m4 bonding i: covalent bonding

M4 Bonding I: Covalent Bonding

M4 Bonding: Covalent Bonding Slide 2 of 54

Learning objectives

• Key Concepts:

– Stable, unstable, bond, chemical bond, molecule, diatomic molecule, molecular element,

– energetic stability, incomplete valence (or outer) shell, complete valence shell,

– covalent bond, electron pair, shared electrons, single bond, double bond, triple bond, ion, molecular ion (polyatomic ion), dot and cross diagram, structural formula

• Skills:

– Describe the formation of covalent bond

– Define a covalent bond

– Describe and draw dot and cross diagrams for diatomic molecular elements


Learning objectives

– Describe and draw dot and cross diagrams for some simple molecules such as CO2, CO, H2O, CH4, NH3, HCl and other

hydrogen halides.

– Be able to explain and give examples of how the properties of covalent compounds can be explained by their bonding

– Be able to determine, for a select covalent compounds, the name when given the formula and determine the formula when given the name

– Be able to identify ionic, covalent, and metallic substance by their chemical formula or their properties

– Be able to name and give the formula of a select group of molecular (polyatomic) ions

– Be able to explain why molecular (polyatomic) ions are charged


Review

• You learned that elements other than the noble gases are energetically unstable and therefore they react with other elements to be stable.

That atoms of elements, apart from the noble gases, try to attain a stable state by

– either losing electrons, or – gaining electrons, or – sharing electrons.

• so that they have a completely filled valence (outer) shell just like the noble gases.


Review

• A while back you observed the following reactions in the lab:

2Na (g) + H2O (l) 2Na2O (s)

Zn (s) + I2 (s) ZnI2 (s)

2Al (s) + 3I2 (s) 2AlI3 (s)

2Al (s) + 3Br2 (l) 2AlBr3 (s)

2H2O (l) 2H2 (g) + O2 (g)

• You learned that when a metal and a nonmetal are involved in a combination reaction, as in the first four reactions above, the metal lose electrons while the non-metals gain electrons.

– You learned that metals would rather lose and non-metals gain because that is the easier of the two option.


Review

– As metals have 3 or less electrons in their valence shell, it’s easier to lose 1 or 2 or 3 electrons than gain 7 or 6 or 5 electrons.

– Similarly, since most non-metals have 5 or more electrons in their valence shell, when they react with metals, it’s easier for them to gain 3 or 2 or 1 electrons than lose 5 or 6 or 7 electrons!

• So, when a metal and a non-metal react the metal transfers electron(s) to the nonmetal.

When two non-metals are involved however, as in the following examples,

2H2 (g) + O2 (g) 2H2O (l)

C (g) + O2 (g) CO2 (g)

• they share electrons between them because neither of the non-metals have a tendency to lose electron(s).


Chemical Bond

• Now the question is what keeps these atoms in the compound together?

What about zinc and the two iodine atoms in zinc iodide?

What about carbon and the two oxygen atoms in carbon dioxide?

And, lastly, what about oxygen and the two hydrogen atoms in water?

What keeps atoms together in in the molecule of a compound (and also in the molecule of some elements) is an attractive electrostatic (electromagnetic) force, referred to as a chemical bond or simply bond.

– Remember an electrostatic force is a force between oppositely charged particles.


Chemical Bond

• But depending on the composition of the substance, the bond between atoms in a compound is classified as either covalent, ionic or metallic.

– The chemical bond in compounds made up only of non-metals is referred to as covalent bond.

• We will consider covalent bonds, and look at how bonding determines the structure as well as the properties of a substance.


Covalent bond in MOLECULAR elements

• Let’s start with the first element in the periodic table: hydrogen.

Hydrogen atoms contains 1 electron in their valence shell.

Electronic configuration: 1 1

outer shell NOT full outer shell NOT full

energetically unstable energetically unstable

What can the TWO hydrogen atoms possibly do to fill both their outer shells and become energetically stable ?


Covalent bond in Hydrogen

• In order to be stable, in order to acquire the structure of a noble gas, hydrogen atoms must gain one electron.

Since both need one electrons each, which means neither can give electron to the other!!

If that were to happen, one would be stable (with two valence electrons) and the other would have no electrons at all!!

What can the TWO hydrogen atoms possibly do to fill both their outer shells and become energetically stable ?


Covalent bond in MOLECULAR elements

• They can combine to SHARE ONE PAIR OF ELECTRONS and form a hydrogen molecule, each atom contributing one electron to the shared pair.

In so doing, both hydrogen atoms acquire completely filled outer shell (2 electrons) and become stable.

