Question

a. What is meant by atomic radius? b. What trend is observed among the atomic radii of main-group elements across a period? c. Explain this trend.

Short answer

a. Atomic radius is the distance from the nucleus to the outer electron shell. b. It decreases across a period. c. This is due to increasing nuclear charge pulling electrons closer.

Step-by-step solution

Define Atomic Radius

The atomic radius is the distance from the center of an atom's nucleus to the outermost shell of electrons. It gives an idea of the size of an atom.

Identify Trend Across a Period

Among the main-group elements, the atomic radius generally decreases as you move from left to right across a period in the periodic table.

Explain the Trend

As you move across a period, the number of protons in the nucleus increases, causing a greater positive charge. This increased nuclear charge pulls the electrons closer to the nucleus, reducing the atomic radius. Additionally, electrons are added to the same energy level, which does not significantly increase shielding.

Key concepts

periodic table

The periodic table is a chart that organizes all known elements by increasing atomic number. Elements are arranged in rows called periods and columns known as groups. This organization helps illustrate periodic trends, like atomic radius, electron affinity, and ionization energy. The periodic table's layout ensures that elements with similar properties fall into the same column or group.
Understanding the periodic table is crucial for studying chemical behavior, as the position of an element provides insights into its atomic structure and reactivity.

atomic size

Atomic size refers to how large an atom is, typically measured by its atomic radius. The atomic radius is the distance from the center of an atom's nucleus to its outermost electron shell. This size can vary among different elements, depending on their position on the periodic table.
As you move from left to right across a period on the periodic table, the atomic size decreases. This trend occurs because increasing the number of protons in the nucleus (nuclear charge) pulls the electrons closer, reducing the size. Conversely, moving down a group increases atomic size since additional electron shells are added, pushing outer electrons farther from the nucleus.

atomic structure

Atomic structure refers to the arrangement of protons, neutrons, and electrons within an atom. The nucleus, holding protons and neutrons, makes up the atom's dense center, while electrons orbit in shells around the nucleus. Each element's properties are defined by its atomic structure.
Protons in the nucleus have a positive charge and attract the negatively charged electrons, which are organized into various energy levels or shells. The more protons in the nucleus, the stronger the attraction pulling the electrons inward, affecting the atomic radius. Understanding atomic structure is essential for explaining the behavior and interaction of atoms during chemical reactions.

nuclear charge

Nuclear charge is the total charge of all the protons in an atom's nucleus. This positive charge plays a significant role in determining the atom's size and electron arrangement. As you move across a period on the periodic table, the number of protons in the nucleus increases, thus increasing the nuclear charge.
With a higher nuclear charge, electrons are pulled closer to the nucleus, which decreases the atom's radius. Despite adding more electrons as you move across a period, these electrons do not significantly increase shielding. Therefore, the dominant effect is the increased nuclear pull, reducing the atomic size. Understanding nuclear charge and its impact on electrons is key to explaining the periodic trends observed in atomic radii.