Ionic and Covalent bonding - Ms. Buicke maths and science

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Transcript Ionic and Covalent bonding - Ms. Buicke maths and science

Ionic and Covalent Bonding
Syllabus
OC42
Recall that ionic bonding is an attraction between positive and negative
ions; describe the bonding in NaCl and MgO as examples.
OC43
State what a molecule is, understand that covalent bonds involve the
sharing of pairs of electrons, and describe the bonding in H2, O2, H2O, CH4
as examples of covalent bonding.
OC44
Investigate the ability of ionic and covalent substances to conduct
electricity
What are molecules?
A group of atoms which are joined together are called molecules.
The chemical formula for water is H20 because a molecule of water
consists of two hydrogen atoms joined to an oxygen atom.
The formula For methane is CH4. It has this formula because a
molecule of methane
Consists of an atom of carbon joined to four atoms of hydrogen.
Methane
molecule
Chemical bonding.
The atoms in a molecule are held together by forces of attraction called
chemical bonding.
Bonding occurs because all atoms try to have a full outer shell, and will
lose, gain or share electrons in order to do so.
Our understanding of chemical bonding is made a lot easier if we first
study a rule called the octet rule.
Octet rule: when bonding occurs, atoms try to have eight electrons in
the outer orbit.
If you look at the periodic table you will see that the noble gases ( group
8 or 0) have 8 electrons on their outer shell as a result noble gases are
very unreactive and will not form a bond with any other element.
Exercise: can you name three noble gases and give a use for one of the
noble gases you have named.
Three noble gases you should know:
Helium: chemical formula He, it is a colorless, odorless, tasteless gas.
Used in blowing up balloons.
Neon: chemical formula Ne, used in signs
Argon: chemical formula Ar, used in light bulbs
Chemical reactions take place so that the elements can obtain the very stable
arrangement of the noble gases.
There a two types of chemical bonding
1) Covalent bonding,
2) Ionic bonding.
Ionic bonding occurs when an atom loses or gains electrons
Covalent bonding occurs when an atom shares electrons.
Ionic bonding
An ionic bond is a bond formed by the force of attraction between two oppositely
charged ions.
An ion is a charged atom ie an atom which has gained or lost an electron.
How is an ion formed?
If an atom loses an electron then it becomes a positive ion.
If an atom gains an electron then it becomes a negative ion.
In ionic bonding positive ions are attracted to negative ions forming an ionic
Bond.
There are two examples of ionic bonding which we need to know:
1) Sodium chloride NaCl
2) magnesium oxide MgO
Sodium chloride NaCl:
The atomic number of sodium is 11
It has an electronic configuration of 2,8,1 this means that
sodium most lose its outer electron to be stable.
The atomic number of chlorine is 17
It has an electronic configuration of 2,8,7 this means chlorine
needs to gain an electron to become stable.
When a sodium atom bonds with a chlorine atom the sodium
atom loses its outer electron to form a positive ion while the
chlorine atom gains an electron to form a negative ion.
The two atoms now have opposite charges and because
opposite charges attract both atoms move off together as a
sodium-chloride (NaCl) molecule.
A sodium atom and a sodium ion:
A chlorine atom and chloride ion:
Sodium chloride:
What is sodium chloride?
Sodium chloride is the chemical term for table salt.
This is the structure
of the sodium chloride
Molecule.
Magnesium oxide MgO
The atomic number of magnesium is 12.
It has an electronic configuration of 2, 8, 2.
This means that it needs to lose the two
electrons which it has in its outer shell in order
to have a complete outer shell.
The atomic number of oxygen is 8.
Oxygen has an electronic configuration of 2, 6.
This means it needs to gain two electrons in
order to have a complete outer shell.
When a magnesium atom bonds with an
oxygen atom the sodium atom loses its two
When a magnesium atom bonds with an oxygen
atom the sodium atom loses its two outer electrons
to form a positive ion while the oxygen atom gains
two electrons to form a negative ion.
The two atoms now have opposite charges and
because opposite charges attract both atoms move
off together as a magnesium-oxide (MgO) molecule
Magnesium atom and magnesium ion:
Oxygen atom and oxide ion:
Magnesium Oxide:
You must know how to draw the ionic bonds of both Sodium Chloride and
Magnesium Oxide.
Covalent Bonding:
A covalent bond is formed when atoms combine by
sharing electrons so that
Each atom has a stable outer shell of electrons.
In covalent bonding neither atom wants to donate
electrons as they both need electrons to achieve a
full outer shell. The solution is they share electrons.
Remember: sharing is caring!! C for caring C for
covalent bond.
We need to know 4 examples of covalent bonding.
1) H2 2) CH4 3)H2O 4) 02
Examples of covalent bonding: (i) A hydrogen
molecule (H2)
The atomic number of hydrogen is 1.
It needs to have two electrons in its outer shell,
and so if it ‘bonds’ with another hydrogen atom
and they both ‘share’ their electron with each
other, it has the effect of allowing both atoms to
have a complete outer shell.
Examples of covalent bonding: (ii) An
oxygen molecule (O2)
The atomic number of oxygen is 8.
It has an electronic configuration of 2, 6.
It needs to have 8 electrons in its outer shell,
and so if it ‘bonds’ with another oxygen atom
and they both ‘share’ two of their electrons with
each other, it has the effect of allowing both
atoms to have a complete outer shell.
