1. Week 2 Cell chemistry: atoms, elements and their ions
Dr Nicole Masters
Introductory Bioscience, LFS103

2. 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

3. 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.

4. Atoms Subatomic Particles
2.1 Arranging electrons in atoms [4.6]
Atoms Subatomic Particles
Today we are going to talk about how the tiny electrons are organised in the electron cloud.
This is called electron arrangement of atoms.
We will study the first 20 elements (H to Ca)

5. Electron Arrangement in Atoms
2.1 Arranging electrons in atoms [4.6]
Electron Arrangement in Atoms
Every electron has a specific amount of energy. Electrons of similar energy are grouped in an energy level called a shell. The shells closest to the nucleus contain electrons with lowest energy.
There are 7 shells that can orbit the nucleus
We will study up to 4 shells

6. Electron Arrangement in Atoms
2.1 Arranging electrons in atoms [4.6]
Electron Arrangement in Atoms
Capacity of the first four electron shells.
Electron Maximum Number
Shell of Electrons
Increasing in energy levels
Note: This is how many electrons each shell can hold in total, not
the order in which the electrons are filled into the shells.

7. Electron Arrangement in Atoms The First Twenty Elements
2.1 Arranging electrons in atoms [4.6]
Electron Arrangement in Atoms The First Twenty Elements
There is a specific order for filling electrons into the shells of the first twenty (20) elements (Table 3.9).
• Fill 2 electrons into shell 1
• Fill 8 electrons into shell 2
• Fill 8 electrons into shell 3
• Fill 2 electrons into shell 4
The electron order you need to remember is:

8. 2.1 Arranging electrons in atoms [4.6]
Shell 1
Hydrogen (Z = 1) H
1e-
Hydrogen has 1 electron
The first shell fills first it has a capacity for 2 electrons
Hydrogen only has 1 shell, this is the outermost shell
This means that in this form hydrogen is unstable i.e. reactive
The outer most shell is where the chemistry takes place

9. 2.1 Arranging electrons in atoms [4.6]
Helium has 2 electrons
The first shell fills first it has a capacity for 2 electrons
Helium has 2 electrons in its shell, this is the outermost shell
This means that this shell is now full.
Full shells are very stable and un-reactive
Helium is a noble gas and that is why noble gases very stable
Shell 1
Helium (Z = 2) He
2e-

10. 2.1 Arranging electrons in atoms [4.6]
Helium has 2 electrons
The first shell fills first it has a capacity for 2 electrons
Helium has 2 electrons in its shell, this is the outermost shell
This means that this shell is now full.
Full shells are very stable and un-reactive
Helium is a noble gas and that is why noble gases very stable
Shell 1
Helium (Z = 2) He
2e-