Electronic configuration: 1 1 1+1 = 2 1+1 = 2

outer shell NOT full outer shell NOT full outer shell full

energetically unstable energetically unstable energetically stable

H HH2


Why stable?

• BOTH hydrogen atoms have BONDED together in order to fill their outer shells and become energetically stable.

The shared electron pair remain between the two atoms because they are attracted by both the positively charged nuclei of the atoms.

1+ 1+ - -

HH

Structural formula

The single line representing a pair of shared electrons


Elements in Group 7• Lets now move on to group 7 element fluorine.

A Fluorine atom, like all the elements in GROUP 7, has 7 valence electrons in its outer shell.

Electronic configuration: 2, 7

outer shell NOT full

energetically unstable


Elements in Group 7: Fluorine

• Or…

Atoms of Fluorine, just like most other elements can become stable in one of two ways:

• either by losing seven electrons (and acquiring the structure of helium gas)


Elements in Group 7: Fluorine

– By gaining one electron (and acquiring the structure of neon gas)

+


Single Covalent Bond • The outer shell of each atom have crossed over each other.

Each atom shares its valence electron with the other atom.

(co-operation)

++

Electronic configuration: 2, (7 +1 = 8) 2, (7 +1 = 8)

outer shell full outer shell full energetically stable energetically

stable All group 7 elements are DIATOMIC due to this BONDING.

Again the two atoms are held together because both the nuclei attract the shared pair of electrons.

FF

Structural Formula


Covalent Bonding

• Covalent bonds form between non-metal atoms.

Covalent bonds involve sharing one or more pairs of electrons between the atoms bonded together.

Noble gases, the elements in the last column of the periodic table, as have been mentioned before, have completely filled valence shells and therefore they are energetically stable.

The rest of the non-metals however have less than 8 electrons (hydrogen less than 2 electrons), incomplete valence shells and therefore are energetically unstable.

To become stable like the noble gases, these non-metal atoms, when they react with each other, share electrons to attain a completely filled valence shell.


Covalent Bond: Definition• The outer shells of each atom have crossed over each other.

Each atom shares its valence electron with the other atom.

(co-operation)

The 2 atoms shared: CO-operated with their VALENT (valence) electrons.

COVALENTBOND

• The definition of covalent bond therefore is the electrostatic force of attraction between the nuclei of the bonded atoms and the shared electrons.


Single Covalent Bond

• Other elements that share two electrons between themselves and form a diatomic molecule are:

– The rest of the halogens (Chlorine, Bromine, and Iodine)

– Oxygen

– Nitrogen

• Hydrogen and halogens (fluorine, chlorine, bromine and iodine) share only two electrons between the atoms bonded together.

A covalent bond consisting of only two shared electrons it is referred to as a single bond.

Oxygen and nitrogen however share 4 and 6 electrons respectively.

Can you guess why? (Hint: How many electrons do oxygen and nitrogen atoms need to have a full valence shell?)


Multiple covalent bonds: Double Bond

• Oxygen is a non-metal. A structure for oxygen is given to the right.

How many electrons does it need to have a completely filled outer shell and become energetically stable?

If two oxygen atoms are to combine and form a diatomic molecule, and become stable, how many electrons must they each contribute to the bond then?

How many electrons must they share between them?

What is the electronic configuration of nitrogen?

How many electrons does its outer shell have?

2, 6 6

2

2

4


Multiple covalent bonds: Double Bond

• Electronic configuration:2, (6 + 2 = 8) 2, (6 + 2 = 8)

outer shell full outer shell full

energetically stable energetically stable

Since the oxygen atoms share 2 pairs of electrons (4 electrons), a molecule of oxygen therefore has a double bond.

Determine the bonding situation in nitrogen. (Hint: It has a triple bond.)

Determine the bonding situation in diatomic molecules of chlorine, bromine and iodine as well.

Additionally, draw structural formula for all the molecules considered thus far.

O2


Multiple covalent bonds: Triple Bond• Nitrogen is another non-metal. A structure

for nitrogen is given to the right.

How many electrons does it need to have a completely filled outer shell and become energetically stable?

If two nitrogen atoms are to combine and form a diatomic molecule, and become stable, how many electrons must they each contribute to the bond then?

How many electrons must they share between them?

2, 5 5

3

6

3

What is the electronic configuration of nitrogen?

How many electrons does its outer shell have?


Multiple covalent bonds

• Electronic configuration:2, (5 + 3 = 8) 2, (5 + 3 = 8)

outer shell full outer shell full

energetically stable energetically stable

Since the nitrogen atoms share 3 pairs of electrons, a molecule of nitrogen therefore has a triple bond.

