Keywords

• group
• metal
• neutron
• non – metal
• Proton
• trends
• electron
• electronic
• arrangement
• atomic number
• diversity
• period

By the end of this unit, you will be able to:

(a) Classify or group elements as metals and non – metals.

(b) Relate the physical properties of metals to their uses.

(c) Know that the Periodic Table is a classification of elements according to the atomic or proton number.

(d) Relate the arrangement of electrons in the first 20 elements to their positions in the Periodic Table.

 (e) Understand the relationship between the position of elements in groups and the charge on the ions that they form.

 Competency

You should be able to investigate the diversity of elements in the Periodic Table.

3.1. Introduction

In this section, you will learn how to group elements as metals non – metals and semi – metals or metalloids, and the reasons for that classification. You will finally learn how elements came] to be arranged in the Periodic Table

In Book One, we discussed elements. Do you recall what elements are?

Several elements in different physical states exist around us

Some are solids while the others are either liquids or gases.

We use the elements in everyday life in many ways. For example, aluminium is used in making saucepans and iron is used to reinforce concrete in construction works.

Since many elements were known to exist, it was important to classify them. The classification is based on certain properties the elements exhibit.

3.2 Classification of elements

As early as the 19th century, there were several attempts by scientists to arrange elements in an orderly manner, particularly after realizing that there existed a relation between their physical and chemical properties.

 In the following activity, you will learn how to classify elements according to their properties.

Physical properties of metals and non – metals

You have seen that elements can be categorized as metals or non – metals on the basis of their properties.

Remember elements are useful to us because of their special properties.

The properties can be classified as physical or chemical.

In this section, you will look at how to tell if a substance is a metal or a non – metal by looking at some of its physical properties. Physical properties are such as density, hardness, melting point, and electrical conductivity, among others.

Now, let’s look at some of the physical properties of elements in detail in the following activity:

Figure 3.1: Testing electrical conductivity of metals and non – metals

Figure 3.2: Testing electrical conductivity of metals and non – metals

Figure 3.2: Testing heat conductivity of metals and non – metals

Table 3.1: Summary of physical properties of elements

Results and discussion

• What general conclusions can you draw from the observations in Activity 3.2 about the physical properties of elements?
• Metals are used in various ways as a result of their physical properties. Using the results in Table 3.1, copy and complete the following table:

• Suggest one use of carbon as a result of its electrical conductivity in Table 3.1 of results.
• Make a summary of your findings. Share your findings with other group members.

Assessment 3.1

1. Make a summary of the physical properties of metals and non – metals in the form of a table.
2. Basing on the knowledge acquired in Activity 3.1 and Activity 3.2 classify the following elements as metals and non – metals:

Iron , sulphur , copper , hydrogen , silver , aluminium , copper , chlorine , argon , sodium , magnesium , calcium , iodine , nitrogen and zinc . (Use a table for your classification. Your table should include the chemical symbols of the element. )

The structure of an atom

Everything you see around you is made up of tiny particles called atoms. Do you recall what an atom is?

The idea that everything is made of atoms (indivisible particles) was first suggested by a Greek philosopher about 2000 years ago.

With scientific advancements, it has been proved that atoms themselves are made up of even smaller particles. The smaller particles are called the fundamental particles (or subatomic particles) of an atom

The three main subatomic particles of interest at this level are electrons, protons and neutrons.

Assessment 3.2

1. The full symbol of an atom of element Q is ₁₆³² Q. State:

a) What the numbers 16 and 32 stand for.

b)The number of neutrons in Q.

c) The number of electrons in Q.

2. An atom of argon has 18 protons and 22 neutrons in its nucleus.

a) What are its atomic number and the mass number?

b) Write the full symbol of argon .

How are electrons arranged in the atoms?

From Activity 3.3, you have found out that:

• Protons and neutrons are in the nucleus of an atom. The nucleus is at the centre of the atom.
• Electrons move in definite orbits (shells / energy levels). The orbits are around the nucleus.
• In an atom, the number of protons is equal to the number of electrons.

Now, the question is: How are the electrons in an atom arranged in orbits?

The way electrons are arranged in an atom of an element is referred to as the electronic structure.

The electronic structure is also called electron configuration.

 Further scientific experiments revealed that:

• There are an infinite number of energy levels around the nucleus.
• The energy levels are at different distances / positions from the nucleus.
• The maximum number of electrons that can be accommodated in the first orbit is 2 and 8 in other orbits for the first 20 elements.
• Electrons are not accommodated in a given shell unless the inner shells are filled. That is, the shells are filled in a step – wise manner

Group discussion

In pairs:

1. Copy and complete the table below:

2. Share your findings with the rest of the class.

 A simple model of an atom showing arrangement of electrons is as follows

 The atom of the element represented has atomic number 20.

We can write this electron arrangement as 2,8,8,2.

Now let us look at how to write the electron arrangement of the first 20 elements in the following activities:

Figure 3.3: Neils Bohr (1885-1962)

Credit goes to the work done by the Danish physicist Neil’s Bohr about the electronic structure

Table 3.2: Electronic structures of the first 20 elements Element

Results and discussion

• Which elements have fully filled outermost shells?
• What name is given to elements whose outermost shells are fully filled?
• What can you say about the reactivity of elements whose outermost shells are fully filled? Give a reason for your answer
• Why do you think some elements combine with other atoms?

In Activity 3.4, we have learnt how to write and draw electronic structures. Now let us look at how to make a static atomic model of the elements.

Group discussion

In pairs:

1. The electronic structure of an element Y is as follows :

a) State the atomic number of element Y.

Write the electronic structure of Y.

2(a) Draw the electronic structures of hydrogen, nitrogen, magnesium and chlorine atoms.

 b) State the atomic number and write the electronic structure of each element in (a).

