Download 11 myp covalent bonding

M4 Bonding I: Covalent
Bonding
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
M4 Bonding: Covalent Bonding
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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
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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.
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Review
• A while back you observed the following reactions in the lab:
2Na
Zn
(g)
(s)
+ H2O (l)  2Na2O
+ I2 (s)  ZnI2
(s)
2Al
(s)
+ 3I2 (s)  2AlI3
2Al
(s)
+ 3Br2
2H2O
(l)
(l)
(s)
(s)
 2AlBr3 (s)
 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.
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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
C
(g)
+ O2
(g)
(g)
 2H2O
(l)
 CO2 (g)
• they share electrons between them because neither of the non-metals
have a tendency to lose electron(s).
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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.
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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.
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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
outer shell NOT full
energetically unstable
1
outer shell NOT full
energetically unstable
What can the TWO hydrogen atoms possibly do to fill both their
outer shells and become energetically stable ?
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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 ?
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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.
H
Electronic
configuration:
1
outer shell NOT full
energetically unstable
M4 Bonding: Covalent Bonding
H
1
outer shell NOT full
energetically unstable
H2
1+1 = 2 1+1 = 2
outer shell full
energetically stable
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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.
Structural formula
1+
M4 Bonding: Covalent Bonding
-
HH
1+
The single line
representing a
pair of shared
electrons
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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
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Elements in Group 7: Fluorine
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)
• Or…
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Elements in Group 7: Fluorine
– By gaining one electron (and acquiring the structure of neon gas)
+
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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:
+
FF
Structural Formula
2, (7 +1 = 8)
outer shell full
energetically stable
2, (7 +1 = 8)
outer shell full
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.
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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.
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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.
COVALENT
BOND
• The definition of covalent bond therefore is the electrostatic force of
attraction between the nuclei of the bonded atoms and the shared
electrons.
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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?)
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Multiple covalent bonds: Double Bond
• Oxygen is a non-metal. A structure for oxygen is
given to the right.
What is the electronic configuration of nitrogen? 2, 6
How many electrons does its outer shell have? 6
How many electrons does it need to have a completely filled outer
shell and become energetically stable? 2
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? 2
How many electrons must they share between them? 4
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Multiple covalent bonds: Double Bond
•
Electronic
configuration:
O2
2, (6 + 2 = 8)
outer shell full
energetically stable
2, (6 + 2 = 8)
outer shell full
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.
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Multiple covalent bonds: Triple Bond
• Nitrogen is another non-metal. A structure for
nitrogen is given to the right.
What is the electronic configuration of nitrogen? 2, 5
How many electrons does its outer shell have? 5
How many electrons does it need to have a completely filled outer
shell and become energetically stable? 3
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? 6
How many electrons must they share between them? 3
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Multiple covalent bonds
•
Electronic
configuration:
N2
2, (5 + 3 = 8)
outer shell full
energetically stable
2, (5 + 3 = 8)
outer shell full
energetically stable
Since the nitrogen atoms share 3 pairs of electrons, a molecule of
nitrogen therefore has a triple bond.
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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.
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Water: Oxygen and Hydrogen
Shared pairs of electrons; each
pair represents a single bond
H
H2O
O
What would the structural formula look like?
H
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Dot and cross diagram
X
X
• 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).
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Covalent bonds in compounds: Ammonia
• 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.
H
H
N
H
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NH3
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Dot and Cross diagram of Ammonia
X
X
X
Three single
bonds between
hydrogen and
nitrogen in
ammonia.
• It is indeed more convenient to just show the outer electrons.
What would the structural formula look like?
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Covalent bonds in compounds:
carbon dioxide
• Carbon needs four electrons to have
a completely filled outer shell and
become stable.
C: 2, 4
Oxygen of course requires two electrons.
O: 2, 6
Therefore, one carbon atom will combine with two oxygen
atoms to form a molecules of carbon dioxide, CO2, each
sharing 4 electrons.
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Covalent bonds in compounds:
carbon dioxide
+
O
M4 Bonding: Covalent Bonding
+
C
O
CO2
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Dot and Cross diagram of CO2
XX
X
X
XX
X
X
XX
Two double
bonds in carbon
dioxide.
XX
• 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.
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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?”
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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.
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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.
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Breaking a covalent bond
+
+
HEAT (thermal)
ENERGY
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Breaking a covalent bond
+
Thermal (heat) energy is
absorbed (taken in) by
the atoms.
M4 Bonding: Covalent Bonding
+
Thermal energy is transferred to
kinetic (movement) energy - so the
electrons and nucleuses start
vibrating in all directions.
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Breaking a covalent bond
+
More thermal energy is transferred
to kinetic energy – causing the
electrons and nucleuses to vibrate
further distances in all directions.
M4 Bonding: Covalent Bonding
+
the distance between the valence
electrons and nucleuses increases –
resulting in the attractive pull
between them becoming weaker.
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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
NH4+ ammonium
Formula
SO42
Common name
sulfate
CO32
carbonate
HCO3
bicarbonate
OH
hydroxide
NO3
nitrate
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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?
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Practice Questions: Multiple Choice
•
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
M4 Bonding: Covalent Bonding
B. CCl4
C. CaCl2
D. CaCl4
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Practice Questions: Multiple Choice
•
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
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Practice Questions: Multiple Choice
• 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
M4 Bonding: Covalent Bonding
B. 2HO
C. 2H2O
D. H2O2
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Practice Questions: Multiple Choice
•
9. J04/1/7. In the diagrams, circles of different sizes represent atoms
of different elements.
Which diagram can represent hydrogen chloride gas?
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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.
M4 Bonding: Covalent Bonding
[3]
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Practice Questions: Structured
• (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.
M4 Bonding: Covalent Bonding
[3]
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Practice Questions: Structured
• (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]
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Practice Questions: Structured
•
2. J05/2/1. The structures of some substances are shown below.
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Practice Questions: Structured
• (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]
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Practice Questions: Structured
•
3. J04/2/1.The diagram shows models of various structures,
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Practice Questions: Structured
• (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
M4 Bonding: Covalent Bonding
hydrogen chloride
methane
water
[2]
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Practice Questions: Structured
• (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]
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Practice Questions: Structured
•
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.
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Practice Questions: Structured
(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.
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Practice Questions: Structured
•
6. N01/3/5b. The diagram shows a possible arrangement of the
valency electrons in a molecule of sulphur dioxide.
O represents an electron
from an oxygen atom
X represents an electron
from a sulphur atom
(i) What type of covalent bond is labelled bond 1?
[1]
(ii) What is unusual about the covalent bond labelled bond 2?
[1]
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