Trends in the Periodic Table

Trends in the Periodic Table

Introduction
Many events in the world have a tendency to occur at regular intervals. Did you know that the full moon appears after every 29.5 days? Similarly, chemical elements have some tendencies. The elements are arranged in a table in a precisely regular manner. This table is referred to as the Periodic Table and it is arranged in such a way that elements with similar properties appear at regular intervals. In this chapter, you will appreciate the diversity of properties of elements and how these properties change across the periods and groups of the Periodic Table.
3.1 Trends in Physical Properties of Elements Across the Periods in the Periodic Table
In Senior Two, you learnt about the arrangement of elements in the Periodic Table.
There is a vertical and a horizontal arrangement. The arrangement of elements in the table is not at random. The way elements are arranged in the Periodic Table gives rise to different trends in the properties of the elements.
In Activity 3. I(a), you will identify the groups and periods of elements in the Periodic Table.

Observation and Analysis

1. What conclusion can you draw about elements in the same:
   a) group?
   b) period?
2. Element Q is in Period 3 and Group IV of the Periodic Table:
   (a) What is the atomic number of Q?
   (b) Write its electronic configuration.
3. The electronic configurations of elements V, W, X, Y and Z (the letters are not
   the actual symbols of the elements) are 2:2, 2:5, 2:8:3, 2:8:8:2, 2:8:7, respectively
   The grid below shows part of the Periodic Table. Put the letters V, W, X, Y and Z in
   the grid to show the positions of the elements the re resent.

Within the Periodic Table, you will observe regular variations (or patterns) o properties of the elements with increasing atomic number; both chemical and physical properties vary in a periodic (repeating) pattern. This is called periodicity. Figure 3.2 shows the Periodic Table. Study it carefully and you will notice several
im rtant features about it.

Now that you know how elements are arranged in the Periodic Table, the patterns in properties can easily be deduced.

Activity 3.1 (b): Investigating the patterns in the arrangement of elements in the Periodic Table
Do this activity individually.
What you need:
Periodic Table
. marker pens
What to do
Study Figure 3.2 carefully. flip charts

• Internet or library materials

Observation and Analysis

1. Using your prior knowledge of the Periodic Table and the elements contained in it, identify the number of periods and groups.
2. How are elements arranged and divided in the Periodic Table?
   Conclusion
3. Explain what periods and groups are, and the principle underlying the arrangement of elements into groups and periods in the Periodic Table.
4. What pattern do you observe in regard to arrangement of the elements across the periods and down the groups?
   Present your findings to the rest of the class.

3.2 Trends in Properties of Simple Compounds of the Elements in the Third Period
Periodic trends are specific patterns in the properties of elements that are revealed in the Periodic Table. Major periodic trends include atomic radius, metallic character and reactivity.

Activity 3.2(a): Identifying Period 3 elements from the Periodic Table using atomic numbers

Do this activity individually.
What you need:
Periodic Table
. Internet
What to do

• relevant Chemistry textbooks
  Study and interpret the Periodic Table provided in Figure 3.2.

1. Identify the elements in Period 3 of the Periodic Table.
2. What do you notice as you move from one element to another across Period 3?
   Present your findings to the rest of the class.
   At this level, we shall only consider the following trends: atomic radius, metallic character, melting points, boiling points and density.

Trend in atomic radius
The atomic radius of an element is a measure of the size of its atom. It is defined as the distance between
the center of the nucleus of an atom and its outermost shell.
For example, in a covalent bonding situation of two individual atoms as shown, atomic radius is equal to
x/2, where x is the distance between the two nuclei of the atoms.

Assignment 3.1
Study the structure of the lithium atom shown and answer the questions that follow.

1. Identify the forces:
2. Which one of the forces above operates to:
   a) decrease the size of the atom?
   b) increase the size of the atom?

Activity 3.2(b): Variation in atomic radius for elements in the same period
Do this activity individually.
What you need:
. notebooks and pens
diagrams of atomic structures
What to do
Study and interpret the structures below and use them to answer questions that follow.

a)Identify the atom with;
i) largest atomic radius
ii) smallest atomic radius
a)Give reasons for your choice in (a)(i) and (a)(ii) above.
b)State the trend in atomic radii of Period 2 elements.

