Chemistry Reference
In-Depth Information
Snapshot Review
❒
The periodic table was originally developed using atomic masses and
chemical and physical properties of the elements, but it is now known
that the atomic number is the real basis of the periodic table.
A. How did Mendeleyev know to leave spaces in his periodic table for
gallium and germanium?
Key Terms
Key terms are defined in the Glossary.
atom (3.2)
atomic mass (3.3, 3.4)
atomic mass scale (3.4)
atomic mass unit (3.4)
atomic number (3.3)
atomic weight (3.4)
atomic weight scale (3.4)
Dalton's atomic theory (3.2)
electron (3.3)
isotope (3.3)
law of constant composition (3.1)
law of definite proportions (3.1)
law of multiple proportions (3.1)
mass number (3.3)
molecule (3.2)
neutral (3.3)
neutron (3.3)
nucleus (3.3)
percent (3.1)
postulate (3.2)
proportion (3.1)
proton (3.3)
relative scale (3.4)
subatomic particle (3.3)
weighted average (3.4)
Symbols/Abbreviations
A
(mass number) (3.3)
n (number of neutrons) (3.3)
p (number of protons) (3.3)
Z
(atomic number) (3.3)
amu (atomic mass unit) (3.4)
Summary
L
avoisier discovered the law of conservation of matter,
which states that matter cannot be created or destroyed
during chemical reactions or physical changes. This gen-
eralization increased chemists' efforts to measure the
masses of elements in compounds and resulted in two
more laws. The law of definite proportions states that the
percentage of each element in any sample of a pure com-
pound is always the same. According to the law of multi-
ple proportions, if the mass of one of the elements in two
or more compounds of the same elements is held con-
stant, the masses of each other element form a small,
whole-number ratio. (Section 3.1)
Dalton suggested that the elements are composed of
indivisible atoms and that the atoms of each element have
a characteristic mass, different from the mass of any other
element. He stated that the atoms combine to form mole-
cules when the elements combine to form compounds.
These postulates explained the laws of chemical combi-
nation known at that time, but most of them have been
amended in light of later discoveries. However, the atom
is still considered to be the fundamental particle of an
element. (Section 3.2)
In the past 200 years, numerous experiments have
shown that the atom is not indivisible but is composed of
electrons plus a nucleus containing protons and neutrons.
The nucleus does not change in any chemical reaction.
The characteristics of the subatomic particles (Table 3.1)
should be memorized. The number of protons, called the
atomic number, governs the number of electrons in the
neutral atom. The sum of the numbers of protons and neu-
trons is called the mass number. All atoms of a given
element have the same atomic number, which differs
from the atomic numbers of other elements. Different
atoms of the same element may have different numbers of
neutrons and thus different mass numbers. Such atoms
are
isotopes
of each other. An isotope is identified by the
symbol of the element, with the mass number as a super-
script to the left. (Section 3.3)
