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Transcript of Periodicity
Welcome to the Wonderful World of Periodicity
Periodicity is a pattern seen across the periods, or rows, of the periodic table. This pattern repeats itself in each of the periods. This is the basis for periodicity.
There are five main physical properties that change across a period:
1. Atomic radii
2. Ionic radii
3. Melting point
4. Electrical Conductivity
5. First ionisation energies
Now we will look at the chemistry of some of the elements of Period 3, focusing on their oxides and chlorides.
Norris, Roger. Cambridge International AS and A Level Chemistry Coursebook. Cambridge: Cambridge University Press, 2001. Print.
String Activity/ Picture
A Brief History:
Dimitri Mendeleev organized the first periodic table in 1869, ordering the elements known at that time by their atomic mass; he left gaps for unknown elements, yet to be discovered.
Now the elements are ordered by atomic number, with eight major groups (numbered with Roman numerals).
Oxides of Period 3 elements
Chlorides of Period 3 elements
The sizes of atoms can be compared using their atomic radii, most commonly obtained using the covalent radius.
Across a period, the number of protons (and hence the nuclear charge), and the number of electrons, increases by one with each successive element. The shielding effect remains roughly constant, so the greater attractive force exerted by the increasing positive nuclear charge on the outer shell electrons pulls them closer to the nucleus (decreasing the atomic radius and making the atoms smaller).
Cations are much smaller than their atoms because they have lost their outer shell, and there is less shielding of the outer electrons. From Na to Si, the ions get smaller.
Anions are larger than their atoms because they gained electrons, increasing the repulsion between them (whilst keeping the same nuclear charge) and making the anion bigger. From P to Cl, the ions decrease in size.
First Ionization Energies
The melting point increases from Na to Si. This is because Na, Mg and Al are metals and have high melting points. Na gives off one electron in the giant metallic lattice, while Al gives three; therefore, the forces of attraction between Al ions and the electrons are stronger, making the melting point higher.
Si has the highest melting point because of its giant molecular structure of very strong covalent bonds.
The melting point decreases from Si to Ar, because the elements to the right of Si are small molecules with weak Van der Waals' forces, requiring little energy to break.
Electrical conductivity increases from Na to Al, because the atoms successively give more electrons to the 'sea' of delocalized electrons, allowing a current to flow more easily and making Al a better conductor than Mg or Na.
Si has a much lower electrical conductivity because it has no delocalized electrons that can move freely about its structure; Si is a semi-metal or metalloid.
Electrical conductivity falls even further to the insulators P and S.
In general, the first ionization energies increase across the period as the nuclear charge increases and the shielding effect is relatively the same.
However, there are small dips between Mg and Al, and P and S.
Between Mg and Al, the distance between the nucleus and outer electron increases slightly, the shielding by the inner shells increases VERY slightly, and these two factors outweigh the increased nuclear charge.
Between P and S, spin-pairing of electrons plays a part: the electron from S is slightly repelled by its neighbor electrons, allowing it to be removed with slightly less energy.
Reactions of Period 3 elements with oxygen
Sodium reacts vigorously when heated in oxygen, burning yellow and producing sodium oxide.
Magnesium also reacts vigorously when heated in oxygen, burning bright white and forming magnesium oxide.
Powdered aluminum reacts well with oxygen, burning bright white and producing aluminum oxide.
Silicon reacts slowly with oxygen to form silicon(IV) oxide.
Phosphorous reacts vigorously with oxygen; a yellow or white flame is seen and clouds of white phosphorous(V) oxide are produced.
Sulfur powder burns gently with a blue flame in oxygen, producing toxic fumes of sulfur dioxide.
Chlorine and argon do not react with oxygen.
Reactions of Period 3 elements with chlorine
When sodium is heated and plunged into chlorine gas, it vigorously forms sodium chloride.
Magnesium and aluminum also react vigorously with chlorine gas.
Silicon reacts slowly, giving silicon(IV) chloride.
Phosphorous also reacts slowly with chlorine gas.
Sulfur does form chlorides, but they don't need to be known.
Argon does not form a chloride.
Reaction of sodium and magnesium with water
Sodium reacts vigorously with cold water, melting into a ball of molten metal. It forms hydrogen gas and a strongly alkaline solution of sodium hydroxide.
Magnesium reacts slowly with water, unless it is hot. Normally, it would take several days to produce a test tube of hydrogen gas, making a weak alkaline solution. If hot, it will react with steam to form magnesium oxide.
The oxidation number for each of the elements in Period 3 rises across the period. This happens because the Period 3 element in each oxide can use all of its electrons in the outermost shell in bonding to oxygen. They all exist in oxidation states which are positive. This is due to the higher electronegativity of oxygen.
The oxidation numbers rise crossing Period 3 until sulfur has been reached.
Why? From sodium to phosphorus, they use all of their electrons in their outermost shell for bonding with chlorine.
Effects of water on chlorides of Period 3 Elements
Chlorides show characteristic behavior when we add them to water (this is linked to their structure and bonding).
Effect of water on oxides and hydroxides of Period 3 elements
The oxides of sodium and magnesium react with water to form hydroxides, making aqueous solutions alkaline.
Aluminum oxide can behave as both an acid or a base; these compounds are known as amphoteric.
Effect of water
Elements, Atomic Radii and the Periodic Table. 2008. Camsoft.co.krWeb. 28 Feb 2013.
1) Describe how the physical properties of atoms vary across Periods 2 and 3.
2) Explain this trend.
Both sodium oxide and magnesium oxide react with water to form hydroxides (alkaline).
Aluminum oxide can behave as an acid or a base, and is called amphoteric.
Silicon dioxide acts as an acid.
Phosphorous(V) oxide reacts vigorously and forms an acidic solution.
The same is true for the oxides of sulfur.
Compounds which can act as both acids and bases, such as aluminium oxide, are called amphoteric.