Distinguishing Between Atoms
• Atomic Number: equals the number of protons
in the nucleus of an atom of that element
• Atoms are electrically neutral, therefore the
number of electrons must equal the number of
protons
• How is the number of electrons for a neutral
atom of a given element related to the atomic
number of that element?
– The atomic number is equal to the number of
electrons
Atoms of the First Ten Elements
Name
Symbol
Atomic
Number
Composition of the
Nucleus
Protons
Mass
Number
Neutrons
Number of
electrons
Hydrogen
H
1
1
0
1
1
Helium
He
2
2
2
4
2
Lithium
Li
3
3
4
7
3
Beryllium
Be
4
4
5
9
4
Boron
B
5
5
6
11
5
Carbon
C
6
6
6
12
6
Nitrogen
N
7
7
7
14
7
Oxygen
O
8
8
8
16
8
Fluorine
F
9
9
10
19
9
Neon
Ne
10
10
10
20
10
Element block
Review
• The element Nitrogen
has an atomic
number of what?
• 7
• How many protons
and how many
electrons are in a
neutral nitrogen
atom?
• 7, number of protons
equals the number of
electrons
How many protons and electrons are in each of
the following atoms?
•
•
•
•
•
•
•
•
•
•
Fluorine
9,8
9,9
8,9
8,8
Aluminum
12,13
12,12
13,12
13,13
•
•
•
•
•
Calcium
19,19
19,20
20,20
20,19
Complete the Table
Element
K
Atomic
Number
19
Protons
Electrons
19
5
16
23
Mass Number: the total number of
protons and neutrons in an atom
•
•
•
•
Mass # = protons + neutrons
Example: Oxygen
16 = 8 + n
n=8
The following is shorthand for writing the # of protons
and neutrons, you don’t need to write the # of
electrons because they are the same as the # of
protons
The large "X" represents where
you will find the atom's
elemental symbol. The mass
number, which is given the
symbol "A", is located in the
upper left-hand corner. The
atomic number (also called
nuclear charge), which is given
the symbol "Z", is found in the
lower left-hand corner.
Number of neutrons = mass number − atomic number
Number of neutrons = mass number − atomic number
• In the case of neon (Ne):
• In the case of carbon (C):
Number of neutrons = 20- 10 = 10
Number of neutrons = 12- 6 = 6
More Examples
Elemental Symbol
# of Protons
(equal to Z)
3
9
# of Neutrons
(equal to A-Z)
4
10
# of electrons
(equal to Z modified by
charge number)
3
9
How many protons, electrons, and neutrons
are in the following atoms?
Atomic Number
Mass Number
(a) Beryllium (Be)
4
9
(b) Neon (Ne)
10
20
(a) Sodium (Na)
11
23
How many neutrons are in each atom?
16 O
8
32 S
16
•
•
•
• 32
• 48
• 16
8
16
24
Express the composition of the following
atom in shorthand form
Carbon-12
Fluorine-19
•
•
12 C
6
9F
19
•
6C
12
•
28 F
9
•
18 C
12
•
28 F
19
•
19 F
9
Isotopes: atoms that have the same number of
protons but different numbers of neutrons
• Isotopes have different mass numbers
because they have different numbers of
neutrons
• They are chemically alike because they
have identical numbers of protons &
electrons, which are the subatomic
particles responsible for chemical
behavior.
Example:
Hydrogen has three isotopes; each isotope has 1
proton in its nucleus.
Two isotopes of carbon are carbon 12 and carbon
13. Write the symbol for each isotope using
superscripts and subscripts to represent mass #
and atomic number
Carbon-12
___
___C
Carbon-13
___
___C
Determine the number of protons, neutrons,
and electrons for each of the following
isotopes of Neon
Neon-20
•
•
•
22
10
11
•
Neon-21
•
•
•
21
10
11
Neon-22
• 22
• 10
• 12
Atomic Mass
Properties of Subatomic Particles
Particle
Symbol
Relative electrical
charge
Relative mass
(Mass of proton =1)
Actual mass (g)
Electron
e-
1-
1/1840
9.11x10-28
Proton
P+
1+
1
1.67x10-24
Neutron
n◦
0
1
1.67x10-24
Actual mass of a proton or a neutron is very small (1.67 x10-24 g)
Since about the 1920s, it has been possible to determine these
masses (protons & neutrons) by using a mass spectrometer
• This is impractical to work with, so a reference isotope
as a standard is used. The chosen isotope is C-12. It
was assigned a mass of EXACTLY 12 atomic mass units
or amu
• Carbon-12 has 6 protons and 6 neutrons, making up its
mass
• Since the mass of an atom mainly depends on the
number of protons and neutrons, shouldn’t the atomic
mass of an element be a whole number? That’s usually
not the case, Cl is 35.453 amu
• The reason for this involves the relative abundance of
the naturally occurring isotopes of the element. Most
occur as a mixture of two or more isotopes
• Each isotope has a fixed mass and a natural percent
abundance
The atomic mass is a weighted average of
all naturally occurring isotopes
• http://www.sisweb.com/referenc/source/ex
actmaa.htm
Example: Chlorine
35 Cl
17
76%
and 37 Cl
17
24%
Three more 35 Cl for every one 37 Cl
17
17
(Remember: the # of protons and neutrons = atomic mass)
% abundance:
# of protons = 17 + 17+ 17+ 17 = 68
# of neutrons = 18 +18 +18 +20 = 74
68 + 74 = 35.5 amu
4
Therefore, the Atomic
Mass is a weighted
average mass of the
atoms in a naturally
occurring sample of the
element
Review
Which isotope of copper is more abundant:
copper – 63 or copper – 65?
(the atomic mass of copper is 63.546 amu)
• Copper-63
• Boron has two isotopes: boron-10 and
boron-11, which is more abundant
(atomic mass is 10.81 amu)?
• Boron -11
• Element X has two natural isotopes. The
isotope with a mass of 10.012 amu (10X)
has a relative abundance of 19.91%. The
isotope with a mass of 11.009 amu (11X)
has a relative abundance of 80.09%.
Calculate the atomic mass of this element.
• 10X = 10.012 x 0.1991 = 1.993 amu
• 11X = 11.009 x 0.8009 = 8.817 amu
10.810 amu
• What is this element?
• Boron
Calculate the atomic mass of Bromine. The
two isotopes of bromine have atomic
masses and relative abundances of 78.92
amu (50.69%) and 80.92 amu (49.31%)
• 79.91 amu