Period Three Elements | Chemistry Form 2

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Period Three Elements - Chemistry Form 2

STRUCTURE AND BONDING

The term structure is applied to any arrangement of things that form a specific pattern. A bond is anything that holds two or more things together.

To understand the nature of the different chemical bonds and structures formed when atoms combine, it is important to remember that atoms are composed of electrons (negatively charged particles), neutrons (without charge) and protons (positively charged particles).Noble gases are chemically stable and therefore do not take part in chemical bonding. Atoms of other elements however, achieve the stable electron arrangement of noble gases by gaining or losing electrons to acquire a noble gas structure.

By the end of the lesson you should be able to

(i) explain the formation of ionic bonding

(ii) use diagrams to illustate ionic bonds

(iii) state properties of ionic compounds

Consider the reaction which takes place when sodium reacts with chlorine to form sodium chloride.Sodium atom reacts by losing its valence electron in order to acquire a stable noble gas electron arrangement. Chlorine atom reacts by gaining one electron on its outermost energy level in order to acquire a stable noble gas electron arrangement.Click the arrow to view the bond formation.

In the reaction between sodium and chlorine, sodium atom loses its outermost electron to form a chloride ion. The sodium ion formed has 10 electrons and 11 protons, therefore, it has net charge of one positive . chlorine accepts a single electron donated by sodium into its outermost energy level, the resulting ion has 18 electrons and 17 protons, therefore, the chloride ion formed has a net charge of one negative.
The positive and negative ions formed attract each other finally forming an ionic bond between them.


COVALENT BONDING

Ionic bond is formed when an electron is transferred from a metal to a non-metal.Generally, reactions between metals and non metals results into the formation of ionic compounds.


Magnesium and oxygen combine ionically to form an ionic bond of magnesium oxide. Magnesium atom requires to lose two electrons to acquire a stable noble gas electron arrangement while oxygen requires to gain two electrons to acquire stable electron arrangement. Click the arrow to view bond formation.

Two valence electrons are transferred from magnesium atom to oxygen atom leading to formation of magnesium ion with a charge of 2+ while oxygen atom gains the two electrons forming oxygen ion with a net charge of 2-. The two ions have opposite charges therefore they attract each other forming an ionic bond.

Calcium reacts with chlorine to form ionic bonds of calcium chloride.Calcium loses two of its valence electrons in order to gain a stable arrangement.Click on the arrow to view the bond formation.

Chlorine requires to gain one electron in order to acquire stable electron arrangement. Therefore, calcium loses its two valence electrons to two Chlorine atoms leading to formation of calcium ion with a net charge of 2+ while each of the two Chlorine atoms gain an electron acquiring a net charge of 1-. The two chloride ions are attracted towards the calcium ion forming bonds.


Other examples of compounds that form ionic bonding include lithium flouride and potassium chloride. The animation below shows the bonding in potassium chloride.

Most ionic compounds form crystals. In the sodium chloride crystal, sodium ions attract the negatively charged chloride ions from all directions.

Similarly, chloride ions attract the positively charged sodium ions from all directions. This type of attraction gives a three dimensional structure. Where each sodium ion (Na+) is surrounded by six chloride ions (Cl-). Similarly each chloride ion is surrounded by six sodium ions.Since the ions are very many the pattern is repeated, then the arrangement results in a giant ionic structure.



1. Solubility in water - Many ionic compounds dissolve in water
2. Ionic compounds have high melting and boiling points due to the strength of the ionic bond
3. Electrical conductivity- ionic compounds conduct electricity when in aqueous or when they are in molten form.

CONCLUSION

Ionic compounds conduct electricity when molted and when in aqueous solution, because the melt or the solution contains mobile ions.

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.Click on the arrow to view the formation of the bond.

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

(i) explain formation of covalent .

(ii) illustrate the covalent bonding using equations

(iii) explain properties of covalent compounds.


Chlorine is in group VII of the periodic table, and therefore has seven electrons in the outermost energy level. The electron arrangement of Chlorine is 2.8.7. It therefore needs to gain one electron to attain stable noble gas configuration of 2.8.8 . If it combines with another chlorine atom and shares an electron with it, a covalent bond is formed.Click on the arrow to view the bond formation.

Both chlorine atoms have an electron with it, then both chlorine atoms have complete outermost energy levels (an octet). The unit formed from the covalent bonding is called a molecule.

