UNIT14:TRENDS OF CHEMICAL PROPERTIES OF PERIOD 3 ELEMENTS AND THEIR COMPOUNDS
UNIT 14: TRENDS IN CHEMICAL PROPERTIES OF PERIOD 3
ELEMENTS AND THEIR COMPOUNDS
Key unit competency: Compare and contrast the properties of the Period 3 elements and
their compounds in relation to their positions in the Periodic Table.
Learning objectives:
By the end of this unit, students should be able to:
• Compare the physical properties of the Period 3 elements.
• Describe the nature of the oxides of the Period 3 elements and the type of
bonding in their chlorides, oxides and hydrides.
• Relate the physical properties of the Period 3 elements to their position in
Periodic Table.
• Relate the physical properties of compounds of the Period 3 elements totheir nature of bonds across the period.
14.1. Physical Properties of the Period 3 elements
Activity 14.1
1. Write the electronic configuration of the following elements in terms of s, p, d
and f…
(i) Sodium (ii) Magnesium (iii) Aluminium (iv) phosphorous (v) sulphur
2. Considering the electronic configuration of magnesium and Aluminium, phosphorus and sulphur. How do you expect their ionization energies to vary?
3. How do you expect the general trend in ionization energy, electron affinity,
melting and boiling point, electronegativity to vary for the elements in the period 3?
4. Considering the electronic configuration of magnesium and Aluminium, phosphorus and sulphur. What can you say about them, how do you expect their
ionization energies to vary?
(a) Variation of First ionization energies (IE) of Period 3 elements
First ionization energy generally increases across Period 3 from left to right. However,
it drops at aluminium and Sulphur (table 14.1 and Fig.14.1). This can be explained interm of more stable electronic structures of the two elements after losing 1 electron:
Going across Period 3 from left to right, the number of protons in the nucleus
increases so, the nuclear charge increases. There are more electrons, but the
increase in shielding is negligible because each extra electron enters the same
principal energy level. Therefore, the force of attraction between the nucleus and
the electrons increases. So the atomic radius decreases as indicated in the Figure
14.2 and table 14.2.
(c)Variation of electronegativity of Period 3 elementsTable 14.3: Variation of electronegativity of period 3 elements
Figure 14.3: Graph showing the variation of electronegativity of period 3 elements
Going across Period 3 from left to right, electronegativity increases almost linearly
due to the nuclear charge increase as atomic radius decreases. There are more
electrons, but the increase in shielding is negligible because each extra electron
enters the same principal energy level so electrons will be more strongly attracted
to the nucleus.
You might expect argon (with 18 electrons) to be the most electronegative element
in Period 3, but its outermost energy level is full. Therefore, it does not form covalent
bonds with other atoms, so it is given an electronegativity value of zero.
d. Variation of melting and boiling points in Period 3
Melting and boiling points generally increase going from sodium to silicon, thendecrease going to argon with a “jump” at Sulphur (Fig 14.4 and Table 14.4).
The delocalized electrons are free to move and carry charge. Going from sodium to
aluminium, the number of delocalized electrons increases, there are more electrons
which can move and carry charge so the electrical conductivity increases.
Silicon is called a semi-conductor because at higher temperatures more electrons
are promoted to the higher energy levels so there are more delocalized electrons to
move and carry charge.
Phosphorus, sulphur and chlorine, the outer electrons are not free to move and carry
charge because they are held strongly in covalent bonds. In argon (mono atomic) the
outer electrons are not free to move and carry charge because they are held strongly
in a stable third energy level and this explains their zero electrical conductivity.
f. Variation of metallic character of period 3 elements
Metallic character decreases as you move across a period 3 in the periodic table from
left to right. This occurs as atoms more readily accept electrons to fill the valence
shell than lose them. Note that as the metallic character decreases across the period,
the reducing power decreases whereas oxidizing power increases.
g. Variation of electron affinity across period 3 elements
The electron affinity [EA] is the energy change for the process of adding an electronto a gaseous atom to form an anion (negative ion).
