2. Are all atoms reactive?
Most of them are!
But …
Atoms of Noble Gases are different …
Atoms of noble gases are unreactive or stable.
This means that they do not combine with other
atoms to form compounds.
They exist as individual atoms.
3. What are Noble Gases?
Noble gases are elements that are found
in Group 8 (also called Group 0) of the
Periodic Table.
Examples:
helium, neon, argon, krypton, xenon,
radon.
4. Why are Noble Gases unreactive?
The outer shells of noble gases are shown here.
What are the electrons in the outer shells also
called?
Valence electrons
What do they have in common?
Each noble gas has a full valence shell.
5. What are valence electrons?
The shell that is furthest from
the nucleus which contains
electrons is called the outer
shell or the valence shell.
The electrons in this shell are called
valence electrons (or valency electrons).
6. Helium atoms are unreactive because …
Helium has two valence electrons; this is the
maximum number that can occupy the 1st (K-) shell.
We say that it as a duplet configuration. This is a
stable configuration (structure).
7. Atoms of the other noble gases have eight valence
electrons.
This is called an octet configuration; this
structure is stable.
8. How do we know if an atom is stable
or unstable?
An atom is stable if it has a duplet
or octet configuration.
A duplet or octet configuration is
also known as a noble gas structure
or
a noble gas configuration.
9. So why are other atoms reactive?
Atoms react in order to have the
noble gas structure (duplet or
octet) (in order to become stable!!)
10. How do atoms achieve the noble gas
structure?
Atoms of elements (besides the noble gases)
react to achieve the noble gas structures.
They do so by losing, gaining or sharing
valence electrons.
When atoms lose or gain electrons, they form ions.
11. What type of ions do metals form?
Metals form positively charged ions (cations).
12. How do atoms of metals become
positive ions (cations)?
Atoms of metals lose valence electrons to
form positive ions (cations).
Why don’t they gain electrons instead?
Hint: Do metal atoms have few or many valence
electrons?
Few; between 1-3 valence electrons.
It takes less energy to lose fewer electrons.
13. Example 1: Formation of a
Sodium Ion
A sodium atom has 11 protons and 11
11+
electrons. It is neutral (has a net charge
of zero).
Its electronic configuration is (2, 8, 1).
11+ The sodium ion that is formed has
one positive charge because it has
11-
11 protons but 10 electrons.
0
14. Formation of a Sodium Ion
To attain an octet configuration,
a sodium atom (Na) loses
1 valence electron.
+
It forms a sodium ion (Na+).
15. Example 2:
Formation of a Calcium Ion
2+
To attain an octet
configuration,
the atom loses 2
valence electrons.
A calcium atom (Ca) It forms a
has an electronic calcium
configuration ion (Ca2+).
of (2, 8, 8, 2) (2, 8, 8)
16. What type of ions do non-metals
form?
Non-metals form negatively charged ions (anions).
17. How do atoms of non-metals
become negative ions (anions)?
Example 1: -
A chlorine atom (Cl) gains an electron
-
to form a chloride ion (Cl ).
The chloride ion has an octet configuration.
18. Example 2:
Formation of an Oxide Ion
2-
The oxygen atom has an electronic
configuration of (2, 6). It gains two
electrons to form an oxide ion (O2–).
19. Forming Ionic Bonds
When metals react with non-metals, an
ionic compound is formed.
Example:
Sodium reacts with chlorine to form an
ionic compound called sodium chloride.
20. When SODIUM meets CHLORINE…
Na (2,8,1) Cl (2,8,7) Na+ (2, 8) Cl- (2,8, 8)
Sodium ion
Sodium atom Chlorine atom Chloride ion
21. How do we show ionic bonding?
The diagram here shows the formation of an ionic bond in
sodium chloride. It is called a ‘dot-and-cross’ diagram.
Positive ions and negative ions
A valence electron is are attracted to each other and
transferred to the outermost move closer – an ionic bond is
shell of a chlorine atom formed. Electrostatic attraction
Na (2,8,1) Cl (2,8,7) Na+ (2, 8) Cl- (2,8, 8)
Sodium atom Chlorine atom Sodium ion Chloride ion
22. Steps involved in the formation of an
ionic compound
1. Formation of positive ions
Example:
Each sodium atom (Na) loses its single valence
electron to form a positively charged sodium
ion (Na+).
