Problem 92 - A Complete Resource Book in Chemistry for JEE Main

Question

The electronegativity of the following elements increases in the order: (a) Si, P, C, N (b) \(\mathrm{N}, \mathrm{Si}, \mathrm{C}, \mathrm{P}\) (c) \(\mathrm{P}, \mathrm{Si}, \mathrm{N}, \mathrm{C}\) (d) \(\mathrm{C}, \mathrm{N}, \mathrm{Si}, \mathrm{P}\)

Step-by-Step Solution

Answer

The correct order is (a) Si, P, C, N.

Step 1: Understand Electronegativity

Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons. The most commonly used scale of electronegativity is the Pauling scale.

Step 2: Recall the General Trend

In general, electronegativity increases across a period (from left to right on the periodic table) and decreases down a group (from top to bottom). Therefore, elements in the same period such as C (carbon) and N (nitrogen) will have increasing electronegativity as you move towards the right.

Step 3: Arrange Elements According to Periodic Table

Find the position of the given elements on the periodic table: Si (Silicon), P (Phosphorus), C (Carbon), and N (Nitrogen). Silicon and phosphorus are in the same period (3rd), while carbon and nitrogen are in another (2nd). Identifying elements' periods helps determine relative electronegativities.

Step 4: Order the Elements by Electronegativity

Based on their positions on the periodic table, we know: 1. Electronegativity order in the second period is C < N. 2. Electronegativity order in the third period is Si < P. To compare elements from different periods, note that elements in the second period generally have higher electronegativities than those in the third.

Step 5: Identify the Correct Order

We can place the elements in increasing order of electronegativity: Si, P, C, N. This gives us the correct and logical order based on periodic trends.

Key Concepts

Periodic table trends

Understanding periodic table trends is key to mastering electronegativity concepts. The periodic table is organized into periods (horizontal rows) and groups (vertical columns).

Electronegativity shows a clear trend across the table:

• From left to right across a period, electronegativity increases. This is because as atoms have more protons, they pull electrons more strongly.
• From top to bottom down a group, electronegativity decreases. Larger atoms have more electron shells, which shield the nucleus and reduce its pull on outer electrons.

These trends help predict and compare the reactivity and chemical behavior of different elements. Always remember, these are general trends and some exceptions can occur due to unique atomic structures of certain elements.

Pauling scale

The Pauling scale is the most widely recognized method for quantifying electronegativity. Developed by chemist Linus Pauling, this scale assigns values to atoms that reflect their tendency to attract electrons.

Here is how it works:

• Each element is assigned a value on the scale, typically ranging from about 0.7 to 4.0.
• Higher values indicate a greater ability to attract electrons.
• For instance, fluorine has a value of 4.0, making it the most electronegative element, while cesium and francium are among the least electronegative without covalent bonds.

Since the Pauling scale provides a comparative measure, it helps chemists easily determine the relative electronegativity of elements when discussing chemical reactions and bond formation.

Electronegativity in periods

Electronegativity patterns within a period are influenced by atomic structure. As you move from left to right across a period in the periodic table, the electronegativity increases.

Here's why:

• Atoms gain more protons as you move across, which increases the positive charge of the nucleus.
• The increased nuclear charge attracts the electron cloud more strongly, making the atom more electronegative.

For example, in the second period of the periodic table, carbon (4C) and nitrogen (7N) demonstrate this trend with C being less electronegative than N. Recognizing such patterns helps you predict how an element might react chemically.

Electronegativity in groups

When exploring electronegativity down a group, the trend is: it decreases as you move downwards. This behavior is due to increasing atomic size and electron shielding.

Consider the following:

• As new electron shells are added, the atomic radius increases.
• This increased distance between the nucleus and the outermost electrons reduces the nucleus's pull on those electrons.
• The added inner shells of electrons also act as a shield, diminishing the nucleus's ability to attract electrons.

Taking elements from the third period, like silicon (8Si) and phosphorus (1P), Si is less electronegative than P. Understanding these group trends is essential when predicting the reactivity and properties of elements in various chemical environments.