John Rogers, age 13, of San Diego, Calif., for his question:
What is meant by atomic weight?
Maybe you have seen a list of the chemical elements beginning with hydrogen. Each element is given an atomic number and an atomic weight. Hydrogen has atomic number one and atomic weight 1.0080. Next comes helium with atomic number two and atomic weight 4.002602. Next comes lithium with atomic number three and atomic weight 6.940.
The list continues with the atomic numbers increasing one by one and the atomic weights increasing bit by bit. The heaviest atoms are the ones with the largest atomic numbers. There are 92 different atoms or chemical elements in nature. The atomic scientists have recently discovered more and brought the list to over 100. The newly found atoms are the heavy ones with high numbers at the end of this list.
The atomic number refers to the number of proton particles in the nucleus of an atom. Each atom has an equal number of proton and electron particle Hydrogen, atomic number one, has but one proton. Helium, atomic number two has two protons. Oxygen, important when it comes to atomic weight, has atomic number eight.
We would expect an atom with eight particles to weigh more than an atom with, say, two particles. This seems to be so since the elements with high atomic numbers are the ones with high atomic weight.
It was decided to use oxygen as a basis for all the atomic weights. Oxygen is a busy element, always ready to enter into chemical compounds with other elements. It is, therefore, an easy element to check, measure and weigh. So oxygen was given the atomic weight of 16 units. This gives the standard atomic weight for all other elements.
The atomic number gives only the number of protons in an element. But atoms are composed of other particles besides protons. There are neutrons and a whole host of newly discovered smaller fragments. All this mass must be included in the total atomic weight. And a neutron weighs slightly more than a proton.
The atomic weight, then, gives a total picture of an atom, since all the particles, large and small, are bulked together. The atomic number gives only the number of protons, or electrons, The atomic weight includes the protons, an equal number of tiny electrons; one or more heavy neutrons and a number of other atomic fragments.
The atomic weight is usually a figure slightly more than twice as large as the atomic number. Nickel, atomic number 28, has an atomic weight of 58.69. Tin, atomic number 50, has an atomic weight of 118.70.
This difference increases with the bigger atoms. Gold, atomic number 75, has atomic weight 197.2. And the large, radioactive atoms are heaviest of all. Radium, number 88, has atomic weight 226.05. Ordinary uranium, 92, has atomic weight 238.07. The more protons an atom has the more neutrons and other particles it seems to have.