Periods, Groups, and Electron Configuration
Quick Notes
- Period number = number of occupied electron shells (energy levels).
- Group number = number of valence electrons (for main group elements).
- Elements in the same group show similar chemical behaviour.
- The electron configuration of an element (up to Z = 36) can be:
- Deduced from its position on the periodic table.
- Used to determine its group and period.
- Know the following classifications:
- Group 1 – Alkali metals
- Group 17 – Halogens
- Group 18 – Noble gases
- Groups 3–12 – Transition elements
Full Notes:
Period Number and Electron Shells
The period number (1 to 7) of an element in the periodic table tells you the highest occupied principal energy level (n) in its atomic structure.
- Example Sodium (Na), Z = 11
1s2 2s2 2p6 3s1 → Period 3
- Example Chlorine (Cl), Z = 17
1s2 2s2 2p6 3s2 3p5 → Period 3
Group Number and Valence Electrons
For main group elements (s- and p-block):
- Group number = number of valence electrons
- Example Group 1 → 1 valence electron (e.g., Li = 1s2 2s1)
- Example Group 16 → 6 valence electrons (e.g., O = 1s2 2s2 2p4)
Transition metals (d-block) show more complex behaviour due to variable oxidation states and partially filled d-orbitals (see here).
Classifications to Know
| Group or Category | Key Features |
|---|---|
| Alkali metals (Group 1) | Very reactive metals, 1 valence electron |
| Halogens (Group 17) | Very reactive non-metals, 7 valence electrons |
| Noble gases (Group 18) | Inert gases, full outer shell, 8 valence electrons (except He) |
| Transition metals (Groups 3–12) | Variable oxidation states, form coloured compounds |
Deducing Electron Configurations (up to Z = 36)
To write electron configurations for elements up to atomic number 36, use the Aufbau principle, which states that electrons fill the lowest available energy orbitals first.
The periodic table is a helpful tool for determining the order of filling based on both period number and block.
Use the periodic table to identify the type and energy level of orbitals:
- s-block (Groups 1–2): Fill the ns orbitals, where n = period number
Example Sodium (Na, Period 3) ends in 3s1 - p-block (Groups 13–18): Fill the np orbitals, where n = period number
Example Chlorine (Cl, Period 3) ends in 3p5 - d-block (Transition metals, Groups 3–12): Fill the (n–1)d orbitals, where n = period number
Example Iron (Fe, Period 4) fills the 3d orbitals
d-block orbitals "lag behind" the period. In Period 4, for example, you fill 3d after 4s.
Examples:
- Calcium (Z = 20)
Full: 1s2 2s2 2p6 3s2 3p6 4s2
Noble gas shorthand: [Ar] 4s2 - Iron (Z = 26)
Full: 1s2 2s2 2p6 3s2 3p6 4s2 3d6
Noble gas shorthand: [Ar] 4s2 3d6 - Bromine (Z = 35)
Full: 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p5
Noble gas shorthand: [Ar] 4s2 3d10 4p5
Using Noble Gas Shorthand Notation
Remember to simplify longer configurations, replace the inner electron configuration with the previous noble gas in brackets. This highlights the valence shell electrons, which are most important in chemical behaviour.
From Configuration to Position
You can work backwards using an electronic configuration to find an element’s position in the periodic table:
- The highest energy level tells you the period the element is in
- The number of electrons in the outermost shell tells you the group the element is in
- If there is a partially filled d-sublevel it is likely a transition metal
Linked Course Questions
How has the organization of elements in the periodic table facilitated the discovery of new elements?
The periodic table arranges elements by atomic number and groups them by similar chemical properties, revealing repeating patterns known as periodicity. This structure helped scientists predict the existence and properties of unknown elements.
Example Mendeleev’s predictions
Dmitri Mendeleev left gaps in his early periodic table for elements that hadn’t been discovered yet. He correctly predicted the properties of gallium and germanium based on trends in the table – and they were discovered later with the properties he predicted.
Today, the periodic table continues to guide the search for new synthetic elements, like tennessine (Ts), by showing where they should appear.
Summary
- The period number corresponds to the highest occupied electron shell.
- The group number corresponds to valence electrons for s- and p-block elements.
- Transition metals have complex configurations with variable oxidation states.
- Electron configurations can be deduced from position and used to classify elements.
- The periodic table arrangement enabled the prediction and discovery of new elements.