GCSE & A level Chemistry Calculations: Defining & calculating relative atomic mass

DEFINING & CALCULATING the RELATIVE ATOMIC MASS Ar of an element

Doc Brown's Chemistry - GCSE/IGCSE/GCE (basic A level) O Level Online Chemical Calculations

1. Defining, explaining and calculating relative atomic mass RAM or Ar, also mention of relative isotopic mass

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(c) is for advanced level chemistry students only and includes how to work out isotopic composition given the relative atomic mass.

(d) Table of relative atomic masses for elements 1 to 92 (2 decimal places)

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Self-assessment Quizzes on relative atomic mass:

Revision notes on how to define relative atomic mass and how to calculate relative atomic mass from the percentage abundance of isotopes, help in revising for A level AQA, Edexcel, OCR 21st century, Gateway science GCSE 9-1 chemistry examinations 1. Explaining and how to calculate the relative atomic mass RAM or Ar of an element (a) Introduction - defining relative atomic mass - carbon-12 scale

• Every atom has its own unique relative atomic mass (RAM) based on a standard comparison or relative scale e.g. it has been based on hydrogen H = 1 amu and oxygen O = 16 amu in the past (amu = relative atomic mass unit).
• The relative atomic mass of an element takes into account the different masses of the isotopes of that element and the abundance of the isotopes in the naturally occurring element (meaning the percentage of each isotope present).
• Relative atomic mass is defined and explained below, and examples of how to calculate it from data.
• The relative atomic mass scale is now based on an isotope of carbon, namely, carbon-12, nuclide symbol , which is given the arbitrary value of 12.0000 amu by international agreement.
• The unit 'amu' is now being replaced by a lower case u, where u is the symbol for the unified atomic mass unit.
• Therefore one atom of carbon, isotopic mass 12, equals 12 u, or,
• 1 u = 1/12th the mass of one atom of the carbon-12 isotope.
• Note that for the standard nuclide notation, , the top left number is the mass number (12) and the bottom left number is the atomic/proton number (6).
• Since the relative atomic mass of an element is now based on the carbon-12 isotope it can now be defined as ...
• ... relative atomic mass equals the average mass of all the atoms in an element compared to 1/12th the mass of a carbon-12 atom (carbon-12 isotope).
• Examples are shown in the Periodic Table diagram above.
• Note
• (i) Because of the presence of neutrons in the nucleus, the relative atomic mass is usually at least double the atomic/proton number because there are usually more neutrons than protons in the nucleus (mass proton = 1, neutron = 1). Just scan the periodic table above and examine the pairs of numbers.
• You should also notice that generally speaking the numerical difference between the atomic/proton number and the relative atomic mass tends to increase with increasing atomic number.
• This has consequences for nuclear stability.
• (ii) For many calculation purposes, relative atomic masses are usually quoted and used at this academic level (GCSE/IGCSE/O level) to zero or one decimal place eg.
• hydrogen H = 1.008 or ~1; calcium Ca = 40.08 or ~40.0; chlorine Cl = 35.45 ~35.5, copper Cu = 63.55 or ~63.5/64, silver Ag = 107.9 or ~108 etc.
• At Advanced level, values of relative atomic masses may be quoted to one or two decimal places.
• Many atomic masses are known to an accuracy of four decimal places, but for some elements, isotopic composition varies depending on the mineralogical source, so four decimal places isn't necessarily more accurate!
• Note that in the case of carbon, there are three isotopes carbon-12 12C the most abundant and small amounts of carbon-13 13C and carbon-14 14C. The average calculated mass of the atoms compared to carbon 12 is 12.01, but for most purposes at pre-university level, 12.0 is sufficient accuracy.
• In using the symbol Ar for RAM, you should bear in mind that the letter A on its own usually means the mass number of a particular isotope and amu is the acronym shorthand for atomic mass units.
• However there are complications due to isotopes and so very accurate atomic masses are never whole integer numbers.
• Isotopes are atoms of the same element with different masses due to different numbers of neutrons.
• The very accurate relative atomic mass scale is based on a specific isotope of carbon, carbon-12, 12C = 12.0000 units exactly, for most purposes C = 12 is used for simplicity.
• For example hydrogen-1, hydrogen-2, and hydrogen-3, are the nuclide notation for the three isotopes of hydrogen, though the vast majority of hydrogen atoms have a mass of 1.
• When their accurate isotopic masses, and their % abundance are taken into account the average accurate relative mass for hydrogen = 1.008, but for most purposes H = 1 is good enough!
• See also GCSE/IGCSE/AS Atomic Structure Notes
• Therefore, a stricter definition of relative atomic mass (Ar) is that it equals the average mass of all the isotopic atoms present in the element compared to 1/12th the mass of a carbon-12 atom.
• AND, the relative isotopic mass of carbon-12 is assigned a numerical value of 12.0000.
• So, in calculating relative atomic mass you must take into account the different isotopic masses of the same elements, but also their % abundance in the element.
• Therefore you need to know the percentage (%) of each isotope of an element in order to accurately calculate the element's relative atomic mass.
• For approximate calculations of relative atomic mass you can just use the mass numbers of the isotopes, which are obviously all integers ('whole numbers'!) e.g. in the two calculations below.
• To the nearest whole number, isotopic mass = mass number for a specific isotope.
• If an element only has one isotope, relative atomic mass = relative mass of this isotope.
• A good example is fluorine.
• All fluorine atoms have a mass of 19 (19F), therefore its relative atomic mass is 19 and no calculation is needed. Above is typical periodic table used in GCSE science-chemistry specifications

