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The Periodic Table – Periodicity
Key terms in this chapter are:
Dmitri Mendeleev
Periodic Table
Group
Period
s, p, d, f Blocks
Alkali Metals
Alkaline Earth Metals
Halogens
Noble Gases
Transition Metals
Periodicity
Classifying the Elements
In this section you are going to learn:
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•
About how Dmitri Mendeleev organised the elements in his Periodic Table,
why he had to leave gaps in his table, and why he swapped around certain
pairs of elements.
That elements are in Groups and Periods in the Periodic Table.
As more and more elements were discovered, in order to understand them better,
scientists were inclined to categorise them is some way. The Russian chemist,
Dmitri Mendeleev placed the elements in rows from left to right in order of
increasing relative atomic mass so that elements with similar chemical and physical
properties occupied the same column.
Dimitri Ivanovitch Mendeleev (1834-1907) was born at
Tobolsk, Siberia, on 7th February, 1834. He became
Professor of Chemistry at the Technological Institute,
St. Petersburg, in 1863.
The discovery of the Periodic Law and the
development of the Periodic Table is possibly
Mendeelev's greatest achievement. Mendeleev
had a great interest in the chemical elements,
which up to his time were distinguished only by
their characteristic atomic weights (now called
relative atomic mass) that had been proposed by
John Dalton in 1805.
By arranging all of the 63 elements known at the
time by their relative atomic masses, he managed
to organize them into groups possessing similar
properties. Where a gap existed in the table, he
predicted a new element would one day be found,
and he predicted its properties. Three of those
elements, gallium, scandium, and germanium,
were found during his lifetime. This added to his
stature in the scientific world. His table did not
include any of the Noble Gases which had not yet
been discovered.
In this modern form of the Periodic Table, the rows are called Periods and the
columns are called Groups. Mendeleev had to swap around certain pairs of elements
(e.g. Ar and K, and I and Te) so that they were in Groups with other elements with
similar properties.
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Q1.
Look at a Periodic Table to discover the characteristic now used to
place elements from left to right across Periods that avoids the
need to swap around certain pairs of elements. What is this?
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Mendeleev also left gaps for elements in his Periodic Table.
Q2.
Why did Mendeleev leave these gaps?
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Q3.
Give an example of an element for which Mendeleev left a gap.
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The modern form of the Periodic Table
In this section you are going to learn:
About the blocks in the Periodic Table and the Groups in two of these.
Which elements are the transition metals and the blocks they form.
That there is an approximate division into metallic and non-metallic
elements.
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An illustration of the Periodic Table is given below:
The modern Periodic Table is divided into blocks: the s block, p block, d block and f
block. The following questions 4-17 refer to the diagram above.
Q4.
Label the blocks in the Periodic Table.
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Q5.
Label the Groups and Periods.
Q6.
Which Groups make up the s block?
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Q7.
Which groups make up the p block?
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Q8.
Write in the chemical symbols of the elements in Groups 1, 2, 3, 4,
5, 6, 7 and Group 0.
Q9.
Write in the chemical symbols for the elements between scandium
and zinc.
Q10. Write in the chemical symbols of some elements you recognise
that belong to the f block.
Q11. What name is given collectively to the elements that make up
blocks d and f?
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Q12. What does the black zig-zag line signify?
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Q13. Approximately what proportion of the elements are metals?
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Q14. Some Groups of elements have other names by which they are
known. What are these for the following Groups?
Group 1...............................................................................................
Group 2...............................................................................................
Group 7...............................................................................................
Group 0...............................................................................................
Q15. Comment generally on the change in metallic character as you go:
Left to right across a Period. ...............................................................
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Down a Group.....................................................................................
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Q16. Consider the elements in Group 4 of the Periodic Table. Comment
on their change in metallic / non-metallic character going down the
Group.
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Q17. Look up the term 'metalloid' and write below what this means.
Where in the Periodic Table are such elements found?
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Electronic Structure and the Periodic Table
In this section you are going to learn:
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How the number of outer electrons of an atom is known from the Group of
that element.
Electronic structure also relates to the organisation of elements in the Periodic
Table.
Q18. How many outer electrons do each of the elements in the following
Groups have?
Group 1...............................................................................................
Group 2...............................................................................................
Group 4...............................................................................................
Group 7...............................................................................................
He and Ne have complete outer energy levels (shells) of electrons. However, for
Argon, the n = 3 level can still take another 10 electrons. This means that for argon
some part of its n = 3 level must be complete with electrons resulting in its
characteristic stability. A more detailed knowledge of electronic structure is required
to appreciate this. This is discussed in a later section.
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Periodicity
In this section you are going to learn:
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•
The meaning of Periodicity and how to define it.
How to illustrate Periodicity in terms of both physical and chemical properties
of elements.
Periodicity refers to repeating patterns in physical and chemical properties
amongst the elements in the Periodic Table.
Take ‘first ionisation enthalpy plotted against atomic number’ as an example. Moving
across Period 2 of the Periodic Table from left to right produces a pattern. This pattern
is repeated across Period 3. Other physical properties to consider are electrical
conductivity, density, melting point, boiling point, and atomic radius. The idea is
illustrated below for first ionisation enthalpies of elements across Periods 1, 2 and 3.
Q19. From the plot above, what do you notice about the elements in
Group 0?
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Q20. From the plot above, what do you notice about the elements in
Group 1?
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Note that elements in the same Group of the Periodic Table occupy similar positions
along the plot. For example, the Noble gases are at the peaks and the alkali metals are
at the troughs. These observations illustrate periodicity.
Periodicity can also be illustrated with chemical properties. The example below plots
the amount in moles of chlorine atoms combined with one mole of atoms of each of
the elements of Periods 2 and 3.
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However, the pattern of periodicity amongst the elements in the Periodic Table does
at times break down. For example, phosphorus forms two chlorides, PCl5 and PCl3.