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The Atom
Atomic Number and Mass Number
Isotopes
Three fundamental parts of an atom are:
protons electrons neutrons
Structure of the atom is:
protons and neutrons are found in the nucleus electrons are located outside of the nucleus
Atomic Theory
Atoms are building blocks of elements
Similar atoms in each element
Different from atoms of other elements
Two or more different atoms bond in simple
ratios to form compounds
Subatomic Particles
There are three subatomic particles that are of
interest to chemists. These particles define the
nature of atoms and how they are put together.
These particles are:
• Neutrons
• Protons
• Electrons
Properties of Subatomic Particles
Particle Mass, gRelative mass to
C-12, amu
Charge, Coulombs
Unit Charge
proton 1.6726 x 10-24 1.00728 1.67 x 10-19 +1
electron 9.1096 x 10-28 0.000549 -1.67 x 10-19 -1
neutron 1.6749x 10-24 1.00867 0 0
Subatomic Particles
Particle Symbol Charge RelativeMass
Electron e- 1- 0
Proton p+ + 1
Neutron n 0 1
Location of Subatomic Particles
10-13 cm
electrons
protons
neutrons
10-8 cm
nucleus
Representation of Atoms
XAZElementSymbol
Mass Number
Atomic Numbe
r
Atomic Number
Counts the number
of
protons
in an atom
Periodic Table
Represents physical and chemical
behavior of elements
Arranges elements by increasing atomic
number
Repeats similar properties in columns
known as chemical families or groups
Periodic Table
1 2 3 4 5 6 7 8
11 Na
Atomic Number on the Periodic Table
11
Na
Atomic Number
Symbol
All atoms of an element have the same number of protons
11
Na
11 protons
Sodium
Counting Protons
State the number of protons for atoms of each of the following:
A. Nitrogen
1) 5 protons 2) 7 protons 3) 14 protons
B. Sulfur
1) 32 protons 2) 16 protons 3) 6 protons
C. Barium
1) 137 protons 2) 81 protons 3) 56 protons
Solution
State the number of protons for atoms of each of the following:
A. Nitrogen
2) 7 protons
B. Sulfur
2) 16 protons
C. Barium
3) 56 protons
Number of Electrons
An atom is neutral The net charge is zeroNumber of protons = Number of electronsAtomic number = Number of electrons
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 sodium-23
atomic number 11
More Atomic Symbols
16 31 65
O P Zn
8 15 30
8 p+ 15 p+ 30 p+
8 n 16 n 35 n8 e- 15 e- 30 e-
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 37Cl17 17
chlorine - 35 chlorine - 37
Counting Protons, Neutrons & Electrons
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 13C 14C 6 6 6
#P __6___ _ 6___ ___6___
#N __6___ _ _7___ ___8___
#E __6___ _ 6___ ___6___
Protons, Neutrons & Electrons
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
Atomic Symbols
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
16OA. 8 p+, 8 n, 8 e- 8
B. 17p+, 20n, 17e- 37Cl 17
C. 47p+, 60 n, 47 e- 107Ag 47
Identifying Elements
An atom has 14 protons and 20 neutrons.A. Its atomic number is
1) 14 2) 16 3) 34
B. Its mass number is1) 14 2) 16 3) 34
C. The element is1) Si 2) Ca 3) Se
D. Another isotope of this element is
1) 34X 2) 34X 3) 36X 16 14 14
Solution
An atom has 14 protons and 20 neutrons.A. It has atomic number
1) 14
B. It has a mass number of3) 34
C. The element is1) 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
Listed on the periodic table
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
Na22.99
Atomic Mass
Using the periodic table, specify the atomic mass of each element (round to the tenths place):
A. calcium __________
B. aluminum __________
C. lead __________
D. barium __________
E. iron __________
Solution
Using the periodic table, specify the atomic mass of each element (round to the tenths place):
A. calcium _40.1 amu _
B. aluminum _27.0 amu _
C. lead _207.2 amu_
D. barium _137.3 amu_
E. iron _55.8 amu__
Calculating Atomic 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 Mass of Isotope Abundance 24Mg = 24.0 amu 78.70%
25Mg = 25.0 amu 10.13%
26Mg = 26.0 amu 11.17%
Atomic mass (average mass) Mg = 24.3 amu
Mg24.3
CalculatingAtomic Mass
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
100
Ga-71 (%/100)
70.9 amu x 39.8 = 28.2 amu for 71Ga
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 XX + Y = 100Y = 100 - X
Solve for X:X (10.0) + (100 - X )(11.0) = 10.8
100 100
Multiply through by 10010.0 X + 1100 - 11.0X = 1080
Collect X terms
10.0 X - 11.0 X = 1080 - 1100
- 1.0 X = -20
X = -20 = 20 % 10B
- 1.0
Y = 100 - X
% 11B = 100 - 20% = 80% 11B
Isotopes of Copper
Copper has two isotopes 63Cu (62.9 amu) and 65Cu (64.9 amu). What is the % abundance of each isotope? (Hint: Check periodic table 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%
Isotopes of chlorine
Naturally occurring chlorine consists of 75.53% of the isotope with mass number 35 and 24.47% of the isotope with mass number 37. What is the average atomic mass of naturally occurring chlorine?
electronsneutronsprotonsCl
electronsneutronsprotonsCl
172017
171817
3717
3517
Solution
(0.7553)(34.97 u) = 26.41 u(0.2447)(36.95 u) = 9.04 u
Atomic mass of Cl = 35.45 u
Where Are the Electrons Located?
This question becomes important since the nuclear model of the atom proposed by Rutherford accounted only for the location of the positively charged particles. The location of the electrons is the key to understanding chemistry.