Year 11 Chemistry Relative Atomic Masses Mass Spectrometry

Masses E.g. What is the average mass of one smartie?

How much will 200 smarties weigh?

What is the mass of a hydrogen atom?

This is not as simple because atoms are tiny and therefore their mass is exceptionally small.

Chemists use relative masses

What is relative mass ? Mass of a $1 coin = 3 mu Mass of 30 $1 coin = 90 mu

What is relative mass ? A 5c coin is 3 times lighter than a $1 coin so its mass relative to the $1 coin is 1 mu A 50c coin has a relative mass of 15mu, so it is 5 times heavier than a $1 coin Mass of a $1 coin = 3 mu

MASSES OF ATOMS An atoms mass is extremely small. Egone atom of carbon has an approximate mass of 2 x g. ie g Chemists don’t use these types of masses because Such small masses cannot be measured accurately in experimental work and are awkward to work with in calculations

RELATIVE MASSES Chemists more than 200 years ago used a relative scale to compare weights of atoms to each other. Dalton assigned a H atom a mass of 1. According to his scale a helium atom has a relative mass of 4 because it is 4 times as heavy.

RELATIVE MASSES Using Dalton’s scale a carbon atom has a relative mass of 12 because a carbon atom is twelve times heavier than a hydrogen atom

RELATIVE MASSES Dalton assigned a magnesium atom a relative atomic weight of 24. A Mg atom is 24 times heavier than a H atom and twice as heavy as a C atom. Magnesium atom Carbon atom

Comparing Masses In 1961 Dalton’s method of comparing masses of atoms was replaced by IUPAC- International Union of Applied Physics and Chemistry.

IUPAC RELATIVE MASSES IUPAC decided that the most common isotope of C which is 12 C would be used as a reference standard and assigned an atom of 12 C a mass of 12 exactly. Using this scale the helium isotope is assigned a relative mass of 4 Comparing a helium atom to a carbon atom The He atom is 3 times lighter

RELATIVE ATOMIC MASSES A Krpton atom that is given a relative mass of 36 A Kr atom is three times heavier than a 12 C atom

RELATIVE ATOMIC MASSES All isotopes of elements are given a relative isotopic mass compared to the 12 C isotope. There are 3 isotopes of Mg 24 Mg 25 Mg 26 Mg These 3 atoms are different because they have different numbers of neutrons

Abundances of Isotopes In a sample of pure Mg you will find the isotopes of Mg always occur in the following quantity 78.7% 24 Mg 10.13% 25 Mg 11.17% 26 Mg Like Magnesium most elements exist as a mixture of isotopes. Eg 1 H, 2 H and 3 H

Finding Relative Atomic Masses Thomson (1913) discovered some elements had atoms with different masses using an instrument called a mass spectrometer.

Mass Spectrometer – Principle Separates using magnetic attraction and charge. If a force is applied at right angles to the path of a moving object, the force will change the object’s direction. A lighter object will be deflected more from its original path than a heavier one. A more highly charged ion will be deflected more than a one with a lower charge.

A Mass Spectrometer EMOS/MassSpec.htmlhttp:// EMOS/MassSpec.html

1.The element is vaporised 2.Atoms are ionised by knocking one or more electrons off to give a positive ion. Positive ions are accelerated to high speeds by a magnetic field so that they all have the same kinetic energy.

3.The ions are then deflected by a magnetic field according to their masses. The lighter they are, the more they are deflected. The amount of deflection also depends on charge on the ion - in other words, on how many electrons were knocked off in the first stage. The more the ion is charged, the more it gets deflected.

4. The collector measures the current due to the different ions and the data is recorded as a mass spectrum

is used to measure relative isotopic masses. Relative height of peak = relative abundance Position of peak on x axis = relative isotopic mass Mass Spectrometer

This element has 2 isotopes. The lightest isotope has a relative atomic mass of 35 & an abundance of 75%. The heavier isotope has a relative atomic mass of 37 & an abundance of 25%. Mass Spec of an element

Summing Up Relative masses of isotopes of an element are determined by an instrument called a mass spectrometer This separates isotopes and determines their mass relative to the 12 C isotope and gives you the relative abundance of the isotopes on a graph called a mass spectrum.

Mass Spectrum of Magnesium Each peak represents a different isotope. The position of each peak on the horizontal axis indicates the relative isotopic mass which tells us how heavy the atoms of each isotope is compared to the 12 C isotope. The relative heights of the peaks correspond to the relative abundance of the isotopes.

AVERAGE RELATIVE ATOMIC MASSES Don’t normally worry about the isotopes of an element because they always occur in the same proportions and behave identically in chemical reactions. Chemists use what is known as average relative atomic masses This is an average mass of all the isotopes of an element compared to 12 C and it is given the symbol Ar.Ar.

AVERAGE RELATIVE ATOMIC MASSES A r (Ti) = A Ti atom on average is about 4 times heavier than a C atom. (47.9 ÷ 12)

CALCULATING A r Calculate the average relative atomic mass of Magnesium given: IsotopeRelative MassAbundance 24 Mg % 25 Mg % 26 Mg % Assume we have 100 atoms of Mg. mass contributed by the 24 Mg isotope is x 78.7 mass contributed by the 25 Mg isotope is x mass contributed by the 26 Mg isotope is x Total mass of 100 Mg atoms = x x

Finding A r Total mass of 100 Mg atoms = x x x A r (Mg)= x x x A r (Mg) = 24.3 This is not the true mass of a Mg atom but its relative mass compared to a 12 C atom.

Finding A r The general rule is: A r = Σ(relative isotopic mass x abundance) 100

Finding A r Find the relative atomic mass of Chorine. Isotope Relative Mass Abundance 35 Cl % 37 Cl % A r (Cl) = x x A r (Cl) = 35.45

Find A r (O) Isotopes Relative Isotopic Mass Abundance 16 O O O A r (O) = x x x A r (O) = 16

Calculating Abundances The relative atomic mass of Rubidium is The relative masses of the two isotopes are and Calculate the relative abundances of both isotopes.

Calculating Abundances Relative mass lightest isotope = Relative mass heaviest isotope = A r = Abundance of lightest isotope = x Abundance of heaviest isotope = 100 – x A r = ∑ (relative isotopic mass x abundance) = × x (100 – x) 100

Calculating Abundances = x x (100 – x) = 84.94x – 86.94x -147 = -2x x = 73.5 Abundance of lightest isotope = 73.5% Abundance of heaviest isotope = 26.5%

Relative Atomic Masses Can be read from the Periodic table or a table of relative atomic masses.

Relative Molecular and Formula Mass We can also find out how heavy a molecule of a compound is. M r – relative molecular mass or formula mass

Find M r of H 2 O To find M r simply add the relative atomic masses of each atom in the molecule. M r (H 2 O) = 2 x A r (H) + A r (O) = 2 x = 18 A water molecule is 1.5 times heavier than a carbon atom. (18 ÷ 12)

Find M r of C 6 H 12 O 6 M r (C 6 H 12 O 6 ) = 6 x A r (C) + 12 x A r (H) + 6 x A r (O) = 6 x x x 16 = 180 A glucose molecule is 15 times heavier than a carbon atom. (180 ÷ 12) glucose

Ionic Compounds Eg NaCl For compounds that don’t consist of molecules we find the formula mass. M r (NaCl) = = 58.5

Course Work Read Chapter 2 pp 37 – 38 Chapter Review Questions pg 49 Q 6, 7, 8, Chapter Questions pg 39 Q 10, 11, 14b, 15, 16, 18 Complete mass spec worksheet