Tuesday, 26 January 2016

Periodicity (1) Ionisation energy and electronegativity of the elements

The Periodic Table is a wonderful thing.

It’s not just that there are these elements in groups that reflect their physical properties like you look at the Alkali Metals (Group 1) and see that they get more reactive with water as you go down the group but the properties are periodic.

The periodic nature of the physical properties is called periodicity.

Here’s a definition of periodicity:  Periodicity is the name given to the regular recurrence of similar physical and chemical properties of the elements and their compounds in the periodic table. 

It’s like this: your school timetable is periodic because the same chemistry lesson with the same teacher occurs at the same time each week. 

Your timetable is a periodic table of sorts!! (See, you just can’t get away from Chemistry its everywhere!!)

In life periodic things take place year on year: we go on holiday in the summer, we give presents to those we love at Christmas, we hope that people remember our birthday when it comes round every year. 

So how do the properties of the elements reveal their periodicity?

Let’s look at some of the obvious properties of the elements and see if we can see periodic patterns in them.

Now if you have a Data Book of elements’ properties you can draw up charts and tables to reveal this periodicity.

Here’s one of them:

1. First Ionisation Energy

Here is a plot of first ionisation energy (Em1) against atomic number (Z).

You can see the regular repetition of similar (not identical) properties.

Every eighth element has the highest Em1 starting with Helium. 

We notice the break for the first transition series at atomic number 21 Scandium.

But the pattern picks up again at atomic number 30 and peaks at Kr Krypton.

The alkali metals are also labelled because they have the lowest Em1 values.

Their values repeat at regular intervals: every eighth element. 

We can see from this 3D image the trends in the first ionisation values:

Em1 decreases down a group, e.g. lithium (Li) to caesium (Cs), because the outer shell electrons get further from the positive electrostatic pull of the nucleus.

Em1 values generally increase across a period, e.g. Lithium (Li) to Neon (Ne), because the size of the positive nucleus increases, increasing the positive pull of the nucleus on the outer electron shell.

2. Electronegativity

What is electronegativity?

This is the power of an atom of an element within a covalent bond to attract the bonding pair of electrons to itself. 

So here is a water molecule which we can represent like this to show how the oxygen atom tends to pull electrons in the two covalent bonds towards itself ( the arrows) and lead to an internal dipole in the water molecule.
The two hydrogen atoms end up slightly positively charged relative to the oxygen atom. 

This ability to attract the bonding electrons to itself is called an element’s electronegativity.

Electronegativity is measured on a scale from 0-4 first developed by one of the greatest ever chemists Linus Pauling. 

Here is how electronegativity varies across the periodic table and as you can see it is a periodic function of the elements.  

The values and trends are easier to see on the next diagram.

The value increases across the periodic table from left to right and it decreases down a group of the periodic table. 

It is highest at Fluorine (F).  (Why are there no values for the Noble Gases?)

It is at its lowest at Caesium and Francium. 

Metals tend to have low values and non–metals have high values, hence the colour densities in the diagram above. 

The reason why the values tend in these directions is due to the dependence of electronegativity on the atomic radii values: the higher the electronegativity the smaller the atomic radius. 

And the atomic radius tends to depend on the size of the nucleus, the number of electron shells and the tendency for the inner shells to shield the outer shell electrons from the influence of the nucleus.

So take the Fluorine atom that has the highest electronegativity, it has two shells with electron arrangement 1s2, 2s2, 2p5.  There is little chance of shielding of the outer shell electrons so it pulls bonding electrons easily to itself. 

But look at Caesium with a very low electronegativity.  It has 6 electron shells.  It is in Period 6.  Its outer shell electron is well shielded from the positive nucleus even though the nucleus is huge (Z=55) with 55 protons. So there is a great tendency of the outer electron to be lost and caesium atoms are easily oxidised.

Things to do:  

So why do the Noble gases not have an electronegativity value? 

Can you construct a chart to reveal the periodicity of the melting points and boiling points of the elements?

Can you calculate the atomic volumes (the volume of one mole of atoms of the element, hint: use the molar mass and the element’s density.) of the elements and show that this is also a periodic property. 

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