Question

Use the second period of the periodic table as an example to show that the size of atoms decreases as we move from left to right. Explain the trend.

Short answer

Atomic size decreases across the second period due to increased nuclear charge without additional shielding.

Step-by-step solution

Understand Atomic Size

Atomic size refers to the distance from the nucleus to the outermost electron shell. It is usually measured in terms of atomic or ionic radii.

Identify Elements of the Second Period

The second period of the periodic table includes the elements: Lithium (Li), Beryllium (Be), Boron (B), Carbon (C), Nitrogen (N), Oxygen (O), Fluorine (F), and Neon (Ne).

Observe the Trend in Atomic Size

As we move across the second period from left to right, we observe that the atomic size decreases. For example, the atomic radius of Lithium is larger than that of Beryllium, which is larger than Boron's, and so on until Neon, which has the smallest atomic radius among them.

Explain the Trend

The decrease in atomic size across a period is due to an increase in the number of protons (positive charge) in the nucleus, which results in a stronger attraction between the nucleus and the electrons. Even though electrons are being added as you move across the period, they are added to the same electron shell and are not effective at shielding each other from the increased nuclear charge, causing the atomic radius to decrease.

Key concepts

Periodic Table

The periodic table is a powerful tool for understanding the properties of elements and their relationships with each other. It is organized in a way that allows us to see trends and patterns in the behavior of different elements. Elements are arranged in order of increasing atomic number, which is the number of protons in the nucleus of an atom.
The table is divided into periods (rows) and groups (columns). As you move across a period from left to right, elements change in predictable ways. In the case of the second period, this includes elements from lithium (Li) to neon (Ne).
Each element in a period has an additional proton and electron compared to the previous one, which affects their atomic size and other properties. This systematic arrangement helps in predicting the chemical behavior of elements and their compounds.

Atomic Radius

The atomic radius is a measure of the size of an atom, specifically the distance from the nucleus to the outermost boundary of the surrounding cloud of electrons.
It is often expressed in picometers (pm) or angstroms (Å), where 1 angstrom is equal to 100 picometers. An atom’s radius determines how it interacts with other atoms, because it influences the space it occupies.

• As you move from left to right across a period on the periodic table, the atomic radius tends to decrease.
• This shrinking effect is due to the increase in the positive charge of the nucleus as protons are added, which pulls the electron cloud closer.

Understanding the atomic radius is crucial for explaining why elements behave differently and form different types of bonds.

Nuclear Charge

Nuclear charge refers to the total charge of the protons in an atom's nucleus, which directly impacts an atom’s size and its electron configuration.
As you move across a period in the periodic table, each element has a higher nuclear charge than the element preceding it. This is because each successive element has an additional proton.

• The increasing nuclear charge pulls electrons closer to the nucleus, reducing the atomic radius despite the addition of more electrons.
• The electrons added in the same shell do not efficiently shield each other from the increased nuclear charge, leading to a smaller atomic size.

Thus, understanding nuclear charge helps explain why ions and elements interact in specific ways.

Second Period Elements

The second period elements on the periodic table, consisting of lithium (Li), beryllium (Be), boron (B), carbon (C), nitrogen (N), oxygen (O), fluorine (F), and neon (Ne), provide an excellent example of atomic size trends.
Across this period, each element has one more proton and one more electron than the previous element. These additional electrons enter the same outer electron shell.

• Despite the increase in electrons, the nuclear charge becomes stronger, pulling the electrons closer and decreasing the atomic radius.
• Neon, by the end of the second period, has the smallest atomic radius, being fully satisfied with a complete electron shell and thus maximizing the compact energy configuration.

Using the second period as a model, you can see a clear demonstration of the atomic size trend where the atomic radius decreases as you move from left to right.