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Chapter 3
Atoms: The Building Blocks of Matter
3.3 Counting Atoms
Objectives
• Define atomic number and mass number, and describe how they apply to
isotopes.
• Given the identity of a nuclide, determine its number of protons, neutrons, and
electrons.
• Explain what isotopes are.
3.3 Counting Atoms
Atomic Number
• Atoms of different elements have different numbers of protons.
• Atoms of the same element all have the same number of protons.
• The atomic number (Z) of an element is the number of protons of each atom of
that element.
3.3 Counting Atoms
Atomic Number
• The number of protons in each atom identifies it as an atom of a particular element
• Each atom has a unique number of protons and, in effect, the number of protons
cannot change for a particular atom
• The Periodic Table is organized by increasing atomic number (left-to-right and
top-to-bottom)
• All atoms are neutral, so the # protons = the # of electrons because the number of
positive charges equals the number of negative charges
• Atomic # = # of protons = # electrons
• For Helium, the atomic # is 2. So, we know that Helium also has 2 protons
and 2 electrons
3.3 Counting Atoms
APE MAN
• Protons; p+
• Electron; e• Neutron; n
A P E
Atomic # = # of p+ = # of eM A N
Atomic Mass – Atomic # = # n
3.3 Counting Atoms
Isotopes
• All atoms of a particular element have the same
number of protons and electrons
• The number of neutrons may differ; the same
element will have different a mass
• In nature most elements are found as a mixture of
isotopes
• Isotopes are atoms of the same element that have
different masses
3.3 Counting Atoms
Designating Isotopes
• Hyphen Notation: The mass number is written with a hyphen after the name of the
element.
uranium-235
• Isotope Notation: The superscript indicates the mass number and the subscript
indicates the atomic number.
235 U
92
3.3 Counting Atoms
Designating Isotopes, continued
• The number of neutrons is found by subtracting the atomic number from the
mass number.
•
mass number − atomic number = number of neutrons
•
235 (protons + neutrons) − 92 protons = 143 neutrons
• Nuclide is a general term for a specific isotope of an element.
3.3 Counting Atoms
Isotopic Notation Practice
Isotope
Notation
Hyphen
Notation
Atomic #
# p+
#n
56
76
3 H
1
Fluorine - 20
24 Si
14
Mass #
# e-
3.3 Counting Atoms
Potassium Isotope
• 93.25% of K atoms have 20 neutrons
• 6.7302% of K atoms have 22 neutrons
• .0117% of K atoms have 21 neutrons
Potassium-39
Potassium-40
Potassium-41
p+
19
19
19
n
20
21
22
e-
19
19
19
3.3 Counting Atoms
Mass of Individual Atoms
• The atomic mass of an element is the weighted average mass of the all the known
isotopes of that element
• To determine the weighted average atomic mass of an element, calculate the mass
contribution of each of the isotopes and add the values together
3.3 Counting Atoms
Average Atomic Mass
35 Cl
17
37
17Cl
Atomic mass: 34.969 amu
Atomic mass: 36.966 amu
% abundance: 75.770%
% abundance: 24.230%
Mass
:
(34.969)(.75770)= 26.496 amu
Mass:
(36.966)(.24230)=8.957 amu
Average atomic mass of chlorine =
(26.496 amu + 8.957 amu) = 35.453 amu
3.3 Counting Atoms
Steps to Solving Weighted Averages of Isotopes
1. Turn the % into a decimal for the % abundance.
2. Multiply the decimal by the atomic mass of each particular isotope.
3. Add the masses of the isotopes together.
4. Your answer will be in amu (atomic mass units).
3.3 Counting Atoms
Practice 1
Calculate the atomic mass of Magnesium. The three Magnesium isotopes have
atomic masses and relative abundances of 23.985 amu (79.99%), 24.986 amu
(10.00%), and 25.982 amu (11.01%).
3.3 Counting Atoms
Practice 2
What is the atomic mass of this sample of Silver?
107 Ag
106.905 amu
52 %
109 Ag
108.905 amu
48 %
3.3 Counting Atoms
Practice 3
What is the average atomic mass of this sample of Hydrogen?
1H
12 %
2H
48 %
3H
40 %