Untitled - Kelly Walsh High School

Atoms, Ions, and Molecules 19
table. They are numbered from 1 to 7. Elements in the same period have consecutive
atomic numbers but differ predictably in their chemical properties.
Groups or families are the vertical rows on the periodic table. They may be
labeled in two ways. The older way involves a Roman numeral and a letter,
either A or B. We call the groups labeled with an A the main-group elements,
while the B groups are the transition elements. Two horizontal groups, the inner
transition elements, belonging to periods 6 and 7 are normally pulled out of the
main body of the periodic table and are placed at the bottom of the table.
A newer way of labeling the groups is by consecutive number from left to right,
1 to 18. The older method is still very commonly used and is the labeling
method we prefer and will use in this book.
Chemists give four main-group families special names:
IA (1) group alkali metals
IIA (2) group alkaline earth metals
VIIA (17) group halogens
VIIIA (18) group noble gases
2-3 Chemical Formulas: Ions and
Molecules
Atoms may combine to form compounds. A compound is a combination of two
or more different elements in a specific ratio. We use a formula to represent this
compound. H 2O is a formula representing water, a compound composed of two
atoms of hydrogen and one atom of oxygen. An empirical formula shows the
atoms found in the compound and the lowest whole number ratio of those
atoms. The empirical formula of water would be H 2O. However, suppose
another compound of hydrogen and oxygen had an empirical formula of HO.
This empirical formula tells us that there is only hydrogen and oxygen in the
compound and the two atoms are in a 1:1 ratio. The molecular or true formula
shows what atoms we find in the compound and the actual number of each
atom. A molecular formula for the empirical formula HO might be H 2O 2, the
compound hydrogen peroxide. The structural formula shows both the kind and
actual number of atoms in the compound but also shows the bonding pattern.
We will show you more about structural formulas when you study molecular
geometry.
A compound may be formed in two major ways. If a metal is reacting with a nonmetal,
then the metal loses one or more electrons while the nonmetal gains those