Trend in atomic radius down the group You will use your responses in Assignment 3.2 to identify and state the trend in the atomic radii of elements down a group.

Assignment 3.2
Study the atomic structure of Group
I elements below and answer questions that follow.
a) Identify the atom with the;
i) smallest atomic radius:
ii) largest atomic radius.
b) Give reasons for your choices in
(a)(i) and (a)(ii) above.
c) State the trend in atomic radii of elements occupying the same group.

Trend in metallic character
Metallic character depends on how readily atoms of elements lose their electrons to form cations (positively charged ions).
Increase in atomic size increases the tendency to lose electrons, hence increasing the metallic character.
Decrease in atomic size decreases the tendency to lose electrons, hence decreasing metallic character

Trend in melting and boiling points of Period 3 elements
Have you ever placed a piece of ice in an open place or lit a candle at home? What did you notice? Some substances require more heat than others to change their physical state (melting). Why is this so?
Elements of different groups and periods exhibit different trends in boiling and melting points. Melting points and boiling points depend on the strengths of forces which exist between the particles which make up a substance.

Thus, in Activity 3.2(c), you will find out how the melting points and boiling points vary across periods.

Activity 3.2(c): Exploring the trends in melting and boiling points of the elements in Period 3 of the Periodic Table
Work in groups.
What you need:
relevant data on melting and boiling points of elements from Na to Ar access to Internet or library

What to do
1.On the same pair of axes, draw a graph of the melting point and boiling point
for the elements from Na to Ar.
2.Study and describe the trends in the melting and boiling points shown on the graph.
3.Which factors contribute to the observed trends in the melting point and boiling point across period 3.
4.Draw a conclusion with respect to the melting points and boiling points of the elements across Period 3 and explain the trends.
5.Share your findings with the rest of the groups.

How densities vary across a period
In Activity 3.2(d), you will explore the trends in the densities of Period 3 elements of the Periodic Table. You will also explain the observed trends.

Activity 3.2(d): Exploring the trend in densities of the elements in Period 3 of the Periodic Table
Work in groups.
What you need:

• internet or relevant chemistry textbooks
• relevant data on the densities of elements of Period 3, as in Table 3.3
  Table 3.3: Densities of elemen

What to do

1. On the same pair of axes, draw a graph of density against atomic number for the elements from Na to Ar.
2. Study and analyse the trends in the densities of Period 3 elements shown on 2 the graph you have drawn. Describe and explain the trends across the period.
3. Which factors contribute to the observed trends in density across the period?
4. How does each of the above factor affect the observed trend?
5. Share your findings with the rest of the groups.
   In Senior One, you learnt about compounds and their chemical composition. You will now learn about compounds of elements of the third period. The simple compounds of elements of the third period include oxides and chlorides.
   Chemical properties of Period 3 elements
   Different observations are made when Period 3 elements participate in chemical reactions.
   In this section, you will learn about the chemical properties of Period 3 elements and their reactions with oxygen, water and chlorine.
   (a) Reaction with Oxygen
   Each of the elements in Period 3 except chlorine and argon, combines directly with oxygen to form an oxide. The trend in the reactions varies across the eri

What to do
Hold a piece of sodium using a clean, dry pair of tongs and place it on a Petri dish.
I.Cut a small portion (the size of a grain of rice ) of the piece of sodium.
2.Place the portion of sodium on a deflagrating spoon and warm it gently until it .starts to burn.
4.lower it (while still on the deflagrating spoon) into a gas jar containing oxygen as shown in Figure 3.4.
5.Remove the deflagrating spoon after the reaction stops.
Measure 10cm3 of distilled water and pour it into the jar and shake. Add 3
6.drops of universal indicator solution into the jar and record your observations. Repeat steps (1)-(6) using a clean piece of magnesium ribbon (5cm long) and a
7.small piece of aluminum foil and heat strongly before lowering them separately, into a gas jar
Repeat step (3) using a piece of white phosphorus, but it should be slightly warm.
8.Once the phosphorus starts smoking , put out the fire and repeat steps (4) – (6),
9.Repeat step (3) using half-filled deflagrating spoon of powdered sulphur. Heat
10.until it starts burning before lowering it in a gas jar of oxygen, Repeat step (6) once.

