Summary of the Atom
• atoms are the smallest particles that can be
uniquely associated with an element
• each element has unique atoms
• atoms are composed of e-, p and n
• atoms are electrically neutral (# of e- = # of p)
• for a single element, isotopes differ only in
number of n (neutrons)
• atoms have characteristic masses (atomic
weights)
• atoms combine with one another in definite,
whole number proportions to make compounds
~ 10-10 m
electron
Mass 9 x 10-31 kg
nucleus
Mass > 10-26 kg
~ 1 – 7 x 10-15 m (1 – 7 fermi)
The Spacious Atom
Microcosms of our solar system, atoms are dominantly
empty space:
electron orbits
If an oxygen atom
had a total radius of
100 km, the nucleus
would be a ~1 m
diameter sphere in
the middle.
Electrons in Orbit
In a simplistic model, electrons float around the nucleus in
energy levels called shells.
electron orbits
As the number of
electrons increases,
they start to fill shells
farther out from the
nucleus.
In most cases,
electrons are lost or
gained only from the
outermost shell.
Atom
Nucleus
The Nuclear Model
of the atom
Subatomic Particles
Particle
Symbol
Charge
Relative
Mass
Electron
e-
1-
0
Proton
p+
1+
1
Neutron
n
0
1
Atomic Number
Atomic Number
Symbol
11
Na
All atoms of an element have
the same number of protons
11 protons
Sodium
11
Na
Number of Electrons
 An atom is neutral
 The net charge is zero
 Number of protons = Number of electrons
 Atomic number = Number of electrons in a
neutral atom
Mass Number
Counts the number
of
protons and neutrons
in an atom
Atomic Symbols
 Show the mass number and atomic number
 Give the symbol of the element
mass number
23 Na
atomic number
11
sodium-23
Notation for Atoms
12C
only one isotope of carbon
13C
only one isotope of carbon
C
all isotopes of
carbon
Basic Definitions
• “atomic number” = number of protons in the
nucleus;
• “atomic weight” = average mass of an atom
calculated from the masses and natural
abundances of all isotopes
(use atomic weights to calculate the molecular
weights of compounds from their constituent
elements!)
• “mass number” = sum of protons + neutrons
in the nucleus
• “isotopic mass” = mass of a single isotope
More Atomic Symbols
16
O
31
P
65
8
15
30
15 p+
16 n
15 e-
30 p+
35 n
30 e-
8 p+
8n
8 e-
Zn
Isotopes
 Atoms with the same number of protons,
but different numbers of neutrons.
 Atoms of the same element (same atomic
number) with different mass numbers
Isotopes of chlorine
35Cl
37Cl
17
17
chlorine - 35
chlorine - 37
Mass spectrometry
Atomic weight measurements
How was the atomic weight measured?
• By mass spectrometry
– This also measures
% natural abundance
for a given isotope
Atomic weight calculation
There are three naturally occuring isotopes
of neon (Ne):
20Ne
21Ne
22Ne
isotopic mass = 19.99244018 amu
isotopic mass = 20.9938467 amu
isotopic mass = 21.9913855 amu
the atomic weight is reported in text as:
20.1797 amu
Learning Check 1
Naturally occurring carbon consists of three
isotopes, 12C, 13C, and 14C. State the number of
protons, neutrons, and electrons in each of
these carbon atoms.
12C
13C
14C
6
6
6
#P _______
_______
_______
#N _______
_______
_______
#E _______
_______
_______
Solution
12C
6
13C
14C
6
6
#P __6___
_ 6___
___6___
#N __6___
_ _7___
___8___
#E __6___
_ 6___
___6___
Learning Check 2
An atom of zinc has a mass number of 65.
A. Number of protons in the zinc atom
1) 30
2) 35
3) 65
B. Number of neutrons in the zinc atom
1) 30
2) 35
3) 65
C. What is the mass number of a zinc isotope
with 37 neutrons?
1) 37
2) 65
3) 67
Solution
An atom of zinc has a mass number of 65.
A. Number of protons in the zinc atom
1) 30
B. Number of neutrons in the zinc atom
2) 35
C. What is the mass number of a zinc isotope
with 37 neutrons?
3) 67
Learning Check 3
Write the atomic symbols for atoms with
the following:
A. 8 p+, 8 n, 8 e-
___________
B. 17p+, 20n, 17e-
___________
C. 47p+, 60 n, 47 e-
___________
Solution
16O
A. 8 p+, 8 n, 8 eB. 17p+, 20n, 17e-
8
37Cl
17
C. 47p+, 60 n, 47 e-
107Ag
47
Learning Check 4
An atom has 14 protons and 20 neutrons.
A. Its atomic number is
1) 14
2) 16
3) 34
B. Its mass number is
1) 14
2) 16
3) 34
C. The element is
1) Si
2) Ca
3) Se
D. Another isotope of this element is
1)
34X
16
2)
34X
14
3)
36X
14
Solution
An atom has 14 protons and 20 neutrons.
