Question

Describe the vertical and horizontal trends in electronegativity (EN) among the main-group elements. According to Pauling's scale, what are the two most electronegative elements? The two least electronegative elements?

Short answer

EN increases across a period and decreases down a group. The most electronegative elements are F and O, and the least are Cs and Fr.

Step-by-step solution

- Define Electronegativity

Electronegativity (EN) is a measure of the tendency of an atom to attract a bonding pair of electrons. The higher the value, the more an element attracts electrons.

- Horizontal Trends in Electronegativity

As you move from left to right across a period in the periodic table, the electronegativity increases. This is because the number of charges on the nucleus increases, attracting the bonding pair of electrons more strongly.

- Vertical Trends in Electronegativity

As you move down a group in the periodic table, the electronegativity decreases. This is because larger atoms have more electron shells, making the bonding electrons farther from the nucleus and thus less strongly attracted.

- Identifying the Most Electronegative Elements

According to Pauling's scale, fluorine (F) and oxygen (O) are the two most electronegative elements. Fluorine has an electronegativity of 3.98, and oxygen has 3.44.

- Identifying the Least Electronegative Elements

The two least electronegative elements, according to Pauling's scale, are cesium (Cs) and francium (Fr). Cesium has an electronegativity of 0.79, and francium has about 0.70.

Key concepts

Electronegativity Definition

Electronegativity (EN) is a fundamental concept in chemistry. It measures an atom's ability to attract and hold onto a bonding pair of electrons.
When two atoms bond, their electrons are not always shared equally. Electronegativity is a way to predict which atom will more likely pull the electrons closer.
The higher an element's electronegativity value, the stronger its attraction for electrons in a chemical bond.
For instance, in a water molecule (H₂O), oxygen has a higher electronegativity than hydrogen; hence, electrons are drawn more towards oxygen.

Periodic Table Trends

Electronegativity shows clear trends in the periodic table, both horizontally and vertically.
Let's start with horizontal trends. As you move from left to right across a period, electronegativity increases.
This is because atoms gain more protons in their nucleus, increasing the positive charge.
This stronger positive charge pulls the bonding electrons closer, increasing electronegativity.
For example, in Period 2, lithium (Li) has a lower electronegativity than fluorine (F).

Now, let's look at the vertical trends. As you move down a group, electronegativity decreases.
This happens because atoms have more electron shells as they go down a group.
The bonding electrons are further away from the nucleus and experience less pull.
For instance, in Group 17, fluorine (F) is more electronegative than iodine (I).

Pauling Scale

The Pauling scale is the most commonly used scale for measuring electronegativity.
Named after the scientist Linus Pauling, this scale quantifies electronegativity on a relative basis.
Pauling assigned dimensionless numbers to elements, starting at 0.7 for cesium (Cs) and francium (Fr), the least electronegative, and going up to 3.98 for fluorine (F), the most electronegative element.
Fluorine's high electronegativity makes it extremely effective at attracting electrons.

• The Pauling scale helps chemists predict bond types and chemical reactions.
• It provides insight into molecule behavior.

The scale consolidates complex atomic interactions into a simple numerical format, providing an easy way to compare elements.
For example, oxygen has a Pauling electronegativity of 3.44, making it the second most electronegative element.