1. Calculating Atomic Mass
Year 11 Chemistry

2. 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

3. ISOTOPES Isotope Notation Mass number Element symbol
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 Element symbol
Atomic number
A- mass number, Z- atomic number

4. Isotope Example 1
Naturally occurring carbon is a mixture of three isotopes: C 14 6
Number of protons 6
Number of neutrons 7 8

5. 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

6. Isotope Example 2 Nitrogen-16
How many protons, neutrons, and electrons does this isotope of nitrogen have?
Protons: from atomic number of N = 7
7 protons
Neutrons: isotope number – atomic number (= 16-7)
9 neutrons
Electrons: number of protons = no. of electrons
7 electrons

7. 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
in this context, "decay" does not mean what it usually does. A radioisotope does not "rot";
There are over 1000 unstable isotopes, both naturally occuring and created in laboratories

8. 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 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 amu. An atom half as heavy is 6.00 amu.

9. Na 22.99 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

10. 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 __________

11. LecturePLUS Timberlake
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

12. Calculating Atomic Mass
Percent(%) abundance of isotopes
Mass of each isotope of that element
Weighted average =
mass isotope1(%) + mass isotope2(%) + …

13. Atomic Mass of Magnesium
Isotopes Mass of Isotope Abundance
24Mg = 24.0 amu %
25Mg = amu %
26Mg = amu %
Atomic mass (average mass) Mg = 24.3 amu Mg
24.3

14. 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?

15. Solution AT7 Atomic mass Ga = 69.7 amu
68.9 amu x = amu for 69Ga
100
Ga-71 (%/100)
70.9 amu x = amu for 71Ga
Atomic mass Ga = amu

16. 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?

17. Assign X and Y values: X = % 10B Y = % 11B Determine Y in terms of X X + Y = 100 Y = X Solve for X: X (10.0) + (100 - X )(11.0) = Multiply through by X X = 1080

18. Collect X terms 10.0 X X = X = -20 X = -20 = 20 % 10B Y = X % 11B = % = 80% 11B

19. Learning Check AT8 1) 30% 2) 70% 3) 100%
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%

20. Solution AT8
2) 70% Solution 62.9X = 64.9X = X = -140 X = 70%

21. Mixtures, Pure Substance and Isotopes

22. 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

23. Categories

24. Definitions Definition 1: Pure substance Definition 1: Mixture
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.

25. Homogeneous vs. Heterogeneous
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 non-uniform. The different components of the mixture can generally be seen.

26. Definitions Pure Substance: Mixture:
is homogeneous, ie, has uniform composition throughout the whole sample
its properties are constant throughout the whole sample
its properties do not depend on how it is prepared or purified
has constant chemical composition
Cannot be separated by physical means
Mixture:
consist of two or more different elements and/or compounds - physically intermingled
Can be separated by physical means

27. Pure Substances

28. Mixtures

29. 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.

30. Electron Arrangements in Atoms

31. 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

32. Electrons tend to ‘fill’ the lowest energy levels first
Each energy level (shell) can only accommodate a certain number of electrons
Electrons tend to ‘fill’ the lowest energy levels first 2
+ 6
5th 50 2
+ 6
4th 32
Energy increases
3rd 18 8
Fill in a way that makes the atom most stable, and in order to do this, the further away from the nucleus the harder it is to stabelise the electron levels.
Too many e- or not enough e- causes an atom to unstablize and results in ions
What ends up happening is the electrons try to partially stabilise the next outermost ‘shell’ before fully filling the current shell.
2nd 8 8
1st 2 2

33. 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

34. 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)

35. Activities Isotope worksheet Text Book P. 190, Ex 20 & 21

36. Glossary

37. Metals, Non-metals and Metaloids

38. 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

39. 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).