 3. Share your ideas with your classmates.

3.3 The Periodic Table of elements

What is the Periodic Table?

With scientific advancements, our knowledge about the elements and their behavior increased. Scientists, therefore, recognized the need to organize this information.

They recorded observed patterns and similarities in the behavior of some groups of elements. After a number of attempts, finally, modern Periodic Table was prepared.

This was a result of the teamwork under the guidance of Moseley .

In this section, you will first look at the early attempts by scientists to organize elements in a tabular form.

 Figure 3.4: The Periodic Table of elements

Assessment 3.3

1. The following sets contain elements arranged in order of increasing atomic masses.

Determine which of these sets form Dobereiner’s triads:

• Lithium, sodium, and potassium
• Nitrogen, phosphorus, and arsenic
• Calcium, strontium , and barium
• Phosphorus, arsenic, and antimony
• Sulphur, selenium , and tellurium
• Iron, cobalt and nickel.

2. What made Dobereneir’s law of triads not to work for all elements?

3. State Mendeleev’s periodic law

4. Describe the characteristics of Mendeleev’s Periodic Table.

5. Discus the achievements and shortcomings of Mendeleev’s Periodic Table.

From Activity 3.6, you must have come across several laws proposed by the different scientists as they were coming up with a Periodic Table of elements.

One such law is Mendeleev’s periodic law that has helped to arrange elements in the modern Periodic Table.

DID YOU KNOW?

Figure 3.5: Dmitri Mendeleev (1834 1907)

In this section, we shall further explore the work of Mendeleev towards the modern Periodic Table.

Table 3.3: The first twenty elements Element

Results and discussion

• What kind of order do you observe when the cards are placed in order of increasing atomic mass?
• What alternative order do you observe when the cards are placed in order of another property of elements?

3.4. The first 20 elements in the Periodic Table

We have already seen that the elements in the modern Periodic Table are arranged in order of their proton number (atomic number) but not their mass number.

The arrangement places elements into:

• Vertical columns called groups. In the groups, the elements have similar properties.
• Horizontal rows of increasing atomic number called periods.

In this section, we shall investigate further about the structure of the Periodic Table using the first 20 elements.

Figure 3.6: Arrangement of the first 20 elements in the Periodic Table

Results and discussion

• What do the elements in each group (column) have in common?
• What do the elements in each period (row) have in common?
• What conclusions can you draw from your activity?

FUN FACT!  

1.The year 2019 was designated by UNESCO as the International Year of the Periodic Table (IYPT), marking the 150th anniversary of the Mendeleev Periodic Table, which is an iconic image and a vital tool to all who learn and work in science, at all stages of their learning and careers.

2. Try this link for an interactive Periodic Table: http: // www.rsc.org/periodic-table.

3.5. Position of elements in the Periodic Table and ions they form

Let us explore what determines the position of an element in a particular group and period in a Periodic Table.

Table 3.4: Electronic configurations of first three elements in group II

Table 3.5: Symbols, atomic numbers and electronic configuration of elements in period 3

Results and discussion

• What pattern do you observe in the electronic structures of the elements in the: 3

a) Group?                                       (b) Period?

• How many valence electrons are there for element in the:

                 a) group?                                    ( b ) period ?

• What is the connection between the valence electrons and the

                a) group number?                                (b) period number?

• What is the trend in the atomic number as you move from one element to another in the

                    a) group?                                                   (b) period?

• What can you say about the number of energy levels occupied by electrons in each elements in the

                   a) group?                                                     (b) period?

• What conclusion can you draw from this activity about the position of elements in the Periodic Table?
• Make a summary of your findings and share with your classmates and then your teacher.

How are ions formed by the elements?

In section 3.2, we learnt about metals and non – metals (classification of elements).

In the Periodic Table, metals are found towards the left – hand side while non – metals are found towards the right – hand side.

Elements normally gain or lose electrons to acquire a stable electronic configuration like the noble gases.

 Let us examine how elements in a group form ions and try to relate the charge on the ion to group number.

Results and discussion

• What can you say about the electronic configuration of group O elements?
• How does the electronic configuration of metals compare with that of group O elements?
• In what way can the three metals acquire an electronic configuration of group O elements?
• How does the electronic configuration of the non – metals compare with that of group O elements
• How best can the non – metals acquire a stable electronic configuration like those of group O elements?
• Give your findings by completing Table 3.6.

Table 3.6: Electronic configuration of ions formed by elements

• Share your findings in presence of your teacher

Assessment 3.4

1. The full symbol of an atom of element Ris

a) State the number of:

I)Protons in R

(ii) neutrons in R

b) Write the electron configuration of R

 c) State the group and period in the Periodic Table to which R belongs.

d) Write:

(i) The formula of the most likely ion R forms.

(ii) The electron configuration of the ion in d (i)

2.The atomic numbers of elements X and Y are 10 and 20 respectively.

 a)Write the electron configurations of the elements.

b)State the periods in the Periodic Table to which X and Y belong.

c) Write the formula of the most likely ion Y forms

 d) Give a reason why element X neither gains nor loses electrons.

Unit Summary

In this chapter, you have learnt that:

• elements are metals , metalloids or non – metals
• on moving from left to right in a period, the metallic character of elements decreases whereas the non – metallic character increases.
• ongoing down in a group of the Periodic Table , the metallic character of elements increases whereas the non – metallic character decreases .
• metals are lustrous , malleable , ductile and good conductors of heat and electricity .
• metals are solids at room temperature , except mercury which is a liquid .
• non – metals have properties opposite to that of metals . They are neither malleable nor ductile . They are bad conductors of heat and electricity , except for graphite , which conducts electricity .
• metals form positive ions by losing electrons .
• non – metals form negative ions by gaining electrons .