Activity 3.2(f): Exploring the trends in physical and chemical properties of oxides and chlorides of Period 3 elements
Work in groups.
What you need:
internet

• relevant Chemistry text books
  Manilla paper and marker pens
  What to do

1. Research and collect information about oxides and chlorides of Period elements.
2. Complete the table below using information and data collected from your

Period 3 elements, except chlorine and argon, react directly with oxygen to form different oxides.

Activity 3.2(g): Investigating the reactions of Period 3
elements with water
Work in groups.
What you need:

Safety Precaution
The size of the sodium to be put in water should be very small, as large pieces would result in an explosion! Sodium reacts with water explosively.

What to do
1.Hold a piece of sodium using a clean dry pair of tongs and place it on a Petri dish.
2.Cut a small piece of the sodium (size of a rice grain) and put it in a beaker containing cold water.
3.Record your observations in Table 3.5.
4.Repeat step using, separately, a small piece of magnesium ribbon, aluminium foil, white phosphorus and sulphur
5.Put a clean piece of magnesium ribbon in a test tube to react with steam, as
shown in Figure 3.6. Test the gas produced using a burning splint.

(b) Reaction with Chlorine
Generally, the anhydrous chlorides of elements in the third period (except chlorine and argon) are prepared by heating the elements in dry chlorine gas, or passing dry hydrogen chloride gas over the heated elements.
In Activity3.2(h), you will investigate the reaction of white phosphorus with chlorine.

Activity 3.2(h): Investigating the reaction of white phosphorus
with chlorine
The teacher will demonstrate this experiment.
What you need:

• jar
• cutting knife
  Petri dish
• deflagrating spoon
  pair of tongs
• water
  Safety Precaution
• White phosphorus catches fire when exposed to air.
  white phosphorus
  source of chlorine
  Chlorine gas is poisonous; so, wear a gas mask during the experiment.
• White phosphous should not be heated before putting it in a gas jar of chlorine.

Safety Precaution

• White phosphorus catches fire when exposed to air.
  Chlorine gas is poisonous; so, wear a gas mask during the experiment.
• White phosphous should not be heated before putting it in a gas jar of chlorine.

Observation and Analysis

1. What happened when white phosphorus was lowered into the gas jar containing chlorine gas?
2. Why was white phosphorus cut while under water?
3. Write down the equation(s) for the reaction(s) that took place.
   Now that you know about the reaction of chlorine with white phosphorus, you will research and discuss the reactions of other Period 3 elements with chlorine in
   Activity 3.2(i).

Activity 3.2(i):
Work in groups.
What you need:

• Internet
  What to do
  Exploring the reactions of Period 3 elements
  with chlorine
• relevant Chemistry textbooks
  Research and report on the reaction of Period 3 elements with chlorine.

C} Analysis and Conclusion
Write the equations for the reactions of chlorine with:
1.a) sodium
b) magnesium
c) aluminium
d) silicon
e) sulphur
Copy and complete Table 3.6.
Arrange the elements starting with the most reactive one. Give reasons to support your arrangement.

3.3 Physical and Chemical Properties of Elements in Group I
How can one predict the physical and chemical properties of any element? The periodic Table can be used to predict the properties of new elements, because the elements in it are arranged according to their atomic numbers and they exhibit periodicity.
The properties of elements change regularly. Periods in the Periodic Table have their own trends and properties, as well as the groups.
Physical properties of Group I elements
Group I elements occupy the first column on the far left of the Periodic Table. The elements in this group are called alkali metals. The alkali metals share similar physical and chemical properties.
(a) Trends in melting points and boiling points of Group I elements

(b) Trend in hardness of metals in Group I
In this section, you will learn about the trend in the hardness of Group I elements.

