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Worked Example 1
Worked Example 2
Worked Example 3

At the end of this topic you should be able to:

• Recall the simple model of the atom in terms of protons. neutrons and electrons.

• Recall the definitions of relative atomic mass, isotopic mass and molecular mass.

• Work out the number of protons. neutrons and electrons in an atom given the atomic number and mass number of an atom.

• Work out the relative isotopic mass of an atom given the mass numbers of its isotopes and their respective percentage abundances.

• Understand how a mass spectrometer can be used to work out relative atomic mass.

• Calculate the relative atomic mass of a substance from mass spectrometric data.

• Predict the electronic configuration of an atom in terms of s, p and d sub-shells.

The Atom

An Atom is made up of protons, neutrons and electrons.

• The protons and neutrons are found in the center of the atom called the nucleus, and the electrons are found in shells outside the nucleus.  The electrons are held in their shells by the attraction of the protons in the nucleus.  This is called the nuclear attraction.  
• The number of protons or electrons in the atom is called the atomic number.  
• The number of protons and neutrons is called the mass number.   
• We can work out the number of protons neutrons and electrons in an atom from these two numbers. [click here for worked example 1].

RELATIVE ATOMIC MASS

This is the mass of one atom of an element compared to that of an atom of Carbon-12.  The relative atomic mass has no units because it a ratio of two masses.

RELATIVE MOLECULAR MASS 

This is calculated by adding up the relative atomic masses of the elements that make up a molecule.  It has the symbol M_r. 

[click here for a worked example]

Isotopes

These are atoms of the same element with the same atomic number but a different mass number.  For example Chlorine has two isotopes, Cl35 and Cl37, in an abundance of 75% and 25%.  The average atomic mass of chlorine is 35.5. [Click here for a worked example 2].  All of the elements in the periodic table have at least 2 isotopes, the atomic mass is therefore the average of the masses of the isotopes it is made up of.

The Mass Spectrometer.

This machine is used to find out the relative atomic mass of an element.  There are 4 main stages in the process:

1.  Ionisation - after a vapourised sample is put into the mass spectrometer, it is ionised - electrons are removed - usually one electron is removed but sometimes two.  The positively charged species then go into the accelerating chamber.

2.  Acceleration - the ions are subjected to an electric field.

3.  Deflection - the heavy ions (ones with the larger atomic mass) are deflected less than the the lighter ions.  Therefore the ions are separated according to their atomic masses and travel a different path in the mass spectrometer (shown by the dotted lines in the diagram below). 

4. Detection - only ions of a certain mass actually end up at this point (the ones taking the green path).  To make sure that all of the ions are detected, you have to vary the strength of the accelerating field.  The detector records each species as a peak on a trace.  

How to Interpret the Results!

The results are plotted on a chart showing the percentage abundance of the isotopes and their mass/charge ratio.  If we assume that the charge on each ion is +1 then the mass/charge ratio is the same as the atomic mass of the ion.  the mass spectrum above is for Boron.  

1. The number of peaks tells you how many isotopes there are - in this case 2.
2. The atomic mass of each is read from the x-axis - in this one isotope has mass of 10 and the other a mass of 11.
3. The height of each peak tells you its percentage abundance - this means that out of 100 atoms of Boron there will be 81.7 atoms of Boron₁₀ and 18.7 atoms of Boron₁₁.
4. To work out the average isotopic mass of Boron: (10 × 81.3/100) + (11 × 18.7/100) = 10.8

The periodic Table

The periodic table of elements is arranged in order of increasing atomic number.

Each box shows the name of the element, the symbol of the element, the mass number and the atomic number. 

The columns are called groups, there are 8 groups.  The rows are called periods.  There are three main blocks in the periodic table, the s block is made up of groups 1 and 2.  The d block is the central block, and the p block is made up of groups 3 to 8.

The electrons in an atom are arranged in orbitals or shells, there are different types of orbitals, each orbital can hold a maximum of 2 electrons, with each electron spinning in opposite directions.

s orbitals         

p orbitals

There are 3 types of p orbitals,             

  px                    py                        pz

d orbitals

There are 5 types of d orbitals, at A level you do not need to know what they look like.

Electronic Configuration of Atoms

Electrons in an atom are filled from the orbital closest to the nucleus until all the electrons are used up.  The orbitals are arranged in the following order....

...1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p etc

For example sodium has 11 electrons, the electronic configuration of sodium is as follows..

1s², 2s², 2p⁶, 3s¹

THE NUMBERS IN RED SHOW THE NUMBER OF ELECTRONS IN THE ORBITAL.

Each electron is given the symbol     if two electrons are in a shell then they are aligned so that they are spinning in opposite directions.

The "electrons in boxes" is also a method of displaying electronic structure.  Each box represents an orbital, and the half arrows inside the box show the electrons.

For sodium

1s        2s                2p             3s

For carbon

1s        2s                2p             

In the 2p orbital the electrons are added to fill each of the three separate orbitals and then paired up. 

[click here to go through some worked examples]

[click here if you are ready to try some exam questions]