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Electronegativity is a measure of how strongly an atom attracts electrons.

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Introduction

Electronegativity

Electronegativity is a measure of how strongly an atom attracts electrons.  This attraction is due to the charge on the nucleus which, being positive, attracts negatively charged species, such as electrons.  As we move along a period of the periodic table the nuclear charge increases and so electrons are attracted more strongly.  However, when we start a new period, the electrons in the filled inner shell are able to shield the outer electronsfrom the nuclear charge.  Consequently, electronegativity decreases as we move down a group.

The noble gases are an exception to this; on the most common electronegativity scale, the Pauling scale, they are not given a value.This is because their exceptional stability makes it difficult to attract electrons.

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Middle

  E+

and the second ionisation energy is the energy needed for

        E+→• E2+

When we refer to "ionisation energy" we usually mean first ionisation energy.

As we move along a row of the Periodic Table, the nuclear charge increases.  This holds all the electrons more strongly, and the ionisation energy increases.  As we start a new period, the electrons in the filled inner shell are able to shield the outer electrons from the nuclear charge, and so there is a large decrease in ionisation energy.  The ionisation energy then starts to increase again as we move along the row.  Consequently, the lowest ionisation energies are found in the bottom right portion of the table.

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Conclusion

When we plot atomic radius against atomic number we get the graph below.

 
            
image02.png 
 

It can be seen from the graph that fluorine has the smallest atomic radius and caesium the largest.  (Francium is not stable enough to measure its size.)

Ionic radius also varies as we move along the group, although in a less obvious manner.  Where elements lose electrons to form cations, the radius decreases along the group.  However, there is then an increase in radius when we get to atoms that gain electrons to form anions.  This is followed by a decrease in radius as we continue moving along the group, due to the increasing nuclear charge.  For example, sulphur has atomic radius 104 pm and the S2- ion has radius 184 pm, while chlorine has radius 99 pm and Cl- has radius 181 pm.

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