Question

Explain why the atomic size varies across the periodic table and down the periodic table.

Short answer

Atomic size decreases from left to right across a period due to increased effective nuclear charge, and increases from top to bottom down a group due to the addition of electron shells and electron shielding.

Step-by-step solution

Understanding Periodic Trends

Atomic size, or atomic radius, is the distance from the nucleus to the boundary of the surrounding cloud of electrons. Atomic size varies across the periodic table (horizontally) and down the periodic table (vertically) due to two principal factors: the number of electron shells (periods) and the effective nuclear charge (groups).

Variation Across the Periodic Table - Horizontal Trend

Moving from left to right across a period, the atomic number increases, meaning more protons are in the nucleus, and more electrons are added to the same electron shell. The increased nuclear charge causes electrons to be pulled closer to the nucleus, resulting in a decrease in atomic size. This is because the effective nuclear charge increases, leading to a stronger attraction between the nucleus and the electron cloud.

Variation Down the Periodic Table - Vertical Trend

Moving down a group in the periodic table, new electron shells are added, increasing the distance between the outermost electrons and the nucleus. Although the nuclear charge also increases, the effect is offset by the increased number of electron shells and electron shielding. This means the outer electrons are less tightly held, which results in an increase in atomic size down the groups.

Key concepts

Understanding the Periodic Table

The periodic table is a comprehensive chart that organizes all known elements by increasing atomic number, which is the number of protons in an atom's nucleus. It is structured in rows called periods and columns known as groups or families. The arrangement is not arbitrary; it reflects the elements’ recurring physical and chemical properties. Elements are ordered from left to right and top to bottom in order of increasing atomic number. This organization allows us to predict the behavior of elements and understand their relationships with one another. For example, elements in the same group typically display similar properties, including similarities in atomic size.

Effective Nuclear Charge

Effective nuclear charge (ENC) refers to the net positive charge experienced by electrons in the valence shell of an atom. It is not the full charge of the nucleus because the shielding effect of inner shell electrons reduces the full nuclear charge. ENC plays a crucial role in determining an atom's size. As you move across a period, the atomic number increases, hence more protons are in the nucleus, which in turn increases the ENC. This stronger ENC pulls electrons closer to the nucleus, generally leading to a smaller atomic size.

Electron Shielding

Electron shielding is the phenomenon where inner layers of electrons block the outer electrons from the full attractive force of the nucleus. The strength of the shielding effect increases with the number of electron shells an atom has. Electrons in these inner shells repel the valence electrons due to like-charge repulsion, lessening the hold the nucleus has on the valence electrons. In a vertical trend, as we move down a group and add more electron shells, the shielding effect becomes more significant, which results in an increase in atomic size despite the increase in nuclear charge.

Atomic Radius Trend

Atomic radius is the measure of the size of an atom. It is typically defined as half the distance between the nuclei of two atoms of the same element when the atoms are joined. Atomic radius decreases across a period from left to right, influenced mainly by the increasing effective nuclear charge. As the ENC increases, the pull on the valence electrons is stronger, making the atom smaller. Conversely, as you move down a group, atomic radius increases due to additional electron shells and the electron shielding effect. Each new shell adds a layer of electrons that increases the distance between the nucleus and the outermost electrons, leading to a larger atomic size.