4.4.6 Position of Unknown Element in the Periodic Table

This topic explains how to determine the position of an unknown element in the periodic table using its electronic configuration and periodic trends.

Definition

You can place an unknown element accurately at the appropriate position in the periodic table, and can predict its properties.

Simple explanation: If you know an element's electronic configuration, you can figure out where it belongs in the periodic table.

Main Idea

The electronic configuration of an element strongly influences its chemical behaviour. Elements within the same group have similar electronic configurations, and therefore similar properties.

By examining the electronic configuration of an unknown element and comparing it to the known elements in the periodic table, its likely position in the periodic table can be determined.

• atomic size
• ionization energy
• electron affinity
• electronegativity
• reactivity
• other properties

By analyzing these trends, it becomes possible to estimate the general properties of an unknown element and make a guess about its position in the periodic table.

Core Concept

Steps to Locate an Unknown Element

• Write its electronic configuration.
• Use valence electronic configuration to locate its position.
• Find its group and period.

Simple explanation: Electronic configuration → group and period → position in periodic table.

Rules for Finding Group and Period

From the book (and previous sections):

Detailed Explanation

Example 4.7 (from the book): Identifying Position of an Unknown Element

Problem: Suppose you have an unknown element having atomic number 19, and you want to determine its position in the periodic table.

• Write its electronic configuration.
• Use valence electronic configuration to locate its position — find its group and period.

Solution:

Electronic configuration: 1s², 2s², 2p⁶, 3s², 3p⁶, 4s¹

• The valence electron is in the 4s sub-shell.
• So the element belongs to the s-block.

• The principal quantum number of the valence electron is n = 4.
• So the element is in the 4th period.

• For s-block elements, group number = number of valence electrons.
• Number of valence electrons = 1.
• So the element belongs to Group 1.

Conclusion: The element with atomic number 19 is potassium (K). It is in Group 1, Period 4, s-block.

Worked Example: Another Unknown Element

Problem: An unknown element has the electronic configuration 1s², 2s², 2p⁶, 3s², 3p³. Find its position in the periodic table.

Solution:

• Valence electron is in the 3p sub-shell.
• So the element belongs to the p-block.

• The highest principal quantum number is n = 3.
• So the element is in the 3rd period.

• For p-block elements, group number = number of valence electrons + 10.
• Valence electrons = 2 (from 3s²) + 3 (from 3p³) = 5.
• Group number = 5 + 10 = **15**.

Conclusion: The element is in Group 15, Period 3, p-block. This corresponds to phosphorus (P).

Using Periodic Trends to Predict Properties

Once you know the position of an unknown element, you can predict its properties using periodic trends:

Intermediate Level

Using Electronic Configuration to Determine Position

Why This Works

• The valence sub-shell determines the block (s, p, d, f).
• The highest n value determines the period (row).
• The number of valence electrons determines the group (column).
• Elements in the same group have similar valence configurations, so they have similar chemical properties.

Predicting Properties from Position

Once you know the position, you can predict:

• It is an alkali metal.
• It is highly reactive.
• It forms a +1 ion.
• It reacts with water to form KOH + H₂.
• It is soft and has a low melting point.

• It is a halogen.
• It is a reactive non-metal.
• It forms a −1 ion.
• It is a gas at room temperature.
• It has high electronegativity.

Important Points

• - Electronic configuration determines the position of an element in the periodic table.
• - Valence sub-shell → block (s, p, d, f).
• - Highest n value → period.
• - Number of valence electrons → group (for s-block and p-block).
• - For s-block: group number = number of valence electrons.
• - For p-block: group number = number of valence electrons + 10.
• - Example: Atomic number 19 (K): 1s², 2s², 2p⁶, 3s², 3p⁶, 4s¹ → s