Year 9 Chemistry – Worksheet 8
2.1 Structure of the electron shell: The simplified orbital model
When arranging the electrons from the inner energy levels towards the outer ones so that
a) they fill up all orbitals in one energy level as a single electron first (Pauli exclusion principle), sitting as far apart from each other as possible
b) a maximum of two electrons can fill any orbital (Hund’s rule)
We can set up an easy model for the electron shell of an atom. The simplified orbital model.
Example: Orbital model for oxygen (O)
Task: Construct four more orbital models for boron (B), phosphorus (P) and magnesium (Mg) as well as a fourth of an element of your choice (from the first three rows).
1. energy level (K-shell) (contains nucleus [red]) 3. energy level (M-shell) 2. energy level (L-shell)
= 1 electron in the orbital
= 2 electrons in the orbital
B P
Year 9 Chemistry – Worksheet 8
Task 2: Chemists are very lazy. They don’t want to draw all the lower energy levels of an atom that are all complete. So they choose a shortcut: They draw a circle in the center of an orbital model that represents all completed lower energy levels as well as the nucleus. A number inside the circle states, how many complete energy levels it represents. Around the central circle they draw all orbitals of the highest energy level that contains electrons. The electrons on the highest occupied energy level are also called valence electrons, their orbitals are called valence orbitals.
Example: Simplified orbital model for oxygen
Task 3: Redraw the other four models from the worksheet in the new way.
The Lewis notation is an even simpler way of drawing an atom: The central circle (completed energy levels) is replaced by the element’s symbol, a valence orbital with one electron is represented by a dot, a valence orbital with two electrons is represented by a short line. The dots and lines are drawn around the element symbol exactly where the orbitals would be drawn in the simplified orbital model.
Task 4: Draw the Lewis notation for the five elements on this worksheet.
Task 5: The periodic table is built around similarities between the elements that are listed in a column and between the elements listed in a row. Draw the simplified orbital models of aluminum (Al) and silicon (Si). Compare the models of boron, aluminum and gallium (one column) and aluminum, silicon and phosphorous (one row). List similarities.
Cl
The first energy level is complete (full of electrons), if oxygen had two complete energy levels the number here would be 2
1
These four orbitals represent the 2. energy level, which is the highest occupied level (these are the valence orbitals of the oxygen atom)The third and fourth energy levels contain no electrons, so they are not drawn
