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these elements conduct electric currents under certain condi-
1–
2– 2– tions. The ability to conduct an electric current is a property of
3+ 3+ + e a metal, and nonmalleability is a property of nonmetals, so as
you can see, these semiconductors have the properties of both
3+ metals and nonmetals.
2–
A The transition elements, which are all metals, are located
Li Li + 1+ in the B-group families. Unlike the representative elements,
which form vertical families of similar properties, the transi-
tion elements tend to form horizontal groups of elements with
similar properties. Iron (Fe), cobalt (Co), and nickel (Ni) in
7– 8–
2– 2– group VIIIB, for example, are three horizontally arranged
9+ + e 9+ metallic elements that show magnetic properties.
A family of representative elements all form ions with the
same charge. Alkali metals, for example, all lose an electron to
9+ form a 1+ ion. The transition elements have variable charges.
10–
B F F – 1– Some transition elements, for example, lose their one outer
electron to form 1+ ions (copper, silver). Copper, because of
FIGURE 8.20 (A) Metals lose their outer electrons to acquire its special configuration, can also lose an additional electron
a noble gas structure and become positive ions. Lithium becomes to form a 2+ ion. Thus, copper can form either a 1+ ion or
a 1+ ion as it loses its one outer electron. (B) Nonmetals gain a 2+ ion. Most transition elements have two outer s orbital
electrons to acquire an outer noble gas structure and become
electrons and lose them both to form 2+ ions (iron, cobalt,
negative ions. Fluorine gains a single electron to become a 1– ion.
nickel), but some of these elements also have special configu-
rations that permit them to lose more of their electrons. Thus,
iron and cobalt, for example, can form either a 2+ ion or
semi conductors (or metalloids). Silicon, germanium, and arse- a 3+ ion. Much more can be interpreted from the periodic
nic have physical properties of nonmetals; for example, they are table, and more generalizations will be made as the table is
brittle materials that cannot be hammered into a new shape. Yet used in the following chapters.

SUMMARY

Attempts at understanding matter date back to ancient Greek philoso- isotopes. The mass of each isotope is compared to the mass of carbon-12,
phers, who viewed matter as being composed of elements, or simpler which is assigned a mass of exactly 12.00 atomic mass units. The mass
substances. Two models were developed that considered matter to be contribution of the isotopes of an element according to their abundance
(1) continuous, or infinitely divisible, or (2) discontinuous, made up of is called the atomic weight of an element. Isotopes are identified by their
particles called atoms. mass number, which is the sum of the number of protons and neutrons
In the early 1800s, Dalton published an atomic theory, reasoning in the nucleus. Isotopes are identified by their chemical symbol with the
that matter was composed of hard, indivisible atoms that were joined atomic number as a subscript and the mass number as a superscript.
together or dissociated during chemical change. Bohr developed a model of the hydrogen atom to explain the
Cathode rays were observed to move from the negative terminal characteristic line spectra emitted by hydrogen. His model specified
in an evacuated glass tube. The nature of cathode rays was a mystery. that (1) electrons can move only in allowed orbits, (2) electrons do
The mystery was solved in 1897 when Thomson discovered they were not emit radiant energy when they remain in an orbit, and (3) elec-
negatively charged particles now known as electrons. Thomson had trons move from one allowed orbit to another when they gain or
discovered the first elementary particle of which atoms are made and lose energy. When an electron jumps from a higher orbit to a lower
measured their charge-to-mass ratio. one, it gives up energy in the form of a single photon. The energy of
Rutherford developed a solar system model based on experiments the photon corresponds to the difference in energy between the two
with alpha particles scattered from a thin sheet of metal. This model had levels. The Bohr model worked well for hydrogen but not for other
a small, massive, and positively charged nucleus surrounded by moving atoms.
electrons. These electrons were calculated to be at a distance from the De Broglie proposed that moving particles of matter (electrons)
nucleus of 100,000 times the radius of the nucleus, so the volume of an should have wave properties like moving particles of light (photons).
atom is mostly empty space. Later, Rutherford proposed that the nucleus His derived equation, λ = h/mv, showed that these matter waves were
contained two elementary particles: protons with a positive charge and only measurable for very small particles such as electrons. De Broglie’s
neutrons with no charge. The atomic number is the number of protons in proposal was tested experimentally, and the experiments confirmed
an atom. Atoms of elements with different numbers of neutrons are called that electrons do have wave properties.

8-19 CHAPTER 8 Atoms and Periodic Properties 221