Periodicity and Electron Configuration
An element's position in the Periodic Table (its Period and Group) directly reveals its electron configuration. This is a fundamental concept that allows you to predict an element's chemical reactivity and bonding behaviour without needing to memorise details for every single one.
Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- The Period number (the horizontal row, 1-7) tells you the number of occupied electron shells in an atom.
- The Group number (the vertical column, 1-18) tells you the number of electrons in the outermost shell (valence electrons).
- For main group elements in Groups 1 and 2, the number of valence electrons is equal to the Group number.
- For main group elements in Groups 13-18, the number of valence electrons is the Group number minus 10 (e.g., Group 16 has 6 valence electrons).
- Elements in the same Group share the same number of valence electrons, which is why they have similar chemical properties and reactions.
- The electron configuration can be deduced from this information, e.g., an element in Period 3, Group 17 has 3 shells, with 7 electrons in the outer shell, giving a configuration of 2.8.7.
Diagram
Definitions
- Period
- A horizontal row in the Periodic Table. The Period number corresponds to the principal energy level (shell) of the outermost electrons.
- Group
- A vertical column in the Periodic Table. Elements within a Group have the same number of valence electrons and thus similar chemical properties.
- Valence Electrons
- The electrons located in the outermost occupied electron shell of an atom. These are the electrons involved in chemical bonding.
Worked example
Element Q is in Period 3 and Group 2 of the Periodic Table. Element R is in Period 2 and Group 16. What is the chemical formula of the compound formed when Q and R react?
- 1
Step 1:
Determine the ionic charge of Q.
It is in Group 2, so it has 2 valence electrons.
It will lose these 2 electrons to form a stable ion with a +2 charge (Q2+).
- 2
Step 2:
Determine the ionic charge of R.
It is in Group 16, so it has 16 - 10 = 6 valence electrons.
It will gain 2 electrons to complete its outer shell, forming a stable ion with a -2 charge (R2-).
- 3
Step 3:
Combine the ions to form a neutral compound.
A Q2+ ion will combine with one R2- ion to balance the charges (+2 + (-2) = 0).
- 4
Step 4:
Write the final formula.
The ratio of Q to R is 1:1, so the chemical formula is QR.
Answer: QR
Common mistakes
- ×For Groups 13-18, incorrectly using the Group number itself as the number of valence electrons instead of subtracting 10.
- ×Confusing the number of valence electrons with the charge of the ion formed. For example, a Group 16 element has 6 valence electrons but forms a -2 ion, not a +6 or -6 ion.
- ×Misidentifying the Period number as the number of valence electrons, or the Group number as the number of shells.
No-calculator tips
- ✓Quickly determine ionic charge: Groups 1, 2, 13 form positive ions (+1, +2, +3). Groups 15, 16, 17 form negative ions (-3, -2, -1).
- ✓To find the atomic number of an element up to Period 3, sum the electrons in the shells you deduce. For Period 3, Group 14: it's 2 (shell 1) + 8 (shell 2) + 4 (shell 3) = 14. So the element is Silicon.
- ✓When forming compounds, use the 'cross-over' method for charges in your head. The magnitude of the charge on one ion becomes the subscript for the other ion (e.g., for Al3+ and O2-, the formula is Al2O3).