Oxygen has a double bond because both
oxygen atoms share two electrons
Examples of covalent bonding: (iii) A water
molecule (H2O)
The atomic number of hydrogen is 1 so it has 1
electron in its outer shell.
The atomic number of oxygen is 8 so it has 6
electrons in its outer shell
Oxygen can share 1 electron with one
hydrogen atom and a second electron with a
second hydrogen atom,which has the effect of
allowing all atoms to have a complete outer
shell.
Covalent bonding in a water molecule
Examples of covalent bonding: (iv) A methane
molecule (CH4)
The atomic number of hydrogen is 1 so it has 1
electron in its outer shell.
The atomic number of carbon is 6 so it has 4
electrons in its outer shell.
Carbon can share 1 electron with each of 4
different hydrogen atoms, which has the effect
of allowing all atoms to have a complete outer
shell.
Covalent bonding in the methane molecule
Properties of Ionic and covalent compounds:
Ionic compounds:
1) Consist of positive and negative ions
2) Solid at room temperature.
this is because of the strong attraction
between oppositely charged ions
bonding in sodium chloride is very strong
3) Usually high melting and boiling points.
This is because there is a strong attraction
between the ions, a lot of energy is needed to
separate them form each other.
4) Usually soluble in water
water molecules are attracted to ions and
cause the ions to pull apart.
5) Conduct electricity when melted or
dissolved in water
when ionic material is molten or dissolved in
water the ions are free to move. These ions
then carry the electric current.
Ionic compounds can’t conduct electricity
when solid the ions are not free to move.
Covalent molecules:
1)Usually liquid or gas at room temperature.
example: water is a liquid and methane is
a gas
2) Usually low melting points
the covalent bond is not that strong and not
much energy is
needed to separate them from each other.
3) Usually insoluble in water
Covalent compounds do not contain ions and
water molecules are not attracted to covalent
3) Usually insoluble in water
Covalent compounds do not contain ions and
water molecules are not attracted to covalent
molecules.
4) Covalent compounds do not conduct
electricity.
There is no ions present.
To investigate the ability of ionic and covalent
substances to conduct electricity.
In this experiment we try to distinguish
between ion and covalent substances. To do
this, we look at the ability of these
Substances to conduct electricity.
If ions are present in the solution , the
solution will conduct an electric current.
If no ions are present in a solution, the
solution will not conduct an electric current
Exam Questions
2012 (HL)
The diagram shows part of a crystal of sodium chloride. Name the
type of bonding in sodium chloride. Describe this type of bonding.
Name :
Describe:
2012 (OL)
Choose the correct words from the list on the right, in each case,
to complete the statements bellow.
Ionic bonding involves an attraction between positive and
negative _____________.
Protons
Electrons
Neutrons
ions
Covalent bonding involves the sharing of pairs of
______________________.
2011 (OL)
Complete the following sentence using the correct word from the
list on the right.
Bonding that involves the sharing of electron pairs is called
____________________ .
Ionic
Covalent
2011 (HL)
The diagram shows the way the atoms bond together in a
molecule of water.
(i) What is a molecule?
(ii) Each hydrogen atom shares two electrons with the oxygen
atom. What name is given to the type of bonding that involves
the sharing of pairs of electrons?
(iii) In the space below, draw a diagram of a methane
molecule, CH , showing the bonding between its atoms.
4
(iv) Describe a second type of chemical bonding and name a
compound which has this type of bonding.
2010 (HL)
A potassium atom has atomic number 19 and a mass
number of 39.
Complete the diagram using dots or crosses to clearly
show the arrangement of electrons in the potassium
atom.
2010 (OL)
Water is a compound composed of two elements.
Name these two elements. 1 _______________________
2_______________________
2009 (HL)
Atoms of elements can combine to form compounds using
chemical bonds between their atoms. There are different types of
chemical bonds.
The diagram shows a group of water molecules with one
enlarged below with its constituent atoms identified by their
atomic symbols.
(i)
Name the type of bonding in the water molecule.
(ii) Describe this type of bonding.
(iii)
Name one other compound with this type of bond bonding
(b) The diagram shows sodium ions (+) and chloride ions (-) in part of a crystal of
table salt, sodium chloride.
(i) How are sodium ions and chloride ions formed from their atoms?
How? __________________________________ _______________________________________
(ii) What force holds the ions together in sodium chloride?
______________________________________________________________
(iii) Name one other compound that is composed of ions.
______________________________________________________________
2009 (OL)
Different types of bond can be formed when atoms combine. What
name is given to the bond that involves an attraction
between positive and negative ions?
______________________________
2008 (HL)
Why are the noble gases, group 8, very chemically unreactive?
2008 HL
Atoms of different elements can form compounds by bonding together.
What is an ionic bond?
True or false?
Use your true/ false card to answer the following
questions.
1)
2)
3)
4)
Carbon has 4 electrons on its outer shell? True/false
Hydrogen has 3 electrons on it outer shell? True/false
H2 is an example of a covalent bond? True/false
The covalent bond structure of water (H20) looks like this?
True/false
5) Methane has the chemical formula CH7? True/false
6) A covalent bond is formed when atoms share electrons?
True/false.
7) Covalent bonding in methane looks like this?
True/false