and I've 'usually' used these values in my exemplar calculations to cover most syllabuses

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(b) Examples of relative atomic mass calculations for GCSE 9-1/IGCSE/AS/A level chemistry students

How do I calculate relative atomic mass?

You can calculate relative atomic mass from isotopic abundances

• For accurate chemical calculations relative atomic mass must be used and not an individual mass number.
• Therefore relative atomic mass takes into account all the different 'stable' isotopes of an element which are naturally present.
• The relative atomic mass is the average mass and is quite easily calculated from the percentage composition (% abundance).
• The presence of isotopes accounts for why some relative atomic masses are not even close to a whole number.
• Some relative atomic masses are nearly whole numbers due to coincidence of % isotopes, others because one isotope might dominate the composition with only tiny amounts of lighter or heavier isotopes.
• Example 1.1 Calculating the relative atomic mass of bromine and • bromine consists of two isotopes, 50% 79Br and 50% 81Br, calculate the Ar of bromine from the mass numbers (top left numbers).
• Think of the calculation in terms of '100 atoms'
• Ar = [ (50 x 79) + (50 x 81) ] /100 = 80
• So the relative atomic mass of bromine is 80 or RAM or Ar(Br) = 80
• Note the full working shown. Yes, ok, you can do it in your head BUT many students ignore the %'s and just average all the isotopic masses (mass numbers) given, in this case bromine-79 and bromine-81.
• The element bromine is the only case I know where averaging the isotopic masses actually works! so beware!
• -
• Example 1.2 Calculating the relative atomic mass of chlorine based on the and isotopes
• Chlorine consists of two isotopes, 75% chlorine-35 and 25% chlorine-37, so using these two mass numbers ...
• ... again think of the data based on 100 atoms, so 75 have a mass of 35 and 25 atoms have a mass of 37.
• The average mass = [ (75 x 35) + (25 x 37) ] / 100 = 35.5
• So the relative atomic mass of chlorine is 35.5 or RAM or Ar(Cl) = 35.5
• Note: 35Cl and 37Cl are the most common isotopes of chlorine, but, there are tiny percentages of other chlorine isotopes which are usually ignored at GCSE/IGCSE and Advanced GCE AS/A2 A level.
• -
• Example 1.3: Calculating the relative atomic mass of copper from its isotopic composition (isotope abundance)
• Naturally occurring copper consists of 69.2% copper-63 (63Cu) and 30.8% copper-65 (65Cu)
• Still think in terms of 100 atoms and don't be put off by decimal fractions, it still works out correctly because 69.2 + 30.8 = 100!
• average mass = relative atomic mass of copper = {(63 x 69.2) + (65 x 30.8)} / 100 = 63.6
• -
• Example 1.4: Silver atoms consist of 51.4% of the isotope 107Ag and 48.6% of the isotope 109Ag
• Calculate the relative atomic mass of silver.
•  (51.4 x 107) + (48.6 x 109) 5499.8 + 5297.4 Ar(Ag) = -------------------------------------- =    --------------------------- = 108.0 100 100
• The relative atomic mass of silver is 108.0 (to 1 decimal place)
• -
• Example 1.5: Europium atoms consist of 47.8% Eu-151 and 52.2% of Eu-153
• Calculate the relative atomic mass of europium.
•  (47.8 x 151) + (52.2 x 153) 7217.8 + 7986.6 Ar(Eu) = -------------------------------------- =    --------------------------- = 152.0 100 100
• The relative atomic mass of europium is 152.0 (to 1 decimal place)
• -
• Example 1.6: Atoms of the element silicon consist of 92.2% silicon-28, 4.7% silicon-29 and 3.1% of silicon-30.
• Calculate the relative atomic mass of silicon.
•  (92.2 x 28) + (4.7 x 29) + (3.1 x 30) 2581.6 + 136.3 + 93.0 Ar(Si) = -------------------------------------------------- =  -------------------------------- = 28.1 100 100
• The relative atomic mass of silicon is 28.1 (to 1 decimal place or 3 significant figures)
• -
• See below and mass Spectrometer and isotope analysis on the GCSE-Advanced A Level (basic) Atomic Structure Notes, with further relative atomic mass calculations.

