Download Mixtures, Pure Substance and Isotopes

Calculating Atomic Mass
Year 11 Chemistry
Lesson Objectives
By the end of this lesson you should be able to…
• Solve isotope questions regarding their sub-atomic
particles, notation and nomenclature
• Calculate atomic mass of elements and their isotopes
• Define and understand the following terms
– Pure substance
–
–
–
–
Mixtures
Homogeneous
Heterogeneous
Isotopes
ISOTOPES
• Isotopes: are atoms of an element that differ in the
number of neutrons in their nuclei.
• All atoms of the one element have the same number
of protons and therefore the same atomic number.
Isotope Notation
Mass number
Atomic number
Element symbol
Isotope Example 1
• Naturally occurring carbon is a mixture of
three isotopes:
14
6
C
Number of
protons
6
6
6
Number of
neutrons
6
7
8
Isotope Nomenclature
• The name of an isotope of an element is
simply the name of the base element,
followed by the mass number of the isotope
Carbon-13
Isotope Example 2
• Nitrogen-16
• How many protons, neutrons, and electrons does this
isotope of nitrogen have?
Protons:
Neutrons:
Electrons:
from atomic number of N = 7
7 protons
isotope number – atomic number (= 16-7)
9 neutrons
number of protons = no. of electrons
7 electrons
Main types of Isotopes
• Two main types of Isotopes are stable or unstable
• Unstable Isotopes: known as radioisotopes and
are subject to radioactive decay.
– This ‘decay’ means that over time, these isotopes turn
into another isotope of the same element
• Stable isotopes: do not experience any change
over time
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.
8
Atomic Mass
Na
22.99
 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
9
Learning Check AT6
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
__________
10
Solution AT6
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__
LecturePLUS Timberlake
11
Calculating Atomic Mass
 Percent(%) abundance of isotopes
 Mass of each isotope of that element
 Weighted average =
mass isotope1(%) + mass isotope2(%) + …
100
100
12
Atomic Mass of Magnesium
Isotopes
24Mg
=
Mass of Isotope Abundance
24.0 amu
78.70%
25Mg
=
25.0 amu
10.13%
=
26.0 amu
11.17%
26Mg
Atomic mass (average mass) Mg = 24.3 amu
Mg
24.3
13
Learning Check AT7
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?
14
Solution AT7
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
=
69.7 amu
71Ga
100
Atomic mass Ga
15
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?
16
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) = 10.8
100
100
Multiply through by 100
10.0 X + 1100 - 11.0X = 1080
17
Collect X terms
10.0 X - 11.0 X = 1080 - 1100
- 1.0 X = -20
X
= -20
- 1.0
= 20 %
10B
Y = 100 - X
% 11B = 100 - 20% = 80% 11B
18
Learning Check AT8
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%
19
Solution AT8
2) 70%
Solution
62.9X + 6490 = 64.9X = 6350
-2.0 X = -140
X = 70%
20
Mixtures, Pure Substance and
Isotopes
Classification of Substances
• Chemists like lists, rules and categories
• As a result chemists like to classify matter
• Matter is classified based on its composition
• Two main classifications of matter
– Pure substances
– Mixtures
Categories
Definitions
Definition 1: Pure substance
Is homogeneous, – cannot be separated into simpler substances by any physical
processes
Definition 1: Mixture
A combination of two or more substances, where these substances are not bonded (or
joined) to each other and no chemical reaction occurs between the substances.
Homogeneous vs. Heterogeneous
Homo-geneous,
one - kind
.
Homogeneous mixtures are samples that contain more than one substance
(similarly to heterogeneous mixtures), but they're mixed uniformly. If you take
multiple samples, each one should be exactly the same. Often referred to as
solutions
Hetero-geneous
many - kind
.
Heterogeneous mixtures contain more than one substance but are nonuniform. The different components of the mixture can generally be seen.
Definitions
• Pure Substance:
o is homogeneous, ie, has uniform composition throughout the
whole sample
o its properties are constant throughout the whole sample
o its properties do not depend on how it is prepared or purified
o has constant chemical composition
o Cannot be separated by physical means
• Mixture:
o consist of two or more different elements and/or compounds physically intermingled
o Can be separated by physical means
Pure Substances
Mixtures
Mixtures: methods of separation
Separation technique
Property used for
separation
Example
Sifting (sieving)
Particle size
gold separated from soil particles using a sieve
Magnetic attraction
Magnetism
Magnetic iron separated from sulphur powder
Distillation
Boiling point
Ethanol separated from water as a result of ethanol’s
lower boiling point
Evaporation
Solubility and boiling point
Sodium chloride separated from water by evaporation
of water.
Electron Arrangements in Atoms
Electron Arrangements in Atoms
• Electrons in atoms exist in discrete energy
levels
• Electrons in energy level 1 have specific amounts of energy
and likewise each electron energy level 2 have a specific,
but higher, energy level than the electrons in level 1.
• The arrangement of electrons in energy levels
is called the electron configuration
• Each energy level (shell) can only
accommodate a certain number of electrons
• Electrons tend to ‘fill’ the lowest energy levels
first
2
Energy
increases
5th
50
4th
32
3rd
18
8
2nd
8
8
1st
2
2
2 + 6
+ 10
+ 6
+ 10
Stable Electron Configuration
It is the life mission of the elements to attain a
stable electron configuration
The most stable elements on the periodic table
are the noble gasses and so it is also referred to
as the nobles gas configuration
Valence Electrons and
Electron Configuration
• Electrons in the incompletely filled (highest) energy levels are known
as valence electrons
• The outermost energy level is known as the valence shell
Example: Nitrogen
Atomic number= 7
1st shell= 2/2
2nd shell= 5/8
Valence shell, with 5 valence electrons
Electron configuration:
Element symbol (orbital 1, orbital 2, orbital 3 etc.)
N (2, 5)
Activities
• Isotope worksheet
• Text Book
P. 190, Ex 20 & 21
P. 192, Ex 23 & 24
Glossary
Metals, Non-metals and Metaloids
Metals and Non-metals
Metals are elements which:
• Are solid at room temperature
• Have a shiny or lustrous appearance
• Are good conductors of heat and electricity
• Are malleable and ductile
Most other elements are called non-metals
Metalloids
There are some elements that have properties
of both metals and non-metals which makes
them difficult to classify and so they are called
metalloids (semi-metals).