As a rule, they are present in definite ratios.. The standard atomic weights of the elements take these ratios and weight variations into account. The atomic weight of' uranium is 233:03 with a fraction allowing for a ratio of other isotopes.
The mass number of any single isotope atom is a whole number. This is because its . weight variation depends upon one or more whole neutrons. Every neutron in every atom is alike and has the same weight. It weighs about as much as a positively charged proton, or 1,836 times as much as a negatively charged electron. But unlike these two oppositely charged particles, the neutron is neutral. It adds nothing to the electrical charge of an atom. But an extra neutron adds one unit of weight and one less neutron reduces the atom by exactly the same mass unit.
A sample of natural tin has an atomic weight of 118.69. Most of its atoms are tin 120; others range from 112 to 124. Some of these tin isotopes are lighter and others heavier than the average. In nature, most elements have extra isotopes and about 50 of them are top heavy enough to be radioactive. Science has created extra isotope atoms for all the natural elements. At least 700 man made isotopes are radioactive. Many of these are very useful, especially in biological studies of the life processes in plants and animals.