Checking up 14.1
1.Explain the variation of the following terms as applied in period 3 of the periodic
table:
(i) Ionization energy, Electronegativity, )
(ii)Explain the anomalous behavior indicated by magnesium and phosphorous in graph 14.1 above
2.The table below shows the melting points of the period 3 elements except forsilicon:
(a)Explain in terms of bonding why the melting point of magnesium is higher
than that of sodium.
(b) Predict the approximate melting point of silicon.
(c) Explain why chlorine has a lower melting point than sulphur.
(d) Explain the variation of metallic character, electronegativity, atomic radii
,first ionization energy, melting and boiling points, electron affinity andelectrical conductivity across the period
14.2. Chemical properties of period 3 elements
Study questions:
1. What do you say about your observations made in experiment above.
2. Write equation for the reaction that occurs in each test tube in procedure 2.
(b) Experiment to investigate the action of heat on period 3 elements
Materials /apparatus:
Water , test tubes, a piece of sodium metal, aluminium power/sheet, magnesium
ribbon/powder, phosphorous and sulphur powder, universal indicator , pair of
tongs, source of heat
Procedure:
1. Hold a piece of magnesium ribbon on a Bunsen flame and record you observation.
2. Repeat experiment 1 for sodium, aluminium, phosphorous and sulphur and
record your observation in each case.
3. For each of the products formed i.e. for metal oxides formed, add water and dip
a litmus paper to test their nature.
Note: if the oxide is gaseous hold a piece of litmus paper on the mouth of the test tube.
Study questions:
1.Write equations to show how the metals react with oxygen.
2. What would you expect to observe when the metal is burned in oxygen.
a) Reaction with water
Reactivity with water generally decreases across the period from left to right because
there is a decrease in metallic properties.
i) Sodium reacts vigorously with cold water to form sodium hydroxide and hydrogengas.
14.3. Compound of period 3 elements
The oxides of period 3 elements:
Activity 14.3(a)
1. Write the formulae of the oxides of period 3 elements
2. What did you consider when writing the formulae of the oxide in 1 above?
3. How do you expect the oxides to behave in water? Explain your answer.4. Suggest the trend of acid- base character of the oxides of period 3
middle, silicon forms a giant covalent oxide (silicon dioxide); the elements on the
right form simple molecular oxides with simple structures. The intermolecular forces
binding one molecule to its neighbors are van der Waals dispersion forces or dipoledipole interactions.
Physical properties of the oxides of period 3 elements
Melting and boiling points: the metal oxides and silicon dioxide have high melting and
boiling points because a large amount of energy is needed to break the strong bonds (ionic
or covalent) operating in three dimensions. The oxides of phosphorus, sulfur and chlorine
consist of individual molecules.
Electrical conductivity: None of the oxides above have any free or mobile electrons,indicating that none of them will conduct electricity when solid.
Acid-base Behavior of the Oxides
Activity 14.3 (b)
1. Classify the oxides in terms physical states of the oxides of period 3.
2. How do you expect the oxides react with water, acids, and sodium hydroxide.
(use equations to justify your answer)
3.(a) Predict the nature of oxides of period 3 elements when dissolved in water.
(b)What would you expect to observe when both blue and red litmus papers
are dropped into each of the solutions formed in question (2) above in water.
Acidity increases from left to right, ranging from strongly basic oxides on the left to
acidic ones on the right, with an amphoteric oxide (aluminum oxide) in the middle.Reaction of oxides with water:
Electrical conductivity: solid chlorides do not conduct electricity because the ions
are not free to move
Sodium, magnesium and aluminium chlorides are ionic and so will conduct
electricity when they are molten or in aqueous solution. The rest of the chlorides donot conduct either in solution or molten state due to absence of ions
Reactions with water
Checking up 14.3(c)
1. a. Distinguish between dissolving and hydrolysis.
b. Name one chloride that dissolves in water, and one chloride that undergo
hydrolysis.
c. State how the bonding in the chlorides changes on crossing the second and
third periods from left to right
Checking up 14.3(c)
1. a. Distinguish between dissolving and hydrolysis.
b. Name one chloride that dissolves in water, and one chloride that undergo
hydrolysis.
c. State how the bonding in the chlorides changes on crossing the second andthird periods from left to right
14.3.3. The hydrides of period 3 elements
Activity 14.4(d)
1. Period 3 elements from sodium to chlorine form different hydrides of different
bond nature, physical properties and structure.
