Physical Periodicity - Application of 1st Ionisation Energy trend

The periodic table often provides us with much information, such as reactivity and physical properties of the different elements. The grouping of these elements is really an enlightened and detailed classification of the numerous elements found on earth.

In physical periodicity, much is mentioned about the trends in physical properties along a period. The following video aims to show how the first ionisation energy trend can be used to predict an unknown species which is found on a nth ionisation energy trend. You will be pleasantly surprised by the pattern(s) that we can observe from trends in the ionisation energies.

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^{Article written by Kwok YL 2011.}

^{Disclaimer and remarks:}

• ^{If you would like to use this source, kindly drop me a note by leaving behind a comment with your name and institution. I am all for sharing as the materials on this blog is actually meant for the education purpose of my students.}
• ^{This material is entirely written by the author and my sincere thanks will be given to anyone who is kind, generous and gracious to point out any errors.}

Periodicity - Electrical Conducitiy Trend

Across the period, there is a change in the electrical conductivity of the elements. Clearly, Na, Mg and Al are metals and hence are good conductors of electricity. While, P₄, S₈, Cl₂ and Ar are simple discrete molecules; the absence of mobile charge particles prevent them from conducting electricity. Lastly, Si as we commonly know is a semi-conductor.

The diagram below depicts the electricial conductivity across Period 3.

Electricial conductivity trend.

In order for us to explain the trend, we need to know the structure the substance exists as. If the substance is a metal, it will have a sea of delocalised electrons available which acts as mobile charge carriers for electricity to be conducted. Thus, simplistically more available the sea of electrons, the better the conductivity. Therefore, structure provides important information to whether there are any charge particles which can move easily.

Flowchart to explaining the conductivity trend.

Interestingly, the explanation to why Si acts as a semi-conductor can be found here. But it is not part of the "A" level syllabus. If you can appreciate the article, you may wish to think about what is the more accurate representation of metallic bonds.

Lastly, there is a slight anonamlly. In this post, I accounted that Al has a higher electrical conductivity as it has a greater pool of delocalised electrons as Al has more valence electrons - Ironically, this contradicts the reasoning to why Al's melting point is not much higher than Mg. However, the details of this reasoning is beyond the scope of this discussion.
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Article written by Kwok YL 2009.}

^{Disclaimer and remarks:}

• ^{If you would like to use this source, kindly drop me a note by leaving behind a comment with your name and institution. I am all for sharing as the materials on this blog is actually meant for the education purpose of my students.}
• ^{This material is entirely written by the author and my sincere thanks will be given to anyone who is kind, generous and gracious to point out any errors.}

Periodicity - Ionisation Energy Trend

Across the period, another trend which we need to explain is how the first ionisation energy varies as atomic number increases. The diagram below shows the first ionisation energy trend.

1st IE trend across the period.

Generally, as we move across the period, the 1st ionisation energy increases. This is because despite an electron is added, the number of inner shells remains the same hence the shielding effect is relatively constant. While the nuclear charge increases. Therefore, the combination of these two reasons result in an overall increase in the 1st ionisation energy.

There are a couple of dips seen and these are exceptions to the trend. Generally, the dips are present because the causes for the dips are more significant than the effect due to increase in effective nuclear charge. The mind map below provides you with a structure which will explain the trend, including the exceptions.

Mindmap to explain trend across Period 3.

In addition, there is another explanation to why the ionisation energy differs between period. We move from Period 2 to Period 3, the size of the atoms get larger. The added inner shell of electrons results in the valence shell of elements in Period 3 to be further away from the nucleus. Hence, electrostatic attraction between nucleus and valence shell becomes weaker, therefore 1st ionisation energy is smaller.

Trends to notice across the period

Across the period, there are 4 main trends to take note. These trends usually appear even in the general trend for 2^nd and 3^rd I.E and their explanation used to explain is similar. However, do take note that sometimes we do not see the dip (e.g. when you compare the 2^nd I.E of S and Cl we don't see the expected drop) and that is because of the bold phrase of the article.

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Article written by Kwok YL 2009.}

^{Disclaimer and remarks:}

• ^{If you would like to use this source, kindly drop me a note by leaving behind a comment with your name and institution. I am all for sharing as the materials on this blog is actually meant for the education purpose of my students.}
• ^{This material is entirely written by the author and my sincere thanks will be given to anyone who is kind, generous and gracious to point out any errors.}

Periodicity - Melting Point Trend.

In the understanding of periodicity, we will need to explain for the melting point trend across the period. Below is a picture of a graph which shows the melting point trend of the elements across the period.

Diagram to show melting point trend.

Across the period from Na to Ar: Na, Mg and Al exist as giant metallic structures. While Si exists as giant covalent molecule (macromolecular structure). While P, S, Cl and Ar exists as simple discrete molecules. These elements' standard states formula of P₄, S₈, Cl₂ and Ar respectively.

Hence, to explain the melting point trend across the period, it is recommended to define the structures the substance exists as first before proceeding further. The following diagram constructs the explanation needed.

Mindmap which constructs the explanation.

Lastly, Al contains 3 valence electrons. Although these 3 should contribute to the sea of electrons, it does not happen because it will be very difficult for Al to lose so many electrons to contribute to the sea of electrons. Therefore, the increase in melting point from Al to Mg is much smaller than the increase form Mg to Na.

^{-- -- -- -- --
Article written by Kwok YL 2009.}

^{Disclaimer and remarks:}

• ^{If you would like to use this source, kindly drop me a note by leaving behind a comment with your name and institution. I am all for sharing as the materials on this blog is actually meant for the education purpose of my students.}
• ^{This material is entirely written by the author and my sincere thanks will be given to anyone who is kind, generous and gracious to point out any errors.}