The example of hydrogen, chlorine and oxygen molecules shown, represent covalent bonding of similar atoms. However, dissimilar atoms can also form covalent bonds. Hydrogen atom for example combines with chlorine atom to form a molecule of hydrogen chloride.Hydrogen and chlorine molecules are formed from covalent bonding.Click on the arrow to view the bonding.

Similarly, two hydrogen atoms will combine with an oxygen atom to form a covalent bond of water molecule.In this case, hydrogen atoms and oxygen atom each contribute one electron for sharing.Click on the arrow to view the bond formation.

Eventually, hydrogen will have two electrons in its energy level similar to that of stable helium atom. Oxygen on the other hand, shares two of its electrons with hydrogen for it to achieve a stable configuration similar to that of Argon 2.8.8.

Carbon atom combines with two oxygen atoms covalently to form carbon (IV) oxide molecule. Click on the arrow to view the bond formation.


Hydrogen, Oxygen and Nitrogen exist as simple molecular structures.The covalent bonds between two atoms for instance Hydrogen-Hydrogen atoms are strong. However, the forces between the molecules are weak. The weak intermolecular forces are known as Van-der-Waals.


Properties of simple molecular structure

Animation

Table showing properties of simple molecular structure of substances

Molecule

Formular

State

Melting point (0c)

Boiling points

(0c)

Electrical conductivity

Hydrogen

H2

GAS

-259

-253

Low

Chlorine

Cl2

Gas

-101

-34

Low

Hydrogen chloride

HCl

Gas

-114

-85

Low

(a)      Melting point and Boiling point

Simple molecular substances have low melting points and boiling points. This is because they have weak Van der Waals forces between the molecules that are easy to be broken.

(b)      Electrical conductivity

Simple molecular substances have very low conductivity. They do not have mobile ions or delocalized electrons because they are tightly bonded.

Simple molecular structures are gaseous in nature and have low boiling and melting point. They do not conduct electricity. The table below shows the properties of simple molecular structures.

ACTIVITY

Simple molecular structures are gaseous in nature and have low boiling and melting point. They do not conduct electricity. The table below shows the properties of simple molecular structures.

Giant atomic (covalent) structures have a large number of covalent bonds formed between atoms.The two examples of giant atomic structures are diamond and graphite.

Each carbon atom in diamond is covalently bonded to four other carbon atoms. The basic unit is repeated in three dimensions to form a giant structure of millions of carbon atoms which are bonded to four other carbon atoms using covalent bonds. The melting point of diamond is very high.

The reason is that diamond structure made of strong covalent bonds between the carbon atoms which require very high temperatures to break.Diamond does not conduct electricity because all its electrons are used for bonding.

Each layer consists of carbon atoms covalently bonded together into hexagons. Adjacent layers are held together by weak Van der Waals forces. These layers readily slide over one another accounting for the greasy texture of graphite.Each carbon atom forms three covalent bonds with other carbon atoms.

Thus, three of its four outermost electrons are paired up to form covalent bonds. The fourth electron is delocalized extending along the layers of carbon atoms. Graphite conducts electricity .This is due to the delocalized electrons extending along the layers and therefore free to move through the layers.

Graphite has a higher melting and boiling points because it has strong covalent bonds between the carbon atoms which requires temperatures to melt.


Co-ordinate bond is a covalent bond in which the shared pair of electrons are donated by one atom.Consider a reaction which takes place when a hydrogen ion and an ammonia molecule (NH3) combine to form ammonium ion.


Hydrogen ion has no electron around its nucleus. On the other hand, the ammonia molecule has unshared pair of valency electrons that has not been used in bonding. The hydrogen ion combines with ammonia molecule, the hydrogen ion accepts the pair of electrons into its empty first energy level. The ammonia molecule donates the unshared pair of electrons.

The animation below shows the bonding in carbon (II) Oxide.Click on the arrow to view how the bond is formed.

ACTIVITY

Graphite is used as

An electrode in electrolysis


As a lubricant.This is because it is greasly and has a slippery feel.

Diamond is used in making jewellery .

Diamond is used in drilling rocks. Click to play the video to observe how Diamond is used in drilling through rocks.


QUIZ

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

(i) explain the nature of metallic bonding
(ii) Explain the influence of intermolecular forces on physical properties of substances

In metallic bonding the metal atoms release their valence electrons and form positively charged ions.The electrons form a pool of negative charge around the ions.The electrostatic attraction between the sea of electrons and the ions is called metallic bonding.The electrons are said to be delocalised. The animation below show the bonding in a metal.

