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study examples carefully1. Explaining and calculating relative atomic mass RAM or Ar*study examples carefully

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study examples carefully1. Explaining and how to calculate the relative atomic mass RAM or Ar of an elementstudy examples carefully

Introduction

  • Every atom has its own unique atomic mass based on a standard comparison or relative scale e.g. it has been based on hydrogen H = 1 amu, oxygen O = 16 amu.
  • The relative atomic mass scale is now based on an isotope of carbon, carbon-12, , which is given the value of 12.0000 amu.
  • 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 not whole numbers.
  • Isotopes are atoms of the same element with different masses due to different numbers of neutrons. The very accurate 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 , and are 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 Atomic Structure Notes
  • The strict definition of relative atomic mass (Ar) is that it equals average mass of all the isotopic atoms present in the element compared to 1/12th the mass of a carbon-12 atom.
    • So, you are taking into account the different isotopic masses of the same elements, but also their % abundance in the element.

Examples of relative atomic mass calculations

  • Example 1.1: bromine consists of 50% 79Br and 50% 81Br, calculate the Ar of bromine.
    • 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.
  • Example 1.2: chlorine consists of 75% chlorine-35 and 25% chlorine-37.
    • 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 AS/A2 A level.
  • Example 1.3: 

The mass number for any isotope is the sum of the protons and neutrons in the nucleus, and is always a whole number.

For Advanced Level Chemistry students only

Relative isotopic mass = 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 mass of is 58.9332!

If we were to redo the chlorine example 1.1 above, which is quite adequate for GCSE purposes, more accurately at A level, we would 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-AS(basic) Atomic Structure Notes, with further RAM calculations.

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(indonesia) Doc Brown Kimia - Perhitungan Kimia 1. Menjelaskan dan menghitung massa atom relatif RAM atau Online Kimia Kuantitatif Perhitungan * (portuguese) Doc Brown de Química - Química Cálculos 1. Explicar e cálculo da massa atômica RAM parente ou um r * Cálculos Química Quantitativa Online Cada átomo tem sua própria massa atômica única baseada em um padrão de comparação ou parente escala, por exemplo, foi baseada no hidrogênio H = 1 u, oxigênio O = 16 u. A massa escala atômica relativa baseia-se actualmente um isótopo de carbono, o carbono-12, , which is given the value of 12.0000 amu . , Que é dado o valor de 12,0 mil amu. * (spanish) Doc Brown Química - Cálculos químicos 1. Explicar y calcular la masa atómica relativa de RAM Los cálculos cuantitativos en línea Química Cada átomo tiene su propia masa atómica único basado en una comparación estándar o escala relativa por ejemplo, se ha basado en el hidrógeno H = 1 uma, el oxígeno O = 16 uma. La escala atómica masa relativa se basa ahora en un isótopo del carbono, el carbono-12, Que se da el valor de 12,0000 uma. Al utilizar el símbolo A r para RAM que debe tener en cuenta que la letra A por su cuenta por lo general significa que el número de masa de un isótopo particular y uma es la taquigrafía acrónimo de unidades de masa atómica) * 

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