N2


Covalent Bonds in Compounds• We’ll start by looking at the compound formed when hydrogen and

oxygen react.

When a burning splint is applied to Hydrogen it will will burn in oxygen to produce water.

The energy in the splint will break the single bonds in hydrogen and the double bond in oxygen molecules.

The atoms of hydrogen and oxygen then recombine to produce water molecules.

Because both elements are non-metals, the bonds in the water molecules are covalent.

Oxygen needs two electrons to have a completely filled outer shell and become stable.

Hydrogen of course requires only one electron.

Therefore, one oxygen atom will combine with two hydrogen atoms to form a molecule of water, H2O.


Water: Oxygen and Hydrogen

H

H

OH2O

Shared pairs of electrons; each pair represents a single bond

What would the structural formula look like?


Dot and cross diagram

• When a molecule is represented as H2O molecule is above, it is referred to as a dot and cross diagram.

Notice a few differences between the last diagram (the one in the previous slide) and this. – The core shells have been omitted – The nucleus is omitted – The electrons from the different atoms are distinguished by

representing then as either circles (electrons originating in and belonging to Oxygen) and crosses (electrons originating in hydrogen).

XX


Covalent bonds in compounds: Ammonia

H

H

H

N

• Nitrogen needs three electrons to have a completely filled outer shell and become stable.

Hydrogen of course requires only one electron.

Therefore, one nitrogen atom will combine with three hydrogen atoms to form a molecules of ammonia, NH3.

NH3


Dot and Cross diagram of Ammonia

• It is indeed more convenient to just show the outer electrons.

What would the structural formula look like?

XX

XThree single bonds between hydrogen and nitrogen in ammonia.


Covalent bonds in compounds:carbon dioxide

• Carbon needs four electrons to have a completely filled outer shell and become stable.

Oxygen of course requires two electrons.

Therefore, one carbon atom will combine with two oxygen atoms to form a molecules of carbon dioxide, CO2, each sharing 4 electrons.

C: 2, 4

O: 2, 6


Covalent bonds in compounds:carbon dioxide

O C O

CO2

++


Dot and Cross diagram of CO2

• Draw dot and cross diagrams for SO2, CH4 (methane), HCl

(hydrogen chloride), and CO (carbon monoxide). If you haven’t already, draw structural formula for all the molecules considered thus far.

X

X

XX

XX

X

X

XX

XX

Two double bonds in carbon dioxide.


Flash Cards

• For a set of covalent compounds, you need to know the formula, the electron structure diagram, the structural formula and the dot and cross diagram.

Additionally, knowing them, you need to be able to apply the principle and methods involved in arriving at them to covalent compounds you haven’t encountered in your studies.

And those compounds are: Hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine, iodine, carbon dioxide, ammonia, sulfur dioxide, water, methane, hydrogen chloride, hydrogen bromide, hydrogen iodide, hydrogen sulfide and silicon dioxide

To help you with that you need to make a few sets of flash cards.

Set 1. name on one side with the question “Formula?” at the bottom and the formula on the other side with the question “Name?”


Flash Cards

• Set 2. Formula on one side and the question “Dot and cross diagram?” at the bottom and the dot and cross diagram on the other side with the question “Formula?” Set 3. Electron structure diagram on one side and the question “Structural formula? ” at the bottom and the structural formula on the other side with the question “Electron structure?” on the other side. Use the flash cards to help you with class work, homework assignments and to revise for tests.


Properties of Covalent Compounds

• Few covalent compounds are solid, some are liquid, but most are gaseous. – Examples of solid covalent compounds are: phosphorus

pentoxide, silicon dioxide. – Examples of liquid covalent compounds are as follows: water

(H2O), ethanol.

– Gaseous covalent compounds are: carbon monoxide (CO), carbon dioxide (CO2), hydrogen chloride (HCl), sulfur dioxide (SO2), nitrogen dioxide (NO2).

• The reason is that, though the bond between atoms within the molecules of covalent compounds might be strong, the attraction between the molecules are weak.

For the same reason, in general they have low melting and boiling points.

They are non-conductors of electricity and heat.


Breaking a covalent bond

++

HEAT (thermal)

ENERGY



++

Thermal (heat) energy is absorbed (taken in) by the atoms.

Thermal energy is transferred to kinetic (movement) energy - so the electrons and nucleuses start vibrating in all directions.



++

the distance between the valence electrons and nucleuses increases – resulting in the attractive pull between them becoming weaker.

More thermal energy is transferred to kinetic energy – causing the electrons and nucleuses to vibrate further distances in all directions.


