Problem 23 - Chemistry: The Central Science

Question

How do the sizes of atoms change as we move (a) from left to right across a row in the periodic table, (b) from top to bottom in a group in the periodic table? (c) Arrange the following atoms in order of increasing atomic radius: \(\mathrm{F}, \mathrm{P}, \mathrm{S}\), As.

Step-by-Step Solution

Answer

The atomic radius decreases as we move from left to right across a row in the periodic table due to increased positive charge in the nucleus. In contrast, the atomic radius increases down a group in the periodic table due to the addition of new electron shells. Based on these trends, the order of increasing atomic radius for the given atoms is: \(\mathrm{F} < \mathrm{S} < \mathrm{P} <\) As.

Step 1: (a) Change in atom sizes across a row in the periodic table)

As we move from left to right across a row in the periodic table, the atomic number increases, and elements acquire more protons and electrons. However, electrons are added to the same energy level or shell. This leads to an increased positive charge of the nucleus, which pulls the electrons closer to the nucleus and, as a result, the atomic size or atomic radius decreases. Due to the increased positive charge, the electrons experience a greater electrostatic attraction force. This trend is known as the horizontal trend in atomic size.

Step 2: (b) Change in atom sizes down a group in the periodic table)

As we move from top to bottom in a group, the atomic number increases. The elements acquire more protons, electrons, and energy levels (shells). As the atomic number increases, the electrons are added to new energy levels, which are more distant from the nucleus. Consequently, the atomic size or atomic radius increases going down a group due to the addition of new electron shells. This trend is known as the vertical trend in atomic size.

Step 3: (c) Arrange the given atoms in order of increasing atomic radius)

To accomplish this task, we need to understand the positions of the given elements, \(\mathrm{F}, \mathrm{P}, \mathrm{S}\), and As, in the periodic table: 1. \(\mathrm{F}\) (Fluorine) is in Group 17 and Period 2 2. \(\mathrm{P}\) (Phosphorus) is in Group 15 and Period 3 3. \(\mathrm{S}\) (Sulfur) is in Group 16 and Period 3 4. As (Arsenic) is in Group 15 and Period 4 Considering the horizontal trend (a), we know the atomic size decreases as we move from left to right. As for the vertical trend (b), we know that the atomic size increases as we move down a group. By combining these trends, we can arrange these atoms in the order of increasing atomic radius: Fluorine (\(\mathrm{F}\)) < Sulfur (\(\mathrm{S}\)) < Phosphorus (\(\mathrm{P}\)) < Arsenic (As)

Key Concepts

Periodic Table Trends

When studying the periodic table, you'll notice that each row and column reveals specific patterns known as periodic table trends. One key trend is the change in atomic size as you move across a row or down a column.
Across a row, or period, atomic size tends to decrease. This occurs because as you move from left to right, each element gains an extra proton and electron. Even though electrons are added, they enter the same energy level. The increased number of protons results in a stronger positive charge at the nucleus. This stronger nucleus pulls electrons closer, leading to a smaller atomic size.
On the other hand, moving down a column, or group, shows an increase in atomic size. As you progress down a group, atoms have more energy levels because more electrons need to be accommodated. Each additional level is farther from the nucleus, making the atoms larger. Understanding these trends helps in predicting how elements behave and interact.

Atomic Size

The atomic size of an element is determined by the space occupied by its electrons. A common way to express atomic size is through the atomic radius, which is half the distance between the nuclei of two atoms of the same element when they are bonded together.
As you might guess, the size of electrons plays a significant role in determining atomic size. The number of electrons and how they arrange around the nucleus affect how much space an atom covers. An increase in protons without corresponding electrons being pushed to a higher energy level usually leads to a smaller radius as in rows. However, adding electron layers increases the radius, hence the increase down a group.
Understanding changes in atomic size is crucial for predicting the chemical and physical properties of elements. For example, elements with larger atomic sizes tend to be more reactive in certain conditions due to the less tightly bound outer electrons.

Electron Shells

Electron shells, or energy levels, are where electrons reside around the nucleus of an atom. These shells are crucial in determining an element's properties and its position on the periodic table.
Each atom can have several shells, and these shells are filled as the atomic number increases. The structure of these shells and their capacity to hold electrons impact the element's chemical behavior. The outermost shell, known as the valence shell, is especially important as it influences how the element bonds with others.
As you go across a period, electrons are added to the same shell, strengthening the nuclear pull due to the additional protons. Conversely, moving down a group introduces new shells, increasing the atomic size. Understanding electron shell arrangement is essential for interpreting and predicting how different elements will interact, bond, and form compounds.