InSight
A to m s an d iso to p e s
Figure 2.3
Atoms and isotopes make up all matter. They are made of basic particles: protons, neutrons, and elec-
trons. The arrangement of these particles in the atom's structure dictates its chemical properties.
a.
Structure of
an
atom
• An atom is the smallest form of
an element that retains the
properties of that element.
• An atom is electrically neutral,
so the number of protons =
number of electrons.
• Atoms of different elements
have different numbers of
protons and electrons.
N ucleus
• The nucleus contains most of the
m ass of the atom.
Electron s
• Electrons orbit the nucleus in a
cloud. The electrons do not have
distinct locations, but they fill
discrete energy levels or “shells.”
(also known as the Bohr model)
^
Protons (p+)
(J> Neutrons (n0)
Electrons (e )
b.
A tom s of com m on elem ents In living th in gs
Each electron shell
can accommodate a
maximum number of
electrons (level 1: up to 2;
level 2: up to 8, and so
on). Each shell above the
first has sub-levels that fill
in a specific order.
• The number of electrons
in the outer (valence)
shell determines the
element's chemical
properties and reactivity.
First
electron
Second v
electron
9
shell
9
" . "X.
shell
g
9 9
1
9
Hydrogen (H)
Atomic number =1
M ass number =1
Carbon (C)
Atomic number = 6
M ass number =
12
Nitrogen (N)
Atomic number = 7
M ass number =
14
O xygen
(O)
Atomic number = 8
M ass number = 16
Third
electron
shell
9
9
9
9
9
9
9
11p0
) 9 9
9
9 9 9
9
O
18 n
9
9
9
«
9
9
Fourth
electron
shell
9
9
9
0 9 9 20,0
^ 9 9
9
9
Sodium (Na)
Atomic number = 11
M ass number =
23
Chlorine (Cl)
Atomic number = 17
M ass number = 35
Po tassium (K)
Atomic number = 19
M ass number =
39
Atomic number
= number of protons in an atom
M ass num ber = number of protons + number of neutrons in an atom (boldface indicates most common isotope)
Elements Are Made of Atoms
Each element is made of
atom s
, the smallest units of mat-
ter that retain the properties and characteristics of the el-
ement. For example, pure coal contains only carbon atoms
and oxygen gas contains only oxygen atoms.
An atom consists of two basic parts: a nucleus and one
or more electrons (
Figure 2.3 a
). The centrally located
nu cleu s
contains positively charged
protons
(p
+
)
and
uncharged (neutral)
neutrons (n
0
)
. Because each proton
has one positive charge, the nucleus is positively charged.
Ele ctro n s (e
-
)
are tiny, negatively charged particles that
move about in a large space surrounding the nucleus.
24 CHAPTER 2
Introductory Chemistry
They do not follow a fixed path or orbit but instead form
a negatively charged “cloud” that surrounds the nucleus.
The number of electrons in an atom equals the number
of protons. Because each electron carries one negative
charge, the negatively charged electrons and the positively
charged protons balance each other. As a result, each atom
is electrically neutral, meaning its total charge is zero.
The number of protons in the nucleus of an atom is
called the atom’s
atom ic n u m b er
. The atoms of each dif-
ferent element have a different number of protons in the
nucleus and, hence, a different atomic number (
Figure
2 .3 b
). The total number of protons plus neutrons in an
atom is its
m ass n u m b e r
. For instance, an atom of so-
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