In ionic bonding electrons are transferred from a specific atom to another atom. In covalent bonding two specific atoms share one pair of electrons.

In metallic bonding the metal atoms lose their valence electrons and form positively charged ions The electrons form a pool of negatively charge around the ions.

Melting and boiling points of group 1 and II metals, decrease down the groups due to decrease in strength of the metallic bonds. The table below gives a comparision of the melting and boiling points of group I and II elements.


Influence of metallic bonding on properties of metal

(i)     Melting points and boiling points

V.O. The melting and boiling points of Groups 1 and II metals decreases down the groups due to decrease in strength of the metallic bonds.

V.O The melting and boiling points of metals in period III increase as you move across the period. This increase is due to increase in strength of the metallic bonds as a result of increase in number of delocalized electrons.

All metals are good conductors of electricity because they contain delocalised electrons in their structure.

HYDROGEN AND VANDER WAALS FORCES

ACTIVITY


In molecular compounds, the different molecules are held together by intermolecular forces called Vander Waals forces. Van der waals forces arise from the attraction between the nuclei of atoms in a molecule and electrons in the outermost energy level of atoms in neighbouring molecules.

Van der Waals forces arise from the attraction between the nuclei of atoms in a molecule and outer energy level electron of atoms in neighboring molecules.Van der waal forces are weak. This account for the low melting points of many molecular compounds.

In some molecular compounds, the molecule consist of two atoms one of which has greater electronegativity than other atom. In the water molecule, oxygen has a stronger attraction for electrons than hydrogen atoms. The bonding electrons are therefore closer to the oxygen atom than the hydrogen atom. The water molecule is therefore polarized.


Ethanol molecules also form Hydrogen bonds. The illustration below shows the Hydrogen bonds between Ethanol molecules.

The attraction between the positively charged hydrogen and the lone pair of electrons. In the negatively charged oxygen atom end is hydrogen bond.

The attraction between the positively charged hydrogen ends and the lone pair of electrons in the negatively charged oxygen end is called a hydrogen bond.
The hydrogen bond is responsible for the relatively high melting and boiling points of water and ethanol (C2H5OH). The bond is stronger than Van der waals forces but weaker than covalent bonds.

PROPERTIES AND TRENDS ACROSS PERIOD THREE


ACTIVITY

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

(i) Name the elements in period 3 and write down their electron arrangement

(ii) state and explain the trends in physical and chemical properties of period 3 elements.


Atoms of period 3 elements have 3 energy levels. As you move across the period from left to right, the elements change from metal to metalloid to non metal.
Trends in atomic size and ionization energy
Study the table below which shows the atomic radii and first ionization energy of period 3 elements.

The atomic radius decreases as you move to the right across period 3 due to increase in effective nuclear charge.The first ionization energy increases across the period due to increase in effective nuclear charge.

Study the table below which shows the boiling and melting points of period 3 elements.

ACTIVITY

Study the table below which shows the melting and boiling points of period 3 elements.

Element

Na

Mg

Al

Si

P

S

Cl

Ar

Melting point (0C)

98

650

660

1410

44

119

-101

-189

Boiling point (0C)

890

1110

2470

2360

280

445

-35

-186

Sodium, magnesium and aluminium are metals. Their melting and boiling points are due to strong metallic bonding.
Silicon has giant atomic structure with strong covalent bonds throughout . This accounts for its high melting and boiling points.
Phosphorous, sulphur and chlorine have molecular structures hence their melting/ boiling points are low due to weak Van der Waals forces.
Sulphur has relatively high melting and boiling points due to its tendency to have larger molecules.

Below is a summary of the trend in electrical and thermal conductivity of period 3 elements.

The electrical and thermal conductivity changes from good to poor conductivity as you move across the period.The metals have delocalized electrons which make them good conductors.
The non-metals are poor conductors because they do not have either ions nor delocalized electrons.

The following experiments were performed to investigate the trend in reactivity of period 3 elements with cold water.

Click to play the video and observe the reaction between Sodium and water.



Sodium reacts vigorously with cold water to form Sodium hydroxide solution and evolve hydrogen gas. The red litmus paper turns blue while the colour of the blue litmus paper remains blue indicating that the solution formed is basic.
Red litmus paper changes to blue implying that the solution is basic.

A small piece of Magnesium ribbon is placed in a beaker containing distilled water. Observe what happens in the following animation.