Other covalent species: Molecular ions

• SO42, NO3

, NH4+ are referred to as molecular ions or

polyatomic ions. – They can be thought of as molecules that have charges

because they have different number of protons and electrons.

– SO42 and NO3

are negatively charged because they have more electrons than protons

– while NH4+ is positively charged because it has one less

electron than protons.

• For now you need to learn the names and formula of only the following above molecular ions.

Formula Common name Formula Common name

NH4+ ammonium SO4

2 sulfate

CO32 carbonate HCO3

bicarbonate

OH hydroxide NO3 nitrate


Practice Questions: Multiple Choice

• 1. J04/1/8. How many electrons are shared between the atoms in the molecules of methane, CH4, and of water, H2O?

• 2. N03/1/5. The table shows the electronic structures of four elements.

Which element is a noble gas?



• 3. N03/1/8. Which element is a solid non-metal?

• 4. N03/1/11. Carbon and chlorine form a chloride.

What is the formula of this chloride?

A. CCl2 B. CCl4 C. CaCl2 D. CaCl4



• 5. N03/1/9 The diagrams show the bonding in three covalent molecules.

Which of these molecules combine to form ammonia?

A. 1 and 2 B. 1 and 3 C. 2 and 3 D. 1, 2 and 3

• 6. The structure of a molecule is shown below.

How many different elements does the molecule contain? A 4 B 5 C 9 D 10



• 7. Two elements represented by and can form a compound.

Which diagram shows molecules of the compound?

• 8. A model of a compound is shown.

What is the molecular formula of the compound?

A. H2O B. 2HO C. 2H2O D. H2O2



• 9. J04/1/7. In the diagrams, circles of different sizes represent atoms of different elements.

Which diagram can represent hydrogen chloride gas?


Practice Questions: Structured

• 1. J01/2/2. Natural gas is largely methane, CH4.

(a) Use information from the Periodic Table to find

(i) the number of electrons in the outer (valence) shell of a carbon atom . . . .

(ii) the number of electrons in the valence shell of a hydrogen atom. . . . . [2]

(b) Draw a dot and cross diagram to show how the electrons are arranged in a molecule of methane. Only the outer electron shells need to be shown.

Use ‘o’ to represent carbon electrons.

Use ‘x’ to represent hydrogen electrons. [3]



• (c) Methane burns in oxygen to produce carbon dioxide, CO2

and water H2O. Draw a diagram to show how the electrons

are arranged in a molecule of carbon dioxide. Only the outer electron shells need to be shown.

Use o to represent carbon electrons.

Use x to represent oxygen electrons. [3]



• (d) With the help of the dot and cross diagrams you have drawn for methane and carbon dioxide explain why carbon bonds to different number of hydrogen and oxygen atoms.

[2]



• 2. J05/2/1. The structures of some substances are shown below.



• (a) Answer these questions using the letters A, B, C, D or E.

(i) Which structure is methane? [1]

• (d) Is substance D an element or a compound? Explain your answer. [1]



• 3. J04/2/1.The diagram shows models of various structures,



• (e) Structure D represents a compound.

(i) State what is meant by the term compound.

(ii) Which one of the following substances is structure E most likely to represent? Put a ring around the correct answer.

ammonia hydrogen chloride methane water [2]



• (f) Hydrogen chloride is a compound.

(i) Draw a diagram to show how the electrons are arranged in a molecule of hydrogen chloride. Show only the outer electrons.

show hydrogen electrons as

show chlorine electrons as x [2]

(ii) State the name of the type of bonding present in hydrogen chloride. [1]



• 4. J05/3/4b. (iii) Draw a diagram to show the arrangement of the valency electrons in one molecule of the covalent compound hydrogen sulphide. Use ‘o’ to represent an electron from a sulfur atom. Use ‘x’ to represent an electron from a hydrogen atom.

5. N03/3/1e. Another compound that contains nitrogen and hydrogen is hydrazine, N2H4.



(i) Draw the structural formula of hydrazine. Hydrogen can form only one bond per atom but nitrogen can form three.

(ii) Draw a diagram that shows the arrangement of the valency electrons in one molecule of hydrazine. Hydrazine is a covalent compound.

Use x to represent an electron from a nitrogen atom.

Use o to represent an electron from a hydrogen atom.



• 6. N01/3/5b. The diagram shows a possible arrangement of the valency electrons in a molecule of sulphur dioxide.

(i) What type of covalent bond is labelled bond 1?[1]

(ii) What is unusual about the covalent bond labelled bond 2?[1]

O represents an electron from an oxygen atom

X represents an electron from a sulphur atom

m4 bonding i: covalent bonding

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