Chemical Bonding:PART 2
Ionic Bonding
As you work through the following keynote, make sure you take notes in your
workbook. !
By the end of the keynote, you should be able to define ionic bonding, and
identify some ionic compounds.
!
The tasks you will need to complete are highlighted in green text. All answers
must be completed in your exercise book. !
Ionic Bonding
The properties and structure of different substances depends on the way their
atoms are joined together, or bonded. The way a substance reacts with other
substances also depends on these chemical bonds.
!
!
!
Remember that all atoms aim to achieve a stable state. An atom is stable if it has a
full outer shell of electrons. !
In covalent bonding, atoms achieve full outer shells by bonding together to share
electrons.
!
In ionic bonding, atoms give up or take electrons from one another to achieve a full
outer shell.
An atom that has gained or lost electrons becomes an ion. Ions have electrical
charges as they do not have an equal number of positively charged protons and
negatively charged electrons.
!
An atom that has lost electrons becomes a cation. It has a positive charge.
!
An atom that has gained electrons becomes an anion. An anion has a negative
charge because it has more electrons than protons.
!
Ionic bonds are created between cations and anions. By losing and gaining
electrons, these atoms become stable.
TASK: Answer the following questions in your workbook. - Define the term ‘ionic bond’.
- Define the term ‘cation’.
- Define the term ‘anion’.
Compounds made of a metal and a non-metal (salts) form ionic bonds.
Sodium is a metal. It has 1 electron in its outer
shell. It loses this electron to drop down to a
full outer shell. It now has one less electron
than protons, so is given the charge +, and we
write it as Na+.
Non-metals have 5 to 7
electrons in their outer shell.
It requires less energy for
non-metal atoms to take 1-2
electrons to fill their outer
shell, rather than losing 5-6.
electrons to drop down to
the next full shell. Nonmetals always form anions.
Metals only have one or two
electrons in their outer shell. It
requires less energy for metal
atoms to give up electrons and
drop down to the next full
electron shell, rather than trying
to find 6-7 electrons to fill the
outer most shell. Metals always
form cations.
Fluorine is a non-metal. It
has 7 electrons in its outer
shell. It gains one extra
electron to achieve a full
outer shell. It now has one
more electron than proton,
so is given the charge -, and
we write it as F-.
Metal Ions
!
- Metals in Group 1 always form ions with a +1 charge.
!
- Metals in Group II always form ions with a +2 charge.
TASK: Copy this
info into your
workbook.
!
- Metals in Group III always form ions with a +3 charge.
!
- Metals in Group IV usually have a +2 charge, but can also have a +4 charge.
Assume a +2 charge unless otherwise stated.
!
- Transition metals have a variety of ionic charges, but most form ions with +2
charge.
!
- Metals in Groups V and VI can also have charges that vary.
!
- If a metal has more than one common ion, the charge it takes is shown with
Roman numerals
Copper (I) = Cu+
Copper (II) = Cu2+
Non-metal Ions
TASK: Copy this
info into your
workbook.
- Elements in Group VII (halogens) always form ions with a -1 charge.
!
- Elements in Group VI always form ions with a -2 charge.
!
- Elements in Group V always form a -3 charge.
!
- Non-metal elements in Group IV (C and Si) may form -4 ions.
!
- Elements in Group VIII (noble gases) either have full outer shells or are happy
with eight electrons in their outer shells. They are extremely stable and do not
form ions.
An example: Formation of table salt (Sodium Chloride)
!
If a sodium atom meets a chlorine atom, the sodium
loses its single outer shell electron to form the sodium
cation Na+. Because is now has one more proton than
electrons, it is given the charge +.
Chlorine takes the electron from sodium to become the
anion Cl-. It now has a new name: chloride. Because is
now has one more electron than protons, it is given the
charge -.
!
Both ions are stable, and are attracted to each other.
They exist together as Na+Cl-... sodium chloride.
!
The attraction between the positive and negative ions is
called the ionic bond.
TASK: Copy this diagram into your
workbook TASK: Copy this diagram into your
workbook - Which element forms the cation? - Which forms the anion?
TASK: Complete the following table on this slide. - You can work in pairs to complete this task.
- You can also use the “smell-o-mints” App to help.
TASK: Complete the following activity on this slide. In order to write the correct formulae for ionic compounds, you need to know the charge
on each ion involved. To help you learn these, complete the tables below.
Ions with opposite charges are attracted to each other, with negative charged
ions and positive charged ions combining to form a neutral compound. This
creates the ionic bond that holds them together. !
Ionic compounds are not made of separate molecules. Instead, the ions gather
in a regular arrangement called an ionic lattice. !
Ionic bonds are very strong - much stronger than covalent bonds. It take a lot
of energy to break ionic bonds, so ionic compounds have high melting and
boiling points.
