Calculating Atomic Mass

Year 11 Chemistry

Lesson ObjectivesBy 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 Element symbol

Atomic number

Isotope Example 1

• Naturally occurring carbon is a mixture of three isotopes:

C146

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: from atomic number of N = 7 7 protons

Neutrons: isotope number – atomic number (= 16-7)9 neutrons

Electrons: number of protons = no. of electrons7 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.

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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

9

Na22.99

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 __________

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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

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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

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Mg24.3

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?

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Solution AT7

Ga-69

68.9 amu x 60.2 = 41.5 amu for 69Ga 100Ga-71 (%/100)

70.9 amu x 39.8 = 28.2 amu for 71Ga 100

Atomic mass Ga = 69.7 amu

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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?

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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

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Collect X terms 10.0 X - 11.0 X = 1080 - 1100

- 1.0 X = -20

X = -20 = 20 % 10B - 1.0Y = 100 - X

% 11B = 100 - 20% = 80% 11B

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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%

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Solution AT8

2) 70%

Solution

62.9X + 6490 = 64.9X = 6350-2.0 X = -140

X = 70%

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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 substanceIs homogeneous, – cannot be separated into simpler substances by any physical processes

Definition 1: MixtureA 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 non-uniform. The different components of the mixture can generally be seen.

Definitions

• Pure Substance: o is homogeneous, ie, has uniform composition throughout the

whole sampleo its properties are constant throughout the whole sampleo its properties do not depend on how it is prepared or purifiedo has constant chemical compositiono Cannot be separated by physical means

• Mixture:o consist of two or more different elements and/or compounds -

physically intermingledo 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

Energy increases

1st

2nd

3rd

4th

5th

2

8

18

32

50

2

8

8

2

+ 10

+ 6 + 10

+ 6 2

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: NitrogenAtomic number= 71st shell= 2/22nd 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 & 21P. 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).