A. It has atomic number
1) 14
B. It has a mass number of
3) 34
C. The element is
1) Si
D. Another isotope of this element would be
3) 36X
14
Masses of Atoms
 A scale designed for atoms gives their small
atomic masses in atomic mass units (amu)
 An atom of 12C was assigned an exact mass of
12.00 amu
 Relative masses of all other atoms was
determined by comparing each to the mass of
12C
 An atom twice as heavy has a mass of 24.00
amu. An atom half as heavy is 6.00 amu.
Atomic Mass
Na
22.99
 Gives the mass of “average” atom of each
element compared to 12C
 Average atom based on all the isotopes and their
abundance %
 Atomic mass is not a whole number
Calculating Atomic Weight or
Mass
 Percent(%) abundance of isotopes
 Mass of each isotope of that element
 Weighted average =
mass isotope1(%) + mass isotope2(%) + …
100
100
Atomic Mass of Magnesium
Isotopes
24Mg
=
Mass of Isotope
24.0 amu
Abundance
78.70%
25Mg
=
25.0 amu
10.13%
26Mg
=
26.0 amu
11.17%
Atomic mass (average mass) Mg = 24.3 amu
Mg
24.3
Atomic mass calculation
How was the atomic mass calculated?
• multiply each isotopic mass by the
reported natural abundance for the
isotope, then:
• add these individual contributions for
each isotope to get the average atomic
mass for the element
Atomic mass calculation
There are three naturally occuring isotopes of
neon (Ne):
20Ne mass # = 19.99244018 amu (90.51%)
21Ne mass # = 20.9938467 amu
(0.27%)
22Ne mass # = 21.9913855 amu
(9.22%)
the atomic mass is reported in text as:
20.1797 amu
18.10 + 0.057 + 2.03 = 20.19 amu
Learning Check 5
Gallium is a metallic element found in
small lasers used in compact disc players.
In a sample of gallium, there is 60.2% of
gallium-69 (68.9 amu) atoms and 39.8% of
gallium-71 (70.9 amu) atoms. What is the
atomic mass of gallium?
Solution
Ga-69
68.9 amu x
60.2
=
41.5 amu for
69Ga
28.2 amu for
71Ga
100
Ga-71 (%/100)
70.9 amu x 39.8
=
100
Atomic mass Ga =
69.7 amu
Finding An Isotopic Mass
A sample of boron consists of 10B (mass
10.0 amu) and 11B (mass 11.0 amu). If
the average atomic mass of B is 10.8
amu, what is the % abundance of each
boron isotope?
Assign X and Y values:
X = % 10B
Y = % 11B
Determine Y in terms of X
X
+
Y
= 100
Y = 100 - X
Solve for X:
X (10.0) + (100 - X )(11.0)
100
100
= 10.8
Multiply through by 100
10.0 X + 1100 - 11.0X = 1080
Collect X terms
10.0 X - 11.0 X
=
1080 - 1100
- 1.0 X = -20
X
=
-20
- 1.0
=
Y = 100 - X
% 11B = 100 - 20% =
20 %
10B
80% 11B
Learning Check 6
Copper has two isotopes 63Cu (62.9 amu)
and 65Cu (64.9 amu). What is the %
abundance of each isotope? (Hint: Check
Zumdahl or any other chemistry text for
atomic mass)
1) 30%
2) 70%
3) 100%
Solution
2) 70%
Solution
62.9X + 6490 = 64.9X = 6350
-2.0 X = -140
X = 70%
Atomic Masses
13C
12C
13.00335 amu (1.11%)
12.0000 amu (98.89%)
atomic weight of C = 12.01115 amu WHY?
Calculating masses of
atoms relative to 12C
(mass of 12C atom) * 1.58320 = mass of F
atom
= 18.99840
reported atomic weight of F = 18.9984
Charged Atoms: Ions
Left to their own devices, atoms are electrically neutral.
That means that they have an equal number of
protons and electrons.
During the course of most natural events,
protons are not gained or lost, but electrons may be.
Atoms with more or fewer electrons than protons are
electrically charged. They are called ions:
an atom that loses electrons takes on a positive charge
(cation);
an atom that gains electrons takes on a negative charge
(anion).
An ISOTOPE is one of a set of nuclides with the same Z and
consequently different A. (ie isotopes are the same chemical
element but different masses). e.g.
12
6C
13
6C
14
6C
An ISOTONE is one of a set of nuclides with the same N
and consequently different A. e.g.
39
40
41
A
,
K
,
18 21 19 21 20 A 21
An ISOBAR is one of a set of nuclides with the same A but
different N and Z.
e.g
14
6C
14
7N
14
8O
• More on atomic notation, which is based on the nuclear
structure:
– Isotope: same Z, different A and N
– Isobar: same A, different Z and N
– Isotone: same N, different Z and A
Example: From the following list of atoms, which are isotopes, isobars, and isotones?
131
54
Component
Atom
Xe
I
Cs
I
Xe
130
53
A
I
132
55
Cs
Z
131
53
I
N