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Structure and bonding

Objectives

By the end of the lesson you should be able to:

(i) explain the formation of ionic bonding

(ii) use diagrams to illustrate ionic bonds

(iii) state properties of ionic compounds.

Covalent bonding occurs mostly between non-metal atoms.Hydrogen atom has one electron in its only energy level. This hydrogen atom combines with another hydrogen atom through sharing of their respective electrons to form hydrogen molecule.

PERIOD THREE ELEMENTS

Hallo ! Welcome to Chemistry for From 2 e learning series. In this CD we will discuss various topics which include; Atomic structure and the periodic table, Chemical families, Structure and bonding, Properties across period three, salts, Effects of an electric current on substances and finally Carbon and its compounds.The content is enhanced by the use of photograhs, Illustrations, animations and videos. You will be expected to attempt all the activities in the CD to ensure that concepts are well understood.

Structure of the atom and the periodic table

In this topic we will discuss the structure of the atom and the periodic table. In Form one an atom was defined as the smallest particle of an element that takes part in a chemical reaction.

Ca2 + CL2 w Ca22Cl2

T tanO0 Q 3P

||||||||||||||||||||||||||||2

 

In this topic we will discuss the structure of the atom.

Ca2 + CL2 w Ca22Cl2

T tanO0 Q 3P

||||||||||||||||||||||||||||2

 

Ca2 + CL2 w Ca22Cl2

T tanO0 Q 3P

||||||||||||||||||||||||||||2

 

Objectives

By the end of this topic you should be able to:

i) Describe the structure of the atom

ii) Explain the electron arrangement of atoms in terms of energy levels

iii) Define atomic number, mass number, isotopes and relative atomic mass.

Answer all the questions given below.

An atom is made up of three sub-atomic particles.Electrons,Neutrons and Protons.The central part of an atom is called the nucleus.The nucleus consists of protons and neutrons and the electrons move around the nucleus in energy levels.


 

Protons are positively charged, electrons are negatively charged, while neutrons are neutral.

Charges of the particles

Protons are positively charged, electrons are negatively charged, while neutrons are neutral.

Electrons surrounding the nucleus occupy regions called energy levels.

Electrons surrounding the nucleus occupy regions called energy levels.

 Example.

ANIMATION1 Show the protons and neutrons coming to the nucleus and the electrons arranging themselves on the energy levels.                                                                    xx

                                                                                                     xx                                                                 xx

                                        x                                                           xx                                                                 xx

                                       xx                                                          xx                                                                 xx

 

                                       3p                                           xx          12p           xx                             xx xx          20p           xxxx

Lithium                   4n                                                          12n                                                               20n

                                                                                                      xx                                                              

                                                                                           xx                                                 0              xx

                                                                                                                                                                           xx

                      

                           2:1                               2:8:2                                         2:8:8:2                                                                           


 

The arrangement of electrons as 2:1, 2:8:2 is referred to as electron configuration.

Audio: Electrons always occupy the unfilled energy level nearest to the nucleus.

: When an energy level is full additional electrons occupy the next energy level until it s full.

Lithium has three electrons. Two electrons occupy the first energy level. The third electrons occupy the second energy level.

For the first twenty elements the first energy level takes a maximum of 2 electrons, second energy level a maximum of 8 electrons,the third energy level takes a maximum of eight electrons. Electrons always occupy the unfilled energy level nearest to the nucleus. When an energy level is full the remaining electrons occupy the next energy level until it is full.

Sodium has 11 electrons. Two electrons occupy the first energy level. Eight electrons occupy the second energy level. One electron occupy the third energy level.

Calcium has 20 electrons. Two electrons occupy the first energy level. Eight electrons occupy the second energy level. Eight electron occupy the third energy level while 2 electrons occupy the fourth energy level.

The electrons in each atom are arranged according to the maximum number of electrons in each energy level. For example .The arrangement of the electrons as 2.1, 2.8.1 and 2.8.8.2 is referred to as electron configuration.

Atomic number is the number of protons in an atom . Mass number is the sum of protons and neutrons in an atom.

It is possible to determine the mass number and the number of electrons.

The mass number is placed as a left superscript of the symbol. The atomic number is placed as left subscript of the symbol.

Isotopes are atoms of the same element which have the same atomic number but different mass numbers. The illustration below shows the Isotopes of Lithium atom. Identify the number of protons and neutrons in each isotope.

