Chemical Symbols Atomic and Mass Numbers

Two very important numbers, the atomic number and the mass number, tell you much of what you need to know about an atom. Chemists tend to memorize these numbers like baseball fans memorize batting averages, but clever chemistry students like you need not resort to memorization. You have the ever-important periodic table of the elements at your disposal. We discuss the logical structure and organization of the periodic table in detail in Chapter 4, so for now, we simply explain what the atomic and mass numbers mean without going into great detail about their consequences. 

Atomic numbers are like name tags They identify an element as carbon, nitrogen, beryllium, and so on by telling you the number of protons in the nucleus of that element. Atoms are known by the numbers of their protons. Adding a proton or removing one from the nucleus of an atom changes the elemental identity of an atom. 

In the periodic table, you find the atomic number above the one- or two-letter abbreviation for an element. The abbreviation is the element s chemical symbol. Notice that the elements of the periodic table are lined up in order of atomic number, as if they ve responded to some sort of roll call. Atomic number increases by 1 each time you move to the right in the periodic table when a row ends, the sequence of increasing atomic numbers begins again at the left side of the next row down. You can check out the periodic table for yourself in Chapter 4. 

The second identifying number of an atom is its mass number. The mass number reports the mass of the atom s nucleus in atomic mass units (amu). Because protons and neutrons have a mass of 1 amu each (as you find out earlier in this chapter), the mass number equals the sum of the numbers of protons and neutrons  

you may wonder, don t we care about the mass of the electrons Is some sort of insidious subatomic particle-ism afoot No. An electron has only 1/1,836 of the mass of a proton or neutron, so to make mass numbers nice and even, chemists have decided to conveniently forget that electrons have mass. Although this assumption is not, well, true, the contributions of electrons to the mass of an atom are so small that the assumption is usually harmless. Electron mass is accounted for at the upper levels of chemistry, however, so don t worry. 

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