Topic 1.5 Atomic structure and the periodic table

a. recall the definitions of relative atomic mass, relative isotopic mass and relative molecular mass and understand that they are measured relative to 1/12th the mass of a 12C atom

b. demonstrate an understanding of the basic principles of a mass spectrometer and interpret data from a mass spectrometer to:

i. Determine isotopic mass of an element ii. measure the relative deduce the isotopic composition of a sample of an

element, eg poloniumiii. deduce the relative atom molecular mass of a compound

c. describe some uses of mass spectrometers, eg in radioactive dating, in space research, in sport to detect use of anabolic steroids, in the pharmaceutical industry to provide an identifier for compounds synthesised for possible identification as drugs

1: Mass spectroscopy

Connector – Define the following:

• Atomic number.• Mass number.• Isotopes

Atomic number & Mass number

NaMass Number 23

Atomic Number 11

Atomic Number is the number of protons only

Mass Number is the number of protons and neutrons

Copy and complete the following table

A to m N u m b er o f

p ro to n sN u m b er o fn eu tro n s

N u m b er o fe lec tro n s

Ir

M o

1 9 1

7 7

9 6

4 2

77 114 77

42 54 42

Isotopes                                                                                                      

Atoms that have the same number of protons but different numbers of neutrons are called isotopes.

BIG picture

• What skills will you be developing this lesson?

• ICT• Numeracy• Literacy• Team work• Self management• Creative thinking• Independent enquiry • Participation• Reflection

• How is this lesson relevant to every day life? (WRL/CIT)

What is relative atomic mass?

New Information for Task 1

• Relative atomic mass:

is the average mass of the atoms of an element relative to 1/12th the mass of C=12 atom.

• Relative isotopic mass:

is the mass of one atom of an isotope relative to 1/12th the mass of C=12 atom.

• Relative molecular mass:

is the sum of all the relative atomic masses of the constituent atoms.

• The species X, Y and Z are atoms or ions.

(i) State the number of electrons, protons and neutrons in X

(ii) What is the relationship between X and Y?

(iii) Identify X, Y and Z

species X Y Zatomic number 16 16 16

mass number 32 35 32net charge 0 0 –2

Question

• Mass of an atom cannot be measured directly as it is far too small.

• A mass spectrometer is the instrument used to determine the mass of an atom.

Edexcel animation

• Mass Spectrometry is the generation, separation and detection of gas phase ions according to their relative mass as a function of charge (m/e)

1. A small quantity of sample is injected and vaporized under high vacuum. Particles in gaseous state are free to move in the machine.

2. The sample is then ionized by being bombarded with high energy electrons. A valence electron is “knocked” off and a positive ion (cation) is formed.

e.g. Fe (g) + Energy Fe+ (g) + e-

gaseous atom gaseous ion

Mass Spectrometry

3. The charged ions are now accelerated by an Electric field.

4. Ions pass through a velocity selector, which makes sure that all ions are traveling at the same speed.

5. Ions enter a uniform Magnetic field where they are deflected.

The amount of deflection of ions depends on the mass of the ion and charge on it.

(Heavier ions are deflected less than lighter ions, and ions with smaller positive charge are deflected less than ions with bigger positive charge).

6. Finally the positive ions are detected.

The detector is basically a counter, that produces a peak with a height proportional to the number of ions that strike it at different magnetic fields. This data is sent to a computer interface for graphical analysis of the mass spectrum

Process of mass spectrometry

Process of mass spectrometry

Parts of the mass spectrometer

Mass spectra of monatomic elements

Mass spectra of diatomic elements

The main steps inside a mass spectrometer - summarised

vaporized

ionized

accelerated

deflected

detected

A mass spectrum

Each peak represents a positive ion that has been detected.

The position on the x-axis gives the relative atomic mass.

The relative heights of the peaks show the relative abundance of each isotope.

Relative atomic mass

• Home Learning task:• Read page 53, 56-59 of Edexcel book

To describe some uses of mass spectrometers, e.g. in radioactive dating, in space research, in sport to detect the use of anabolic steroids, in the pharmaceutical industry to provide and identifier for compounds synthesised for possible identification as drugs

Why does sodium have a whole number atomic mass, and chlorine doesn’t?

There is only one type of sodium atom, but chlorine atoms exists as two isotopes.

Calculating relative atomic mass from mass spectra

Suppose an element E had two isotopes E1 and E2

then relative atomic mass of E is given by:

Ar (E) = (mass x abundance of E1) + (mass x abundance of E2)

100

Write the expression to calculate the relative atomic mass of element Z that has three isotopes Z1, Z2 and Z3.

Ar (Na) = 23

(or m/e value)

What is the relative atomic mass of this element?

Ar (Cl) = (35 x 75%) + (37 x 25%)/100 = 35.5

(or m/e value)

What is the relative atomic mass of this element?

(or m/e value)

55.91

What is the relative atomic mass of this element?

(or m/e value)

Explain this mass spectrum

Isotopes of chlorine

molecules of chlorine with different isotopes

• For a particular sample of sulphur atoms the following isotopic composition was recorded.

• Calculate the relative atomic mass of this sample of sulphur. Give your answer to two decimal places.

Isotope Percentage composition

32S 95.0033S 0.7634S 4.24

= [95.0 × 32 + 0.76 × 33 + 4.24 × 34] / 100

= 32.0924 = 32.09

So 31 + 37x = 142

x = (142 – 31)/37 = 3

Questions

(a) (i) Write the equation for the reaction of lithium with water.

(ii) Describe what you would expect to see during the reaction.

(b) State the number of protons, neutrons and electrons in a lithium ion.

(c) The mass spectrum of lithium shows two peaks. Their mass/charge ratios and percentage abundance are shown below.

Calculate the relative atomic mass of lithium, giving your answer to three significant figures.

 

(d) Describe a test that you would do to distinguish between solid lithium chloride and solid sodium chloride. Clearly state what you would do and what you would see with both substances.

Mass/charge % Abundance6.02 7.397.02 92.61