Periods, Groups, and Blocks in the Periodic Table
The Structure of the Periodic Table: Periods, Groups, and Blocks
The periodic table is divided into periods, groups, and blocks, each serving a specific role in organizing elements based on their electronic structure and properties.
Periods: Horizontal Rows
- The horizontal rows of the periodic table are called periods.
- Each period corresponds to the principal quantum number $n$, which represents the highest energy level occupied by electrons in an atom of an element in that row.
- As you move across a period, the number of protons in the nucleus increases.
- This stronger nuclear attraction pulls the electrons closer, leading to trends like decreasing atomic radius and increasing ionization energy.
In period 2, elements like lithium ($n = 2$) and oxygen ($n = 2$) have their valence electrons in the second energy level.
Tip- Remember, the period number indicates the outermost energy level of an element’s electrons.
- For example, elements in period 3 have valence electrons in the $n = 3$ energy level.
Groups: Vertical Columns
- The vertical columns of the periodic table are called groups.
- Elements in the same group have the same number of valence electrons, which explains their similar chemical properties.
- Group Numbers:
- Groups are numbered from 1 to 18.
- For groups 1 and 2, the group number equals the number of valence electrons.
- For groups 13–18, the last digit of the group number corresponds to the number of valence electrons.
- Chemical Similarity:
- Elements within a group exhibit similar reactivity.
- For instance, all alkali metals react vigorously with water to produce hydrogen gas and a basic solution.
Group 1 elements (alkali metals) have 1 valence electron, while group 17 elements (halogens) have 7 valence electrons.
Example- Consider phosphorus (P), located in group 15 and period 3.
- Its group indicates it has 5 valence electrons, while its period tells us these electrons are in the $n = 3$ energy level.
- From this, we can predict its electron configuration: $[Ne] 3s^2 3p^3$.
Blocks: s, p, d, and f
The periodic table is also divided into blocks, which correspond to the type of atomic orbital being filled with electrons.
- s-block: Includes groups 1 and 2 (alkali and alkaline earth metals). The outermost electrons occupy the s sublevel.
- p-block: Includes groups 13 to 18 (non-metals, metalloids, and some metals). The outermost electrons occupy the p sublevel.
- d-block: Contains the transition metals (groups 3 to 12). The outermost electrons occupy the d sublevel.
- f-block: Comprises the lanthanides and actinides (the two rows at the bottom). The outermost electrons occupy the f sublevel.