(c) Examples for Advanced A Level Chemistry students only

How to calculate relative atomic mass with accurate relative isotopic masses

Using data from modern very accurate mass spectrometers

(a) Very accurate calculation of relative atomic mass (need to know and define what relative isotopic mass is)

Relative isotopic mass is defined as the accurate mass of a single isotope of an element compared to 1/12th the mass of a carbon-12 atom e.g. the accurate relative isotopic mass of the cobalt-5 is 58.9332

This definition of relative isotopic mass is a completely different from the definition of relative atomic mass, except both are based on the same international standard of atomic mass i.e. 1 unit (1 u) = 1/12th the mass of a carbon-12 isotope (12C).

If we were to redo the calculation of the relative atomic mass of chlorine (example 1.1 above), which is quite adequate for GCSE purposes (and maybe A level too), but more accurately at A level, we might do ....

chlorine is 75.77% 35Cl of isotopic mass 34.9689 and 24.23% 37Cl of isotopic mass 36.9658

so Ar(Cl) = [(75.77 x 34.9689) + (24.23 x 36.9658)] / 100

= 35.4527 (but 35.5 is usually ok in calculations pre-university!)

See also Mass Spectrometer and isotope analysis on the GCSE/A level Atomic Structure Notes, with further RAM calculations.

(b) Calculations of % composition of isotopes

It is possible to do the reverse of a relative atomic mass calculation if you know the Ar and which isotopes are present.

It involves a little bit of arithmetical algebra.

The Ar of boron is 10.81 and consists of only two isotopes, boron-10 and boron-11

The relative atomic mass of boron was obtained accurately in the past from chemical analysis of reacting masses but now can sort out all of the isotopes present and their relative abundance.

If you let X = % of boron 10, then 100-X is equal to % of boron-11

Therefore Ar(B) = (X x 10) + [(100-X) x 11)] / 100 = 10.81

so, 10X -11X +1100 =100 x 10.81

-X + 1100 = 1081, 1100 - 1081 = X (change sides change sign!)

therefore X = 19

so naturally occurring boron consists of 19% 10B and 81% 11B

(the data books actually quote 18.7 and 81.3, but we didn't use the very accurate relative isotopic masses mentioned above!)

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Revision notes on how to define relative atomic mass and how to calculate relative atomic mass from the percentage abundance of isotopes, help in revising for A level AQA, Edexcel, OCR 21st century, Gateway science GCSE 9-1 chemistry examinations

On other pages on and Relative Formula Mass

Self-assessment Quizzes on relative atomic mass

or  multiple choice QUIZ

APPENDIX 1. A typical periodic table used in pre-university examinations Above is typical periodic table used in GCSE science-chemistry specifications in doing chemical calculations, and I've 'usually' used these values in my exemplar calculations to cover most syllabuses

Revision notes on how to define relative atomic mass and how to calculate relative atomic mass from the percentage abundance of isotopes, help in revising for A level AQA, Edexcel, OCR 21st century, Gateway science GCSE 9-1 chemistry examinations

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(d) APPENDIX 2. Table of relative atomic masses for elements 1 to 92

Notes: (i) The list of relative atomic mass are in alphabetical order by element name, together with chemical symbol and proton/atomic number.

(ii) The relative atomic masses are quoted to two decimal places, though it is essential to be aware that values in pre-university examinations might be rounded to the nearest integer or one decimal place.

(iii) Trans-uranium elements have been eliminated because their isotopic composition varies depending on the source e.g. cyclotron, nuclear reactor etc. AND all their isotopes are highly radioactive and most are very unstable (so your relative atomic mass changes all the time!)

