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2
Atoms and the
Atomic Theory
Section Objectives
1. List the numbers of protons, neutrons, and electrons present in atoms and ions,
using the symbolism A
ZE.
The complete symbol for an atom or ion consists of the elemental symbol surrounded by
subscripts and superscripts.
a) The leading superscript (upper left) is the mass number. This is also the number
of nucleons; a nucleon is a proton or a neutron.
b) The leading subscript (lower left) is the atomic number or proton number.
c) The trailing superscript (upper right) is the charge or the number of protons
(atomic number) minus the number of electrons. The sign (+ or -) always must be
included. The number is zero for a neutral atom, but the zero is written only for
Mass
emphasis.
number
Charge
235 U3
92
Atomic number
EXAMPLE 2-1
What is the atomic number, mass number, and charge of 19 F  ?
19 F  has an atomic number of 9; a mass number of 19; and a charge of –1.
2. Use the periodic table to predict the charges of ions of main group elements.
Elements in the same column have similar properties. Each column is referred to as a
periodic family or group. The horizontal rows are called periods. Elements on the right
side of the periodic table are nonmetals; they form anions, or negatively charged ions.
Elements on the left side of the periodic table are metals; they form cations, or positively
charged ions. Elements within the same group will form ions with the same charge.