In this topic of IBDP Chemistry, we will learn about the arrangement of electrons in an atom.
Electron Arrangement
Electrons are arranged in shells (energy levels), which contain sub shells (s,p,d,f).
In IB Chemistry, the Hydrogen Spectrum is used to provide evidence for electron shells and sub shells.
- A continuous spectrum contains all thewavelengths/frequencies/colour/energies of electromagnetic radiation.
- Hydrogen in a discharge tubeshows a line spectrum, which containsonly certain wavelengths/frequencies/colours/energies of electromagnetic radiation.
- The electron in hydrogen can be given extra energy by a fast-moving electron in a discharge tube, which causes it to be promoted to a higher energy level.
- The energy which the electron absorbes in order to be promoted to higher energy levels are presented as black lines on a absorption spectrum.
- The energy which the electron emits when it falls back to ground state (i.e. its original energy level) are presented as coloured lines on an emission spectrum.
- As we only see lines, not a continuous spectrum, this proves that electrons can only be in one place at one time, so shells must exist.
Each element has their own specific line spectrums, which consist of discrete lines at specific wavelengths. These line spectras can be used to identify an element by comparing the seen line spectrum to recorded ones in a database.
Sub Shell Structure
There are 4 different types of sub-shells (known as orbitals).
- An orbital is region of space where there is a high probability of finding an electron. It represents a discrete energy level. They contain a maximum of 2 electrons in each orbital.
- These orbitals exist within main energy levels - the number of orbitals in each main energy level differs as the energy level increases.
- The first main energy level (n=1) is closest to the nucleus and only consists of one S-orbital, therefore can consist of a maximum of 2 electrons in the main energy level.
- The second level (n=2) contains 1 S orbital and 3 P orbitals, thus can consist of a maximum of 8 electrons in the main energy level.
- The electrons in the outer most energy level are known as valence electrons - these define the group number of the element.
In the periodic table:
- The period number (row number) shows the highest main energy level occupied by the element.
- The blocks indicate the highest occupied sub shell of that element. They are named from left to right: s, d, p blocks.
- The group number (coloumn number) which starts from one in each block, indicates the number of electrons in each sub-shell.
- e.g. Magnesium,in the s block, group number 2 and in period 3, will end its electronic configuration with 3s2 (Readoff previous full sub shells, starting from Hydrogen, to get the full configuration)
Rules for Arranging Electrons
In IB Chemistry, there are 3 rules that determine the arrangement of electrons.
- The Aufbau ("building up") Principle - Electrons enter the lowest available energy level.
- Hands Rule - When in orbitals of the same energy level, electrons will try to remain unpaired.
- i.e. Electrons fill orbitals of the same energy(subshell) one at a time, when each orbital has one they begin to double up.
- Pauli's Exclusion Principle - Every orbital can contain 2 electrons, provided that they have opposite spins.
Electron distribution can be represented by orbital diagrams:
In IB Chemistry, there are 2 exceptions to Hand's filling pattern in the d-block - these are Chromium (Cr) and Copper (Cu)
- The expected electron distribution (following Hand's Rule) for Chromium is as follows:
- However, the actual electron distribution for Chromium is:
- Similarly the expected electron distribution (following Hand's Rule) for Copper is as follows:
- However, the actual electron distribution for Copper is:
This is because whenever a subshell is completely or half filled, it is more stable and has a lower potential energy.
S and P Orbitals
In IB Chemistry, you will need to know how to draw a diagram of S and P orbitals. The diagram below illustrates the shape of each orbital relative to the x, y and z axis.
This is the end of this topic.