• Activity 3.3(b): Finding the trends in the hardness of Group elements
  The teacher will demonstrate to you the hardness of the elements in Group l.
  What you need:
• knife or razorblade
• sodium metal
• dry white tile
• lithium metal
• potassium metal
• plastic gloves
• pair of forceps

Observation and Analysis
Which of the three Group I metals is the easiest to cut?
Explain the appearance of the metals before cutting and immediately after cutting.
Briefly explain why sodium and potassium are kept under oil or paraffn.
Describe the trend in the hardness of Group I elements from your findings.

(c) Trend in densities of Group I elements
In Activity 3.3(c), you will learn about the variation in densities of Group I elements.

chemical properties of Group I elements
Knowing the chemical properties of elements helps chemists to make predictions about the types of reactions of those elements.
In this section, you will study the reactions of Group I elements with oxygen, water and chlorine.

Safety Precaution
The size of the metal to be heated should be very small, as burning large
pieces would result in an explosion!
What to do
1.Hold a piece of lithium metal using a clean, dry pair of tongs and place it on a Petri dish.
2.Cut a small piece of the lithium (the size of a rice grain) metal.
3.Put the cut piece on a deflagrating spoon and heat it until it starts to burn.
4.Record your observation.
5.Repeat steps (7)-(4) using sodium and potassium metal instead of lithium.

(b) Reaction with water
In Senior Two, you learnt about how metals like magnesium, iron and aluminium react with water
In Activity 3.3(d), you will learn about how Group I metals react with water.
Activity 3.3(e): Exploring the trend in the reactions of Group I elements with cold water
This activity will be demonstrated by the teacher.
What you need:

• cold water
  glass beakeis
• sodium metal
• potassium metal
  pair of forceps
  gloves
  lithium metal
  litmus paper (red and blue)
  Safety Precaution
  The size of the metal to be put in water should be very small, as large pieces would result in an explosion!!
  What to do
  Fill a beaker with cold water until it is half-full.

1. Carefully observe as the teacher uses a pair of forceps to pick a sample of
   lithium metal and then dropping it into the beaker containing water.
2. Record your observations.
3. Test the resulting solution with litmus paper (red and blue, but separately).
4. Repeat steps (7)-(4) with samples of sodium and potassium metals.

Activity 3.3(f): Exploring the trend in reactions of Group I elements with chlorine
Work in groups.
What you need:

• Internet
  . notebooks
• relevant Chemistry textbooks pens
  Safety Precaution
  The size of the metal to be put in water should be very small, as I pieces would result in an explosion!
  What to do

1. Research and make summarised notes about the effects of burning Li, Na, and K in chlorine gas.
2. Discuss as a group and present to the class.

Discussion questions
What are the colours of the flames produced when Group I elements burn in chlorine gas?
What happens when hot lithium is lowered into a gas jar of chlorine?
What is the product formed when an alkali metal reacts with chlorine?
Write equations for the reactions between alkali metals and chlorine gas.

Chapter Summary
In this chapter, you have learnt that:

• the Periodic Table is a tabular chart that shows the arrangement of chemical elements in order of their atomic number, in rows and columns, so that elements with similar atomic structure (and hence similar chemical properties) appear in the same column (group).
• when arranged by atomic number, the elements show periodicity.
• all atoms of elements in the same group have the same number of electrons in their outermost energy levels. So, all elements in a group have similar reactions, because they have the same number of valency electrons.
• the period number tells you the number of energy levels in an atom.
• the arrangement of elements in the Periodic Table gives rise to trends in the properties of the element, for example, across a period, metallic character decreases.
• all the elements of Period 3, except chlorine and argon, combine directly with oxygen to form oxides.
• all, the elements of Period 3, except argon, combine directly with chlorine to form chlorides.
• elements in Group I are very reactive, and great care needs to be taken when carrying out experiments involving these elements—as the reactions can be explosive.

explicit language.

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