The relative atomic mass is the number of times an atom of an element is heavier than one twelfth of carbon-12 atom.One atom of Carbon weighs exactly 12.000 atomic mass units (a.m.u) .The table below illustrates relative atomic masses of different atoms.

The following illustration shows the isotopes of Helium atom.Note that the isotopes have tha same atomic number but different mass numbers.

The relative atomic mass is calculated as the sum of the product of the isotopic masses and their fractional abundance or percentage abundance. Click on the following animation to view how to work out the RAM.

Examples of relative atomic mass calculations.

The element Boron consists of two isotopes.Boron 10 and Boron 11 with isotopic abundance of 20% and 80 % respectively. Calculate the relative atomic mass of Boron. Click on the button below to follow the worked out example.

Chlorine consists of 75% chlorine-35 and 25% chlorine-37.Calculate the relative atomic mass of Chlorine.Click on the arrow to follow the method used.

In this lesson we will discuss the periodic table

Objectives

By the end of this lesson you should be able to

: Explain the position of an element in the periodic table in terms of electron arrangement

QUIZ

Elements are grouped together based on the number of electrons in the outermost energy levels. Elements with the same number of electrons in the outermost energy level form a group. Lithium is in group one because it has one electron in the outermost energy level. Sulphur is in group six because it has six electrons in the outermost energy level. Magnesium is in group two because it has two electrons in the outermost energy level.Roll over the mouse on each of the first twenty elements to view their atomic number and electron arrangement.

There are eight groups in the periodic table. The groups are placed in a vertical column.Groups are indicated using Roman numbers. Elements between groups II and III are known as transition elements. Helium is in group VIII though it has 2 outermost electrons . This is because the first energy level can hold a maximum of two electrons. This means that the energy level is full and the element is stable.

Elements in a period have the same number of energy levels.Roll over the mouse on each of the first twenty elements to view their atomic number and electron arrangement. Identify the periods to which they belong.

Element X is in period one because it has one energy level. Element Y is in period two because it has two energy levels. Element Z is in period three because it has three energy levels.

In the periodic table each of the rows is called a period. Every element in the first period has has one energy level. Elements in the second period have two energy levels and so on.

From the periodic table the basic information about each element is given. In the symbol of the element, the superscript represent the R.A.M while the subscript represents the atomic number.

In this lesson we will discuss how ions are formed.

Sodium has electronic arrangement 2:8:1. for it to be stable it can either lose one electron in the outermost energy level to have electron arrangement of 2:8, or gain seven electrons to have electron arrangement of 2:8:8.
More energy is required to gain seven electrons than to lose one electron. Therefore it is easier for sodium to lose one electron than to gain seven electrons. When sodium atom loses one electron it will have 10 negative charges. The positive charges still remain 11. The net charge therefore becomes +1

Objectives

By the end of this lesson you should be able to:

Predict the type of ion formed from a given electron arrangement of an atom.

Sodium has electron arrangement 2.8.1. For it to be stable it can either lose one electron in the outermost energy level to have electron arrangement of 2.8 or gain seven electrons to have electron arrangement of 2.8.8. More energy is required to gain seven electrons than to lose one electron.

Therefore it is easier for sodium to lose one electron than to gain seven electrons. When sodium atom loses one electron it will have 10 negative charges. The positive charges still remain 11. The net charge therefore becomes +1

Sulphur atom has six electrons in the outermost energy level. When it reacts it gains two electrons to be stable since this requires less energy than losing six electrons.

Sulphur atom has six electrons in the outermost energy level. When it reacts it gains two electrons to be stable since this requires less energy than losing six electrons.

In an atom the number of protons is equal to the number of electrons. Group VIII elements have filled up outermost energy levels and are therefore stable.An atom however may lose or gain electrons in the outermost energy level.The following illustration shows this atom is neutral..

Chlorine has an electron arrangement of 2.8.7 Chlorine atom has seven electrons in the outermost energy level. It is easier to gain one electron to be stable than to lose seven. When Chlorine atom gains one electron it will have 18 negative charges. The positive charges will be 17.The net overall charge of the Chloride ion is -1.

Magnesium atom with electron arrangement 2.8.2 will lose two outer electrons to form magnesium ion with electron arrangement 2.8. It is easier for the Magnesium atom to lose two electrons than to gain 6 electron.The Magnesium ion formed will have 12 postive charges and 10 negative charges.The net overall charge of the Magnesium ion is +2

Sulphur atom has six electrons in the outermost energy level. When it reacts it gains two electrons to be stable since this requires less energy than losing six electrons.

