Question

Describe the changes in atomic size and ionization energy across a period and down a group.

Short answer

Atomic size decreases across a period and increases down a group; ionization energy increases across a period and decreases down a group.

Step-by-step solution

Atomic Size Across a Period

As you move from left to right across a period in the periodic table, the atomic size decreases. This is because the number of protons in the nucleus increases, which results in a greater positive charge. The increased effective nuclear charge pulls the electrons closer to the nucleus, resulting in a smaller atomic radius.

Atomic Size Down a Group

When you move down a group in the periodic table, the atomic size increases. This increase is because each element adds a new electron shell, making the atom larger. The added shell outweighs the increase in nuclear charge, thus increasing the size.

Ionization Energy Across a Period

Ionization energy increases as you move from left to right across a period. This occurs because atoms are smaller, more compact, and the effective nuclear charge is stronger, holding the electrons more tightly. More energy is required to remove an electron.

Ionization Energy Down a Group

As you move down a group, ionization energy decreases. Although the nuclear charge is higher, the electrons are further from the nucleus and experience more shielding from inner electron shells. Thus, it takes less energy to remove the outermost electron.

Key concepts

Atomic Size

Atomic size essentially refers to how large an atom's radius is. It's influenced by the number of electron shells and the attraction between electrons and the nucleus. One key aspect to remember is that as you move across a period on the periodic table—from left to right—the atomic size decreases.
This is due to the increasing number of protons. A higher proton count enhances the nucleus's positive charge, which pulls electrons closer. Consequently, the atom's radius shrinks.
However, moving down a group on the periodic table introduces a different trend. Atomic size increases as each new element in the group contains an additional electron shell. This effectively makes the atom larger, despite the increasing nuclear charge, because the inner shells create a shielding effect and reduce the pull on the outermost electrons.

Ionization Energy

Ionization energy is the energy required to remove an electron from an atom. When it comes to changes across the periods and groups, there are some clear patterns to remember. Moving from left to right across a period, the ionization energy generally increases.
This is because atoms become smaller and more tightly packed with electrons being more strongly attracted to an increasingly positive nucleus. Therefore, more energy is needed to remove an electron.
Yet, as you descend down a group in the periodic table, the ionization energy tends to decrease. As atoms have additional electron shells, the outer electrons are further away from the nucleus and are better shielded by these inner layers.

• This reduces the effective pull of the nucleus.
• It makes it easier to remove electrons.

Periodic Table

The periodic table is a crucial tool in understanding electron configurations, atomic size, and ionization energy. It organizes elements in a manner where similar properties recur at intervals.
Here's how you can view the table in terms of trends:

• Periods are horizontal rows where moving left to right shows a decrease in atomic radius and an increase in ionization energy.
• Groups are vertical columns where moving down results in an increase in atomic size and generally a decrease in ionization energy.

This systematic organization not only helps in anticipating chemical behaviors of elements but also reveals patterns that are essential for predicting trends like metallic or non-metallic character.

Nuclear Charge

Nuclear charge is the total charge of the protons in the nucleus. It's a crucial component in understanding why elements behave the way they do on the periodic table.
The effective nuclear charge, often abbreviated as Z*, accounts for the actual influence the electrons experience. It becomes increasingly evident as you move across a period, where the nuclear charge is stronger due to the addition of protons without added shielding.

• Consequently, atomic size shrinks.
• Ionization energy, too, will increase.

In contrast, moving downward through a group, despite an increase in nuclear charge, the effect is less noticeable. Additional electron shells come into play that dampens the pull of the nucleus, due to increased electron shielding.