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 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

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

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.

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

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 __________

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

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 Mg 24.3

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?

Solution AT7 Atomic mass Ga = 69.7 amu 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 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 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

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

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%

Solution AT8 2) 70% Solution 62.9X + 6490 = 64.9X = 6350 -2.0 X = -140 X = 70%

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 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.

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.

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

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

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 + 10 4th 32 Energy increases 3rd 18 8 + 10 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

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

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).