2.35. Atomic Number http://www.ck12.org
2.35 Atomic Number
- Define atomic number.
- Explain what mass number represents.
Athletes wearing the same-colored jerseys are all on the same team. In addition, each player’s jersey has a unique
number to distinguish him from his teammates. Imagine how confusing it would be if members of both teams wore
the same-colored jerseys or all the members of a team had the same number on their jerseys. How could you tell the
athletes apart?
Telling Atoms Apart
It’s often useful to have ways to signify different people or objects like athletes on teams. The same is true of
atoms. It’s important to be able to distinguish atoms of one element from atoms of other elements. Elements are
pure substances that make up all other matter, so each one is given a unique name. The names of elements are also
represented by unique one- or two-letter symbols, such as H for hydrogen, C for carbon, and He for helium. You
can see other examples in theFigure2.65.
Q:The table shown in theFigure2.65 is called the periodic table of the elements. Each symbol stands for a different
element. What do you think the symbol K stands for?
A:The symbol K stands for the element potassium. The symbol comes from the Latin name for potassium, which
iskalium.
The symbols in theFigure2.65 would be more useful if they revealed more information about the atoms they
represent. For example, it would be useful to know the numbers of protons and neutrons in the atoms. That’s where
atomic number and mass number come in.