Magnesium reacts very slowly with cold water to form magnesium hydroxide and evolve hydrogen gas as shown by the equation

Equation

Mg(s) + H20(l) w Mg(OH)2(aq) + H2(g)


The colour of the blue litmus paper remains blue while the red litmus paper turns blue.


Reaction of Magnesium with water
- Cut a small piece of magnesium
- Hold the piece of magnesium using dry pair of tongs
- Put it in a beaker containing distilled water
-Dip a red litmus paper and a blue litmus paper in the solution formed.
Magnesium reacts very slowly with cold water to form magnesium (II) hydroxide and evolve hydrogen gas as shown by the equation below.
The colour of the blue litmus paper remains blue while the red litmus paper turns blue.
Magnesium reacts vigorously with steam to form magnesium (II) Oxide and hydrogen gas as shown in the equation below.
When Magnesium oxide is dissolved in water the solution formed turns red litmus paper blue while the blue in the solution.

Reaction of Aluminium with water

Aluminium does not react with cold water or steam because it first forms an insoluble coating of Aluminium oxide which prevents further reaction.
Aluminium will only react with steam at very high temperatures of over 700 degree celcius.

Reaction of phosphorous with water
Phosphorous does not react with water. Phosphorus is stored in water. This is because it is very reactive in air.


Reaction of sulphur with water.
Sulphur does not react with water.

Click to play the video clip to observe the reaction between Chlorine and water.


Chlorine dissolves in cold water to form hydrochloric acid and chloric (I) acid .
The solution formed changes blue litmus paper red while the red litmus paper remains red.

Reaction of period Three Elements with dilute acids

When period 3 elements react with dilute acid, the following observations are made. Click to play the video to observe the reaction between magnesium and dilute hydrochloric acid.

Insert video of Magnesium and HCL

Magnesium ribbon reacts vigorously with dilute hydrochloric acid to form a salt i.e. Magnesium (II) Chloride and hydrogen gas.

Mg(s) + HCl(aq) w MgCl2(aq) + H2(g)

Aluminium foil reacts with hydrochloric acid to form a salt Aluminium Chloride and hydrogen gas.
Silicon, phosphorous, sulphur, chlorine and argon do not react with dilute acids.

All the oxides of elements in period 3 except those of sulphur and chlorine are solids. The table below shows the Bond types and properties of oxides of elements in period 3.

ACTIVITY

Types of bonds of oxide
Oxides of Sodium, Magnesium and Alumininum form Giant ionic structures, while oxides of silicon forms giant atomic structure. Phosphorus (V) Oxide and sulphur (IV) Oxide form simple molecular structures.


Oxides of Sodium, Magnesium, Aluminium and Silicon have high melting and boiling point compared to those of phosphorous and sulphur. This is because Sodium oxide, Magnesium oxide and Aluminium oxide have giant ionic structures with strong ionic bonds which require high temperatures break. Silicon (IV) Oxide has giant atomic structure with strong covalent bonds between the atoms which requires high temperature to break. Phosphorous (V) oxide and sulphur (IV) oxide have relatively lower melting point and boiling point since they have simple molecular structure with weak vander waal forces.


Metal oxides are basic. However, Aluminium oxide is amphoteric (Reacts with acids and bases). Non metallic oxides are acidic.



Aluminium Oxide


Metal oxides react with acids to form salt and water as the only products.The following equations shows the products formed in some period three elements.

Na2O(s) + 2HCl(aq) w 2NaCl(aq) + H20(l)

MgO(s) +2HCl(aq) w MgCl2(aq) + H20(l)


Chlorides of sodium and magnesium form giant ionic structures. Aluminium chloride is both ionic and covalent, while chlorides of silicon and phosphorous are molecular.
Nature of chlorides
The chlorides of sodium and magnesium on dissolving in water form neutral solutions and therefore have pH of 7. However, chlorides of aluminium, silicon and phosphorous are hydrolyzed in water to form acidic solutions with pH of 3. Hydrolysis means breaking down of a substance with water.


Types of bonding
Chlorides of sodium and magnesium form giant ionic structures. Aluminium chloride is both ionic and covalent, while chlorides of silicon and phosphorous are molecular.

Nature of Aluminium Chloride.Aluminium chloride have coordinate (dative) bond. This structure is called a dimer.

Nature of Aluminium Chloride

Aluminium chloride have coordinate (dative) bond. This process is called a dimmer

 

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