Na Na+ + e
23. Steps involved in the formation of an
ionic compound
2. Formation of negative ions
Example:
Each chlorine atom gains an electron from a
sodium atom to form a negatively charged
chloride ion.
Cl + e Cl -
24. Steps involved in the formation of an ionic
compound
3. Formation of ionic bonds
Positive sodium ions and negative chloride ions
are attracted to one another by electrostatic
attraction to form sodium chloride.
- NaCl
Na + Cl
+
25. Dot-and-Cross diagram of an ionic
compound, e.g. sodium chloride
The dots represent the electrons of one atom,
while crosses represent the electrons of another
atom.
This is the electron transferred from
sodium to chlorine.
26. Formation of Ionic Compounds:
Example 2: Magnesium Chloride.
Each magnesium atom loses
two electrons to form a
magnesium ion. Each chlorine atom gains an
electron to become a chloride ion.
Electrostatic attraction arises between
magnesium and chloride ions.
Magnesium chloride is formed.
27. How to write Chemical Formulae of
Ionic Compounds
The formula of an ionic compound is constructed by
balancing the charges on the positive ions
with those on the negative ions.
All the positive charges must equal all
the negative charges in an ionic
compound.
28. How to write Chemical Formulae of Ionic
Compounds (Method 1)
Example 1: Formula of Magnesium Oxide
Mg2+ O2–
Magnesium ion Oxide ion
Since the charges balance, the formula is
MgO.
29. How to write Chemical Formulae of Ionic
Compounds: Method 1
Example 2: Formula of Aluminium Oxide
Al3+ O2–
Aluminium ion Oxide ion
To balance the charges, there must be 2 Al3+
and 3 O2– .
So, the formula is Al2O3
30. How to write Chemical Formulae of Ionic
Compounds: Method 2
1. write down the ions with the charges:
X m+ Yn–
2. Move the values m and n diagonally (but
without the charges).
X m+ Yn–
The formula is XnYm.
31. How to write Chemical Formulae of Ionic
Compounds (Method 2)
Example 1: Formula of Magnesium Oxide
Mg2+ O2–
Magnesium Oxide ion
ion
Mg2 O2
Reduce to the
simplest ratio Mg1 O1
Write the formula as MgO.
32. How to write Chemical Formulae of Ionic
Compounds: Method 2
Example 2: Formula of Aluminium Oxide
Aluminium ion Al3+ O2– Oxide ion
Al2 O3
So, the formula is Al2O3
33. Sodium ions (Na+) and Chloride ions (Cl-) arrange
into a 3-D lattice structure (crystal).
chloride ion
sodium ion
The formula NaCl for sodium chloride shows that for every
sodium ion present in a piece of salt, there is one chloride
atom present (ratio of Na+ : Cl- = 1: 1)
34. Structure of Ionic Compounds
Ionic compounds form
giant ionic structures /
giant lattice structures/
crystal lattice structures
In a lattice, millions of sodium
and chloride ions are arranged
in an orderly manner.
These ions are held in place by
strong forces of electrostatic
attraction (ionic bonds)
throughout the entire lattice.
35. Physical Properties of Ionic
Compounds
• Volatility (non-volatile)
- high melting and boiling points
2. Solubility
- most are soluble in water but not in organic
solvents (e.g. ethanol)
3. Electrical conductivity
- conducts electricity in molten and aqueous
states, but not in the solid state.
36. Why do ionic compounds have high melting
and boiling points (i.e. low volatility)?
Millions of ions in the lattice are held together by
strong forces of electrostatic attraction between
the ions.
A large amount of energy is needed to overcome
these many strong bonds.
As a result, ionic compounds:
- are solids at room temperature and pressure.
- Have very high melting points and boiling points.
37. Solubility
Ionic compounds are usually soluble in
water.
(Ionic compounds are insoluble in organic
solvents.
Examples of organic solvents : ethanol, petrol and
turpentine.)
38. Electrical Conductivity of ionic
compounds
Bulb does not light up.
SOLID ionic compound,
e.g. sodium chloride
Ionic compounds do not conduct electricity in
the solid state because the ions are not free to
move about.
39. Electrical Conductivity of ionic compounds
Bulb lights up.
Molten or aqueous ionic
compound
When an ionic compound is melted or dissolved in water,
it can conduct electricity.
This is because the ions are FREE to move in the molten
state or in aqueous solution.
