1. Week 2: Cell chemistry: atoms and their ions
2.1 Arranging electrons in atoms [4.6]
2.2 Valence electrons [4.7]
2.3 Electron dot structures [4.7]
2.4 The formation of ions [6.1]
2.5 Cations and anions [6.1]
Readings: Chemistry Textbook. Timberlake (2015). Chemistry (12th edn.). Pearson Australia.
LFS103 Vol. 2 Custom textbook
Ch 4: Concepts 4.6, 4.7
Ch 6: Concept 6.1

2. Week 2: Learning objectives
Describe for the first 20 elements of the periodic table; the Z value, the valance electrons, the electron configuration and the electron dot structures.
Explain if an element is stable or reactive using its electron configuration.
Understand how an ion is formed.
Use the octet rule to write the symbols of the simple ions for the first 20 elements.
Explain which elements form cations and which elements form anions, and why.

3. 2.2 Valence electrons [4.7]
Valence Electrons
The electrons in the outer shell have an important role in determining the chemical properties of an atom.
These electrons are referred to as valence electrons.

4. Hydrogen (Z = 1) Helium (Z = 2) Shell 1 H 1e- 1 valence electron
2.2 Valence electrons [4.7]
Shell 1
Hydrogen (Z = 1) H
1e-
1 valence electron
Shell 1
Helium (Z = 2) He
2e-
2 valence electrons

5. Lithium (Z = 3) Shell 2 Li 2e- 1e- 1 valence electron
2.2 Valence electrons [4.7]
Shell 2
Lithium (Z = 3) Li
2e-
1e-
1 valence electron
Lithium has 3 electrons
The first shell fills first it has a capacity for 2 electrons
The third electron is placed in the second shell, this is the
outermost shell
This second shell has the capacity for 8 electrons
This means that in this form lithium is unstable ie reactive
The outer most shell is where the chemistry takes place

6. Boron (Z = 5) Boron has 5 electrons
2.2 Valence electrons [4.7]
Boron has 5 electrons
The first shell fills first it has a capacity for 2 electrons
The fourth and fifth electrons are placed in the second
shell, this is the outermost shell
This second shell has the capacity for 8 electrons
This means that in this form boron is unstable i.e. reactive
The outer most shell is where the chemistry takes place
Shell 2
Boron (Z = 5) B
2e-
3e-
3 valence electrons

7. Neon (Z = 10) Neon has 10 electrons
2.2 Valence electrons [4.7]
Neon has 10 electrons
The first shell fills first it has a capacity for 2 electrons
Neon has 2 electrons in this shell,
This means that this shell is now full.
The second shell has the capacity for 8 electrons, this shell
is full also
Full shells are very stable and un-reactive
Neon is a noble gas and that is why noble gases very stable
Shell 2
Neon (Z = 10) Ne
2e-
8e-
8 valence electrons

8. Sodium (Z = 11) 1 valence electron Shell 3 Na 2e- 8e- 1e-

9. 2.2 Valence electrons [4.7]
Calcium (Z = 20)
Remember shell 3 has the capacity to hold 18 electrons
2 valence electrons
Shell 4 Ca
2e-
8e-
8e-
2e-
Electron configuration?
2,8,8,2

10. Electron Arrangement in Atoms The First Twenty Elements
2.2 Valence electrons [4.7]
Electron Arrangement in Atoms The First Twenty Elements
A couple of points to note:
notice that elements in the same Group of the Periodic Table have the same number of valence electrons. They also have similar chemical properties.
you can use the old system for numbering the Groups in the Periodic Table (1A, 2A, 3A ….. ) to easily work out the number of valence electrons for the first 20 elements.

11. Write the electron configuration for Fluorine

12. Q. Which group of elements on the Periodic Table have 2 valence electrons?
Alkali metal
Alkali earth metal
Halogen
Nobel gas
ANSWER: Group 2A: alkali earth metals