(iv) * radioactive, mass number of most stable isotope quoted

#### Atomic No. Z

Relative atomic mass
Ac Actinium 89 227.03
Al Aluminium 13 26.98
Sb Antimony 51 121.75
Ar Argon 18 39.95
As Arsenic 33 74.92
At Astatine 85 210 *
Ba Barium 56 137.33
Be Beryllium 4 9.01
Bi Bismuth 83 208.98
B Boron 5 10.81
Br Bromine 35 79.90
Cs Caesium 55 132.91
Ca Calcium 20 40.08
C Carbon 6 12.01
Ce Cerium 58 140.12
Cl Chlorine 17 35.45
Cr Chromium 24 52.00
Co Cobalt 27 58.93
Cu Copper 29 63.55
Dy Dysprosium 66 162.50
Er Erbium 68 167.26
Eu Europium 63 151.97
F Fluorine 9 19.00
Fr Francium 87 223 *
Ga Gallium 31 69.72
Ge Germanium 32 72.60
Au Gold 79 196.97
Hf Hafnium 72 178.49
He Helium 2 4.00
Ho Holmium 67 164.93
H Hydrogen 1 1.01
In Indium 49 114.82
I Iodine 53 126.90
Ir Iridium 77 192.22
Fe Iron 26 55.85
Kr Krypton 36 83.80
La Lanthanum 57 138.91
Li Lithium 3 6.94
Lu Lutetium 71 174.97
Mg Magnesium 12 24.31
Mn Manganese 25 54.94
Hg Mercury 80 200.59

#### Atomic No. Z

Relative atomic mass
Mo Molybdenum 42 95.94
Nd Neodymium 60 144.24
Ne Neon 10 20.18
Ni Nickel 28 58.69
Nb Niobium 41 92.91
N Nitrogen 7 14.01
Os Osmium 76 190.20
O Oxygen 8 16.00
P Phosphorus 15 30.97
Pt Platinum 78 195.08
Po Polonium 84 209 *
K Potassium 19 39.10
Pr Praseodymium 59 140.91
Pm Promethium 61 145 *
Pa Protactinium 91 231.04
Re Rhenium 75 186.21
Rh Rhodium 45 102.91
Rb Rubidium 37 85.47
Ru Ruthenium 44 101.07
Sm Samarium 62 150.36
Sc Scandium 21 44.96
Se Selenium 34 78.96
Si Silicon 14 28.09
Ag Silver 47 107.87
Na Sodium 11 23.00
Sr Strontium 38 87.62
S Sulfur 16 32.07
Ta Tantalum 73 180.95
Tc Technetium 43 98.91
Te Tellurium 52 127.60
Tb Terbium 65 158.93
Tl Thallium 81 204.38
Th Thorium 90 232.04
Tm Thulium 69 168.93
Sn Tin 50 118.71
Ti Titanium 22 47.88
W Tungsten 74 183.85
U Uranium 92 238.03
Xe Xenon 54 131.29
Yb Ytterbium 70 173.04
Y Yttrium 39 88.91
Zn Zinc 30 65.39
Zr Zirconium 40 91.22

Revision notes on how to define relative atomic mass and how to calculate relative atomic mass from the percentage abundance of isotopes, help in revising for A level AQA, Edexcel, OCR 21st century, Gateway science GCSE 9-1 chemistry examinations relative atomic mass calculations how to calculate the relative atomic mass of bromine from the % percent abundance of isotopes, how to calculate the relative atomic mass of chlorine from the percentage abundance of isotopes, how to calculate the % composition of isotopes in an element given its relative atomic mass

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Keywords and phrases: What is the relative atomic mass of an element? What scale is relative atomic mass based on? What is the formula to work out the relative atomic mass of an element? Quantitative Chemistry calculations online Help for problem solving in relative atomic mass calculations. Definitions of relative atomic mass and relative isotopic mass (A level students only) Practice revision questions on working out relative atomic mass from isotopic composition (% isotopes, A level students will learn about very accurate mass spectrometer data). What is relative atomic mass? How do you calculate the relative atomic mass of an element. What is the standard mass unit? Relative atomic mass is explained below, with reference to the carbon-12 atomic mass scale and the relevance of isotopes and 'u' the unified atomic mass unit is explained. Detailed examples of the method of how to calculate relative atomic mass from the isotopic composition are fully explained with reference to the definition of the relative atomic mass of a compound. For A level students, how to define and use relative isotopic masses to calculate relative atomic mass. These notes on defining, explaining and calculating relative atomic mass and defining relative isotopic mass are designed to meet the highest standards of knowledge and understanding required for students/pupils doing GCSE chemistry, IGCSE chemistry, O Level chemistry, KS4 science courses and A Level chemistry courses.

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