Atomic Weight

The atomic weight represents the sum of the masses of the particles that make up the atom, protons, neutrons, and electrons. But since the mass of the electron is so small and essentially all the weight of the atom comes from the protons and neutrons, the atomic weight is considered to represent the sum of the masses of the protons and neutrons present in the atom. These weights were given in relative units called atomic mass units (abbreviated u or, in older notation, amu) in which the protons and neutrons have nearly equal masses. Consequently, the sum of the protons and neutrons in the nucleus would be the same as the atomic weight of the atom.

Today, a very sophisticated instrument, called a mass spectrometer, is used to obtain accurate measurements of atomic masses. In this instrument, atoms are vaporized and then changed to positively charged particles by knocking off electrons. These charged particles are passed through a magnetic field which causes them to be deflected different amounts, depending on the size of the charge and mass. The particles are eventually deposited on a detector plate where the amount of deflection can be measured and compared with the charge. Very accurate relative masses are determined in this way.

When atoms of various elements were analyzed with the mass spectrometer, scientists were surprised to find that not all atoms of the same element had exactly the same mass. Oxygen, for example, was found to exist in three different forms, each differing by one atomic mass unit or about the mass of one proton or one neutron. Since the number of protons in the nucleus was known because of their association with a +1 charge, the three different masses for oxygen had to be caused by different numbers of neutrons in the nucleus. Atoms of this type were called isotopes. Both the identity of the element (since the number of protons remains the same) and the chemical properties (since the electrons remain unchanged) are identical in isotopes of the same element. However, the mass is different because of the different number of neutrons in the nucleus, and this sometimes makes the atom unstable and radioactive. Radioactive isotopes are frequently used in research because the radioactivity can be followed using a Geiger counter. They can be administered to living systems like plants or animals and the isotope is observed as it moves and reacts throughout the system. Oxygen has three isotopes with masses of 16, 17, and 18 (often written as oxygen-16, oxygen-17, and oxygen-18 [¹⁶O, ¹²O, ¹⁸O]). Similarly, carbon exists as carbon-12, carbon-13, and carbon-14 and hydrogen as hydrogen-1, hydrogen-2, and hydrogen(¹²C, ¹³C, ¹⁴C)-3(H, ²H, ³H). Each of these successive isotopes have one more neutron in the nucleus than the preceding one.