Question

a. What trend is observed among the atomic radii of main-group elements down a group? b. Explain this trend.

Short answer

The atomic radius increases down a group due to the addition of electron shells, which increases the distance from the nucleus.

Step-by-step solution

- Identify the trend

Observe the trend in atomic radii as you move down a group in the periodic table. Notice that the atomic radius increases as you move from top to bottom within the same group.

- Provide an example

Consider Group 1 elements: Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), and Cesium (Cs). As you go down the group from Li to Cs, the atomic radius increases.

- Explain the trend

The increase in atomic radius down a group is due to the addition of electron shells (energy levels). With each subsequent element down the group, an extra electron shell is added. This increase in distance from the nucleus weakens the effective nuclear charge experienced by the valence electrons, allowing them to move farther away from the nucleus.

- Summarize

In summary, as you move down a group in the periodic table, the atomic radius increases due to the addition of more electron shells, which results in a greater distance between the nucleus and the outermost electrons.

Key concepts

Periodic Table Trends

When examining the periodic table, you might notice certain patterns, known as periodic trends. One key trend among main-group elements is the change in atomic radius as you move down a group.
More specifically, the atomic radius increases from top to bottom within a group.
For example, looking at Group 1 elements such as Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), and Cesium (Cs), you'll see that the atomic radius becomes larger with each step down the group.

• Lithium (Li): Smallest atomic radius
• Cesium (Cs): Largest atomic radius

This trend is consistent throughout other groups too. Understanding this pattern helps predict the properties and behaviors of different elements as you move through the periodic table.

Atomic Structure

To fully grasp why atomic radii change, it's important to understand atomic structure.
At the core of an atom lies the nucleus, containing protons and neutrons. Electrons orbit the nucleus in shells, which are energy levels.

• The number of protons (positive charge) in the nucleus defines the element and its atomic number.
• Electrons (negative charge) occupy electron shells around the nucleus.
• Neutrons are neutral particles also housed in the nucleus.

The balance of these particles determines the size and energy level of the atom. More electron shells mean larger atomic radii because the electrons are further away from the nucleus.
This principle is crucial when analyzing periodic trends in atomic radii.

Electron Shells

The concept of electron shells is significant in explaining the trend in atomic radii. Electron shells, also known as energy levels, are the paths that electrons follow around the nucleus.
Each shell can hold a certain number of electrons:

• The first shell holds up to 2 electrons.
• The second shell holds up to 8 electrons.
• The third shell holds up to 18 electrons.

As you move down a group in the periodic table, an extra electron shell is added for each element.
For example, in Group 1:

• Lithium (Li) has 2 shells.
• Sodium (Na) has 3 shells.
• Potassium (K) has 4 shells.

Each added shell increases the distance between the nucleus and the outermost electrons (valence electrons), which makes the atomic radius larger. Consequently, the effective nuclear charge felt by the electrons decreases because they are farther from the nucleus, thereby increasing the atomic radius down a group.