The Element Keys Each element is repre- sented on the periodic table by a box called the element key. An enlarged key for hydrogen is shown in Figure 4. An element key shows you the name of the element, its atomic num- ber, its symbol, and its average atomic mass. Element keys for elements that occur naturally on Earth include a logo that tells whether the element is a solid, a liquid, or a gas at room temperature. All the gases except hydrogen are on the right side of the table. They are marked with a balloon logo. Most of the other elements are solids at room temperature and are marked with a cube. Two elements on the periodic table are liquids at room temperature. Their logo is a drop. Elements that do not occur naturally on Earth are marked with a bull’s-eye logo. These are synthetic elements. Element Atomic number Symbol Atomic mass
Figure 4 As you can see from the element key, a lot of informa- tion about an element is given on the periodic table. Identify the two elements that are liquids at room temperature.
What does periodic mean in the periodic table?
Ratio of atoms
E lements often combine with oxygen to form oxides and chlorine to form chlorides. For exam- ple, two hydrogen atoms combine with one oxygen atom to form oxide, H2O or water. One sodium atom combines with one chlorine atom to form sodium chloride, NaCl or table salt. The location of an element on the periodic table is an indication of how it combines with other elements. Periodicity 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 H He Li Be B C N O F NeNa MgAl Si P S Cl Ar K Ca Oxides Atomic number/Element Chlorides
Identifying the Problem The graph shows the number of oxygen atoms (red) and chlorine atoms (green) that will combine with the first 20 elements. What pattern do you see?
Solving the Problem
Find all of the elements in Group 1 on the graph. Do the same with the elements in Groups 14 and 18. What do you notice about their positions on the graph?
This relationship demonstrates one of the properties of a group of elements. Follow the elements in order on the periodic table and on the graph. Write a statement using the word periodic that describes what occurs with the elements and their properties.
SECTION 1 Introduction to the Periodic Table K ◆