Chem 109 - General Chemistry - Spring 2015

Lecture Notes 25: 30 March

DEMONSTRATION: Look at Alkali metals, properties and reaction with water (Li, Na, K); Look at the elements of Period 3 (Na - Cl) Note how their properties change from metallic to non-metallic; Look at Halogens (Cl, Br, I [warm to vaporize crystals]).

Electronic Configurations & Periodicity

There are a number of different notation conventions for electronic configurations:

Spectroscopic notation

In this convention we indicate shells (main energy levels) by numbers, orbitals within these shells by letters (s, p, d, or f), and the number of electrons in each orbital type by superscript. For example:
- H: 1s¹
- He: 1s²
- B: 1s² 2s² 2p¹
- P: 1s² 2s² 2p⁶ 3s² 3p³
- V: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d³

Note that when we get to the d electrons they are added to the next inner orbital - they are added inside the atom and are not outermost! Note also that you may write them in the order they show up on the Periodic Table, or, if you prefer, you may group them by shell. You may also wish to use a tool to help you remember this pattern of filling:

The Aufbau Principle

Pattern of electron addition to atoms. Electrons fill atoms by sequentially filling hydrogen-like orbitals in order of energy. Use to predict electron patterns in atoms. But there are some variations, since in fact we are also adding charge to nucleus.

Aufbau Pattern: This is a useful aid for remembering order - simply list the orbitals in each Period (don't need to go past f since known elements only use up to f orbitals in ground state atoms):

7s 7p

6s 6p 6d

5s 5p 5d 5f

4s 4p 4d 4f

3s 3p 3d

2s 2p

1s

Then draw diagonal arrows with the points on the upper left through the diagram parallel to the diagonal through 2p and 3s. Following the arrows in order starting with the 1s gives the predicted filling order:

Aufbau Diagram

Spectroscopic notation using the Noble gas core convention.

Notice that at the end of each period the outermost shell (s & p) is filled, and when you go to the next element (e.g. Na) its as if you are adding onto the electronic configuration of the Noble gas. So we can save a lot of writing if we substitute its symbol for these inner electrons (note we are not really assuming an inner noble gas, we are just creating a type of short-hand). For example:
- P: [Ne] 3s² 3p³ (instead of 1s² 2s² 2p⁶ 3s² 3p³ where [Ne] = 1s² 2s² 2p⁶)
- V: [Ar] 4s² 3d³ (instead of 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d³ where [Ar] = 1s² 2s² 2p⁶ 3s² 3p⁶)
- U: [Rn] 7s²5f⁴ The savings here is really obvious! Note that if you use the Periodic chart you need to follow the atomic numbers beginning with Period 6 in order to fill the f's before the d's of a given Period.

Orbital Filling Diagrams

In orbital filling diagrams we provide a little more information - noting that the electrons come in two different spins and that they fill into orbitals with their spins paired in opposite directions. I have no preference for the direction of unpaired arrows other than that they should be the same. For example:
- Notice how the electrons first fill into empty orbitals before they pair up
- Now when we add one more electron it goes back to pair up with the first p electron.
Note that the same rules for pairing works for d and f orbital sets as well—all orbitals of a set must be helf-filled before any electrons in the set pair up.

Orbital Filling Diagrams using the Noble gas core convention.
- You can also use the Noble gas core convention with orbital filling diagrams, just like the spectroscopic notation, but using arrows etc.

Syllabus / Schedule