Question

Of the elements magnesium, calcium, and barium, which forms the ion with the largest radius? The smallest? What periodic trend explains this?

Short answer

Barium forms the ion with the largest radius (Ba^2+), while magnesium forms the ion with the smallest radius (Mg^2+). The periodic trend responsible for this observation is the increase of atomic and ionic radii down a group due to the addition of new electron shells and a weaker attractive force between outermost electrons and the nucleus. This results in the ionic radii increasing down the group: Mg^2+ (smallest) < Ca^2+ < Ba^2+ (largest).

Step-by-step solution

Identify the group of the given elements

Magnesium (Mg), calcium (Ca), and barium (Ba) are elements in Group 2 (alkaline earth metals) in the periodic table.

Determine the charges of the ions formed by these elements

Elements in Group 2 form ions with a charge of +2. This happens because they lose two electrons from their outermost electron shell to achieve a stable electron configuration.

Consider the periodic trend of atomic radii

As we move down a group in the periodic table, the atomic radius increases. This happens because a new electron shell is added to the atom at each period, increasing the distance between the nucleus and the outermost electrons. The shielding effect from inner electron shells also weakens the attractive force between the nucleus and outermost electrons.

Determine the trends of ionic radii

When forming ions, the ionic radius also generally follows the same trend. When we move down a group, the ionic radius increases due to the addition of a new electron shell. For the given elements, the order of ionic radii will be: Mg^2+ < Ca^2+ < Ba^2+

Identify the ion with the largest and smallest radius

Since barium (Ba) is lowest in the group, it forms the ion with the largest radius (Ba^2+). Magnesium (Mg), being the highest in the group, forms the ion with the smallest radius (Mg^2+).

Explain the periodic trend

The periodic trend responsible for these observations is the increase of atomic and ionic radii down a group due to the addition of new electron shells and a weaker attractive force between outermost electrons and the nucleus. This causes the ionic radii to increase down the group: Mg^2+ (smallest) < Ca^2+ < Ba^2+ (largest).

Key concepts

Group 2 Elements

Group 2 elements, also known as alkaline earth metals, are found in the second column of the periodic table. These elements include beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). One key characteristic of group 2 elements is that they all have two electrons in their outer electron shell.
This electronic configuration makes them quite reactive, although not as reactive as group 1 elements, the alkali metals. To attain a stable electronic state, they readily lose these two outermost electrons.

• This results in the formation of ions with a +2 charge.
• The general electron configuration for these elements is \(\text{[Noble gas]} ~ \text{ns}^2\).
• They are all shiny, silvery-white metals at room temperature.

These elements have several uses in everyday applications including the construction of buildings, the manufacturing of fireworks, and various medical purposes. Understanding their position in the periodic table helps predict their reactivity and properties.

Alkaline Earth Metals

The term "alkaline earth metals" refers to group 2 elements on the periodic table. The name comes from their oxides, which are earth-like substances that have a basic or alkaline nature. These metals are known for certain distinct properties:

• They are relatively soft metals but harder than the alkali metals of group 1.
• They have higher melting points and boiling points compared to group 1 metals.
• They are less reactive than alkali metals, but still react readily with water and acids, especially when heated.

Alkaline earth metals form ionic compounds in which they exhibit a consistent oxidation state of +2. This is due to the loss of the two electrons in their outer shell. As you move down the group from beryllium to barium, reactivity increases due to the decrease in ionization energy, which makes it easier for these metals to lose electrons.

Ionic Radius

The ionic radius is a measure of an ion's size, which is particularly important in understanding the behavior of ionic compounds. For group 2 elements, this radius changes predictably across the periodic table. When these elements lose their two outermost electrons, they become positively charged ions with a +2 charge.

• As you move down the group from magnesium to barium, the ionic radius increases.
• This is primarily due to the addition of new electron shells, which increases the distance between the nucleus and the outermost electrons.
• The shielding effect from inner electron shells also plays a role, reducing the effective nuclear charge felt by the outermost electrons, allowing them to drift further apart.

Consequently, this trend explains why in the order of ionic radii, \(\text{Mg}^{2+} < \text{Ca}^{2+} < \text{Ba}^{2+}\), with barium's ion having the largest radius. Awareness of ionic radius trends is crucial for predicting the behavior and characteristics of ions in chemical reactions.