(a) Write the formula of the hydrides formed by period 3 elements.
(b) Predict the nature of bonding based on your knowledge of periodicity of elements in the periodic table.
(c ) Basing on the nature of bonding predicted in (b) above. How would you expect their boiling and melting point vary across the period?
(d) Predict the nature of solutions formed by hydrides when dissolved in water.
What would you expect to observe if red and blue litmus papers were separately dropped into each solution?
Hydrides are commonly named after binary compounds that hydrogen forms with
other elements of the periodic table. Hydride compounds in general form with
almost any element, except a few noble gases. The common hydrides of period 3elements are as shown in the table 14.6 below
The hydrides above are examples of period 3 elements with some of their properties
summarized in the tabl. As we can see the hydrides of period 3 vary from ionic
hydride such as NaH at the left side to polar covalent hydride such as HCl at the rightside of the period.
a. In terms of crystal structure and bonding, explain in each case why the melting
points of sodium oxide and silicon dioxide are high.
b. Predict whether the melting point of lithium oxide is higher than, the same as,
or lower than the melting point of sodium oxide and explain your prediction.
c. Phosphorus (V) oxide has a lower melting point than sodium oxide.
i. State the structure of and bonding in phosphorus (V) oxide.
ii. Explain why the melting point of phosphorus(V) oxide is low.
d. Samples of phosphorus(V) oxide and sodium oxide were reacted with water.
In each case, predict the pH of the solution formed and write an equation for
the reaction.
4. Sodium chloride is a high melting point solid which dissolves in water to make
a colorless solution. Silicon (IV) chloride is a liquid at room temperature which
fumes in moist air, and reacts violently with water.
a. Draw a diagram to show the arrangement of the particles in solid sodium
chloride, making clear exactly what particles you are talking about.
b. Explain why this arrangement leads to a high melting point.
c. Draw a simple diagram to show the structure of silicon (IV) chloride, and explain
why silicon (IV) chloride is a liquid at room temperature.
d. Why is there such a big difference between the chlorides of sodium and silicon?
e. Briefly describe and explain the difference in electrical conductivity between
sodium chloride and silicon (IV) chloride in both solid and aqueous molten
state.
f. Write an equation to show what happens when silicon (IV) chloride reacts with
water.
g. Name another Period 3 chloride which behaves similarly to sodium chloride,
and one which behaves similarly to silicon (IV) chloride.
5. With the help of equation describe how the hydrides of period 3 react withwater.
14.4. End unit assessment1. Use the information in the following table to explain the statements below
3. The elements Sodium, Magnesium, silicon, phosphorous and chlorine are
members of the third period of the periodic table
a. i. Write down the formula of the principal oxides and chlorides of the elements
listed above and in each case indicate the type of bonding.
ii. Explain what happens when each of the above oxides and chloride is added to
water and indicate whether the resultant solution will be acidic, basic or neutral.
c. The melting points of Mg , Si and S are 6500
C, 14230
C respectively. Explain the
differences in the melting points of the elements.
d. Name the type of bonding that exists in the hydrides of the elements Sodium,
Phosphorous and sulphur and write the equations to show the reactions if any
of the hydrides with water.
4. Choose from the elements: Sodium, magnesium aluminium, silicon, phosphorous, chorine and argon
a. List the elements that react readily with cold water to form alkaline solutions.
And write the equations for the reactions.
b. List the hydrides that have hydrides with low boiling points/temperatures and
explain why.
c. List the elements that form nitrates and write the formulae of nitrates.
d. What is the most ionic compound that can be formed by the combination of
two of these elements.
e. Which element has both metallic and non metallic properties?
f. Name the elements that normally exist as molecules