TASK: Answer the following questions in your workbook. - Define the term ‘ionic lattice’.
- Which type of elements forms cations?
- Which type of element forms anions?
Ionic Bonding
Task: complete the following activity. “Ions get together” in your
workbook.
You can cut and paste the jigsaw
pieces, or you can draw them
Polyatomic Ions
!
Some covalent compounds have charges - they are made up of more than
one type of atom and have a charge. They are called polyatomic ions. These
ions have special names.
TASK: define the term “polyatomic ion’ your
workbook, and list 2 examples.
Naming Ionic Compounds
To name ionic compounds, simply follow these rules:
!
1. The positive ion (cation) is named first and the negative ion (anion)
second.
!
2. A simple positive ion (cation) takes its name from its parent element.
For example, Na+ is called sodium.
!
3. A simple negative ion (anion) is named by taking the first part of the
elements name and adding the suffix -ide. Cl- was originally a chlorine
atom, and is given the new name chloride.
!
We do not use prefixes when naming ionic compounds, unlike when
we name covalent compounds.
!
For example, an ionic substance made of one magnesium cation and
two fluorine anions would have the formula MgF2 and the magnesium
fluoride.
Writing Ionic Formulae
When trying to determine the chemical formula of an ionic compound (i.e. the
number of cations and anions needed to give a neutral charge), you can draw your
jigsaw pieces...
!
Or you can make it easier by “crossing over”...
!
Ca2+ and Cl-
!
!
Ca2+1Cl-2
CaCl2
Calcium chloride
!
Cr3+ and O2-
!
!
Cr3+2
O2-
3
Cr2O3
Chromium oxide
Place the charge at the
top of each element
Cross over each charge
as subscripts
Remove charges at top
of element for final
formula
Place the charge at the
top of each element
Cross over each charge
as subscripts
Remove charges at top
of element for final
formula
When more than one
polyatomic ion is required in
a formula, brackets are used. !
In magnesium sulfate, MgSO4,
only one sulfate ion is needed.
!
Mg2+
SO42-
!
MgSO4
In aluminium sulfate, Al2(SO4)3,
three sulfate ions are required and
so brackets are used.
!
Al3+ SO42- !
Al2(SO4)3
TASK: Complete the following activity.You can fill in the table on this slide, but you may
need to do your “cross-overs” in your workbook. Use the cross-over method to help write formulae for some ionic compounds.
+
Na
+
Cl
+
Na
Cl
1 sodium ion joins with 1 chloride ion to form sodium chloride.
The formula is NaCl.
Na+ Cl-
!
NaCl
2+
Mg
+
Cl
+
Cl
Cl
2+
Mg
Cl
1 magnesium ion joins with 2 chlorine ions to form magnesium chloride.
The formula is MgCl2.
Mg2+ Cl-
!
MgCl2
TASK: Answer the following questions in your workbook. Write the formulas for the following ionic compounds:
1. Sodium bromide
2. Magnesium sulfide
3. Calcium fluoride
4. Lithium nitride
5. Aluminium carbide
6. Magnesium hydroxide
7. Lithium nitrate
8. Sodium sulfate
TASK: Answer the following questions in your workbook. Write the chemical names for the following covalent compounds:
1. FeCl2
2. Cu(NO3)2
3. NH4OH
4. MgO
5. KBr
6. ZnCl2
7. Li3N
8. SrS
Extension Questions
Level 1 - Hard [knowledge extension activity]
Level 2 - Challenging [application question, linked to Year 11 Chemistry syllabus]
If you are considering studying Physics or Chemistry next year, or just want a
challenge, attempt the following activities.
!
And if you need help, make sure you ask Ms Belshaw.
In today’s keynote, Physics and Chemistry students should attempt to complete
BOTH LEVELS.
Level 1: More ionic compounds
TASK: Complete the following table.
!
Level 2: Valency
TASK: Use information here to answer the question on the following slide. Write your
answers in your workbook
The number of electrons an atom needs to gain or lose to form a stable outer shell of electrons is
called its valency or combining power.
An atom with a stable outer shell
of electrons has a valency of zero.
If an atom needs to gain or lose
just one electron, it has a valency
of 1.If an atom needs to gain or
lose 2 electrons, is has a valency of
2, and so on ...
The valency of an ion is the same size of its
charge. For example, the oxide ion (O2-) has a
charge of 2- and a valency of 2.
!
Some elements can form different ions, so can
have more than once valency. Iron, for example,
forms Fe2+ and Fe3+. Roman numerals after the
name, for example iron (II) and ion (III) indicate
the valency.
Using the internet, fill in the table below, identifying the electron configuration and valency
for the first 20 elements in the periodic table.
Element
Symbol
H
He
Li
Be
Be
C
N
O
F
Ne
Na
Mg
Al
Si
P
S
Cl
Ar
K
Ca
Electron Configuration
Valency