1. Quiz:
Protons, neutrons and electrons are subatomic particles
What is the charge on proton/neutron/electron?
Where do you find each in an atom?
What is the difference between carbon 14 and carbon 12?
What does atomic number indicate?
What does mass number show?

2. Electrons don’t just whiz around randomly, instead they move in orbital.

3. Electrons are involved in many chemical reactions, and in fact they are highly ordered around an atom forming ‘shells’ of energy in which they orbit the nucleus. These shells are located at different distances from the nucleus and a shell has a certain capacity for a given number of electrons. (So if the bus is full, you have to wait for another one) For electrons it means that if the previous layer (closer to the nucleus) is full, electrons will be placed in layers further away.
The general rule for the number of electrons occupying shell is:
Energy level (shells)
Number of electrons in the shell 1 2 8 3 4
Note we only do the first 20 elements, up to calcium.

4. Steps to determining electron configuration:
Identify how many electrons are there in you atom.
Place the symbol of the element in the middle (you don’t have to draw protons and neutrons).
Place the electrons in orbital (draw the orbit) in the order of 2,8,8,2 until all the electrons are placed.
For example, an element has 14 electrons.
Place the first 2 electrons in first shell. 12 left.
Place 8 in the second shell. 4 left.
Place the last 4 in the third shell. 0 left.
The shorthand way of writing electron configuration is
Si: 2,8,4
Chemical reaction: a war of taking electrons.

6. Quiz
Isotopes are atoms with __________ number of protons and electrons but different number of _________.
Write the shorthand electron configuration for Aluminum, oxygen and sodium.
Name the elements whose atoms have the following electron arrangements
2, , ,8, ,8,8,2
4. What are the similarity and differences of these atoms below?

7. What you can do By now you should be able to:
Draw atomic structure of the first 20 elements.
Write shorthand electron configuration of the first 20 elements.
Identify atomic and mass number of elements from the periodic table
describe what an isotope is.

8. The periodic table

10. PT is arranged in groups (down the column) and the elements in those groups share similar properties.
The number of electrons in the outer energy level of an atom determines how it reacts.
Go through groups and periods on PPT, down the group atoms are larger but similar periods, across period properties change gradually (the first and the last are very different).

11. Electron arrangement and reactivity:
The number of electron in the outer energy level of an atom determines how it is going to react, atoms are grouped based number of electrons in their outer energy level and elements within the same group have similar properties.
Group
Name (group)
Electrons in outer energy level 1
Alkali metals 2
Alkali earth metals 17
Halogens 7 18
Noble gases 8
(except helium)

12. Formation of ions:
Ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, resulting in a net positive or negative charge.
Atoms with three or less electrons in their outer energy shell tend to lose electron to reach full outer shell for stability. Thus they form into a positively charged ions. (more protons than electrons)
Atoms with 5 or more electrons tend to gain electrons to form a full outer shell for stability. Thus they turn into negatively charged ions.
Having a full outer shell means stability to the atom.
e.g. Na and Na Cl and Cl-