Chlorine has an electron arrangement of 2.8.7 Chlorine atom has seven electrons in the outermost energy level. It is easier to gain one electron to be stable than to lose seven. When Chlorine atom gains one electron it will have 18 negative charges. The positive charges will be 17

SUMMARY.

When an atom Loses or gains electrons it becomes electrically charged.

Charged atoms are called IONS.

Positively charged ions are called CATIONS.

Negatively charged ions are called ANIONS.

During chemical reactions, metals LOSE electrons Non metals GAIN electrons.

Activity: lithium, fluorine, aluminium, magnesium and sulphur have atomic numbers 3,9,13,12 and 16 respectively.

 

Protons in the nucleus, which are positively charged (+), attract electrons which are negatively charged (-) in the energy levels.For an atom to lose an electron, this force of attraction must be overcome. This is achieved by supplying energy to pull off the electrons. This energy supplied is called IONISATION ENERGY. It is measured in JOULES(J).Ionisation energy is thus defined as the energy required to remove an electron(s) from an atom in gaseous state to produce an ion.

Non- metals gain electrons to become stable since electrons are negatively charged, when an electron attempts to get into the outermost energy level of an atom, it will be repelled by the electrons which are already there. Therefore some force is needed to move the electrons into the energy level.When the electron finally settles in the outermost energy level heat energy is lost. The heat energy lost is more than that used to force the electron into the energy level. The net heat change when a gaseous atom of an element gains an electron is called electron affinity.

When an atom loses or gains electrons it becomes electrically charged. The resultant Charged particles are called ions. Positively charged ions are called cations. Negatively charged ions are called anions.. During chemical reactions, metals lose electrons and therefore form cations. Non-metals gain electrons and therefore form anions.

Magnesium atom loses two electrons and therefore magnesium has a valency of TWO.

The number of electrons an atom gains or loses is called its valency. For example lithium atoms loses one electron and therefore lithium has a valency of one.

Sulphur atom gains two electrons and therefore sulphur has a valency of two.

The table below illustrates some compounds and their valencies. Click on the arrow to view the valencies of different compounds.

The table below illustrates some compound and their valencies.

Some elements have varying valencies. For example copper can have a valency of one or two. The valencies of such elements are indicated in roman numbers in brackets when naming compounds.

When an atom loses or gains electron or electrons, the charge on the resulting ion is referred to as oxidation number. Below are some oxidation numbers of some common radicals. Click on the arrow to view Oxidation numbers.

A group of atoms that exist together and react as a unit with a net charge, and which cannot exist on its own is called a radical. The valency of a radical is the same as the value of its charge.

By the end of the lesson you should be able to

derive the formula of some simple compounds from valencies of elements and radicals.

In this lesson, we will discuss how to derive the formular of some simple compounds from valencies of elements and radicals.

When two or more elements combine, they form compounds. Chemical formula is a short notation of writing names of compounds. It shows the atoms present in the compound. When writing the chemical formula of a compound, its necessary to know the symbols and valencies of the elements and radicals that form the compounds.

Chemical formula of potasium sulphate .Potasium has 1 projection since it has a valency of 1 and Sulphate has 2 grooves representing a valency of 2.Potassium fits in one groove of the sulphate and 1 will be left free for it to be complete it needs another projection of potassium.Note that we need two cards of potassium and one card of sulphate. The chemical formula of potassium sulphate is therefore

Chemical formula of Calcium Chloride.

Writing chemical formula using a game of cards Sodium chloride From the illustration, sodium has one projection since it has a valency of 1. Chlorine has one groove since it has a valency of 1. The two will fit and therefore the formula will be

1.      Potassium sulphate.

 

 

 

 

 

 

 

 

 

Animate the 2 cards coming together and the K will fit in one groove.

 

Note that one groove of the sulphate is still free for it to be complete it needs another projection of potassium.

Animation.

Fix another card of potassium.

 

 

 

 

 

 

 

 

 

 

VOICE.

Note that we need two cards of potassium and one card of sulphate. The chemical formula of potassium sulphate is therefore K2SO4.

