always has 11. To restate this, all the atoms of a particular element have the exact same
number of protons, and the number of protons is that element’satomic number.
Figure 2.7: An atom of Helium (He) contains two positively charged protons (red), two
uncharged neutrons (green), and two negatively charged electrons (yellow). ( 17 )
The Periodic Table
Each element also has unique properties, such as density, boiling point, and how well it
dissolves (”solubility”). Densityis the mass of the substance per unit of volume. That
means that if you take an equal volume of different elements, each different sample will
weigh a different amount. For example, a liter of the metal mercury weighs 13 times as
much as a liter of water. Theboiling pointis the temperature at which an element will
change from a liquid to a gas. For example, the boiling point of water is 100 degrees Celsius.
Once you heat water to this temperature, you see bubbles form as the water turns into
vapor. Each element has a different boiling point. Solubilityis how well a substance will
dissolve in water. You can dissolve more sugar in a liter of water than salt, because sugar
is more soluble than salt. Density, boiling point, and solubility have unchanging values for
each element.
In 1869, Dmitri Mendeleev constructed the periodic table in 1869, organizing all the elements
according to their atomic number, density, boiling point, solubility, and other values. As
mentioned above, each element has a one or two letter symbol. For example, H stands for
hydrogen and Au for gold. The vertical columns in the periodic table are known as groups
and elements in groups tend to have very similar properties. The table is also divided into
rows, known as periods.
Group 1 (seeFigure2.8) contains the highly reactive metals, such as sodium (Na) and
lithium (Li). Just a small amount of these metals will explode into flames when put into
water. Another group are the less-reactive metals, such as gold (Au) and platinum (Pt).
Since they will not react readily with air and tarnish, these metals are highly valued for