 

Formular of water
To write the correct formula we must write:

Formula of carbon (IV) oxide

Steps

1. Write the symbols of C and O

2. Write the valencies of C and O above and to the right side of each

3. Exchange the valencies by writing them below the symbols.

4. Divide the valencies by common factor 2 to present the atoms in the simplest ratio

5. Write the symbols close together CO2.

Formula of carbon (IV) oxide

Formula of Iron (III) oxide

Steps

1. Write the symbols of Iron and Oxygen

2. Write the valencies of Fe and O above and to the right side of each

3. Exchange the valencies by writing them below the symbols.

4. Write the symbols close together Fe203. The valencies have no common factor which can divide and give a whole number.

Therefore, the number is just reversed. Therefore the formula is Fe2O3

Formula of Iron (III) oxide

Formula of Ammonium carbonate

Formula of iron (iii) oxide.

Step I.

Write the symbols of the elements.

Fe                    O                     animate the symbols to come 1 by 1.

Step II.

Write the valencies of the elements above and to the right side of each.

Fe3                   O2.                   animate the valencies.

Step III.

Exchange the valencies by writing them below the symbols as shown by the arrows.

Fe3                   O2                    animate the arrows and put colours.

Blue                             yellow

Step IV.

Write the symbols close together.

Fe2O3

TEXT: The valencies have NO common factor/number which can divide and give it a whole number. Therefore the number is just reversed.

 

In this topic, we will discuss how to write chemical equations and balance them.

Objective

By the end of the lesson you should be able to

write simple balanced equations.

A chemical equation is a statement that shows what happens during the chemical reaction. The following steps are used when writing a balanced chemical equation

Magnesium reacts with oxygen to form Magnesium oxide. The following is the procedure of writing a balanced chemical equation.Click on each box to follow the procedure.

Sodium granules react with oxygen to form sodium oxide. The following is the procedure of writing a balanced chemical equation.Click on each box to follow the procedure.

Sodium granules react with oxygen to form sodium oxide.

Step 1: write the word equation for reaction between sodium and oxygen.

Sodium + oxygen                    sodium oxide.

Animate to appear one by one.

VOICE OVER.

STEP II: Write the equation using correct chemical formulae of reactants and products.

ANIMATION.

Na +    O2                    Na2O.              Animate to appear one by one from the left.

VOICE OVER.

Step III: count the number of atoms of reactants and product and check whether they are equal.

Illustration.      Animate numbers to appear one by one begin with reactants.

Use different colours for Na and O.

 

Element

Reactants

Products

Na

1

2

O

2

1

 

 

VOICE OVER.

Sodium atoms are not equal. We have 1 atom on the left and 2 on the right we notice also that oxygen atoms are not equal. There are 2 atoms on the left and 1 on the right.

VOICE OVER.

To make sodium atoms equal, insert 2 in front of Na and count the atoms again.

2Na + O2                     Na2O

ANIMATION

Animate 2 in front of Na to blink and in colour.

ILLUSTRATION.

Use different colours for Na and O.

Element

Reactants

Products

Na

2

2

O

2

1

 

VOICE OVER.

Sodium atoms are now equal, two on the left and two on the right. Oxygen atoms however are not equal. There are two on the left and one on the right. To make oxygen atoms equal, we insert 2 infront of Na2O and count the atoms again.

2Na + O2                     2Na2O

ANIMATION.

Animate 2 in front of Na2O to blink and in colour.

Illustration/animation.

Animate numbers to appear one by one begin with reactants.

Use different colours for Na and O.

Element

Reactants

Products

Na

2

4

O

2

2

 

 

 

 

VOICE OVER.

Notice oxygen atoms are equal, two on each side, but sodium atoms are not equal, as there are two on the left but four on the right to make sodium atoms equal, we replace 2 in front of Na on the left with 4 and count the atoms again.

4Na     +          O2                       2Na2O

ANIMATION.

Animate 4 in front of Na and 2 in front of Na2O to blink and in colour.

Illustration.

Element           reactants          Products

Na                   4                      4

O                     2                      2

VOICE OVER.

Sodium atoms are now equal, four on the left and four on the right. Oxygen atoms are also equal, two on the left and two on the right. The equation is now balanced. The state symbols have also been inserted.

4Na(s)   +          O2(g)                 2Na2O(s) BALANCED.

ANIMATION

Show numbers 4 on Na and 2  in front of Na2O in colour. Animate BALANCED to blink.

VOICE OVER/TEXT.

Always remember that we balance an equation by making sure that the number of atoms of each type are equal on both sides of the equation, on the left and on the right of the arrow. Never change the formula of a compound to balance the equation.

 




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