Question

Which element in the periodic table has the smallest ionization energy? Which has the largest?

Short answer

Francium has the smallest ionization energy, and helium has the largest.

Step-by-step solution

Understand Ionization Energy

Ionization energy is the amount of energy required to remove an electron from an atom in the gas phase. It generally increases across a period (left to right) and decreases down a group (top to bottom) in the periodic table.

Identify Trends in Ionization Energy

When moving across a period in the periodic table, ionization energy increases as the nuclear charge increases, making it harder to remove an electron. Conversely, moving down a group, ionization energy decreases because the electrons are farther from the nucleus and experience greater electron shielding.

Determine the Element with the Smallest Ionization Energy

The element with the smallest ionization energy is the one that is furthest down and to the left on the periodic table. Francium (Fr), located at the bottom of Group 1, has the smallest ionization energy because it easily loses an electron due to weak attraction to the nucleus.

Determine the Element with the Largest Ionization Energy

The element with the largest ionization energy is the one that is at the top right corner of the periodic table, excluding noble gases. Helium (He) has the largest ionization energy because its electrons are very close to the nucleus with no shielding effect.

Key concepts

Periodic Table Trends

The periodic table organizes elements in a manner that showcases repeating trends or patterns in their properties. These patterns are known as periodic trends. One of the key periodic trends is ionization energy. As you move from left to right across a period, the ionization energy tends to increase. This occurs because atoms have more protons, which increases the nuclear charge and attracts electrons more strongly, requiring more energy to remove an electron. Conversely, as you move from top to bottom in a group, ionization energy decreases. This is due to electrons being further away from the nucleus and experiencing more electron shielding.

Electron Shielding

Electron shielding is an essential concept in understanding how atoms interact with their electrons. It refers to the effect that inner electrons have on reducing the full attractive force of the nucleus on outer electrons, or valence electrons.
As atoms grow larger, with additional shells of electrons, the shielding effect becomes more pronounced. This shielding effect makes it easier for the outermost electrons to be removed, thereby decreasing ionization energy. In simple terms, more electron shells mean greater shielding, which weakens the nucleus’s hold on the outermost electrons.

Nuclear Charge

Nuclear charge is related to the number of protons in an atom's nucleus—the more protons, the higher the nuclear charge. This charge plays a significant role in an atom's attraction to electrons.
The effective nuclear charge is often discussed, considering the balance between the positive charge of the protons and the electron shielding by inner electrons. As this effective nuclear charge increases, the attraction between the nucleus and the electron also increases, usually resulting in a higher ionization energy. In periods, the nuclear charge increases from left to right, making it progressively harder to remove an electron.

Francium

Francium (Fr) is an element in Group 1 of the periodic table, also known as the alkali metals. It is located at the very bottom of this group, making it the element with the lowest ionization energy in its group.

• Group 1 elements are characterized by having only one electron in their outermost shell.
• This single valence electron can be easily removed since Francium exhibits heavy electron shielding and a larger atomic radius than lighter alkali metals above it.
• This results in Francium having a weak nuclear attraction for this outer electron, leading to its very low ionization energy.

Due to its extreme rarity and radioactivity, Francium is not typically encountered outside of specialized scientific research.

Helium

Helium (He) is a noble gas, found in Group 18 of the periodic table. It is positioned at the top of this group and is distinct due to its extremely high ionization energy.

• Helium has only two electrons, which are greatly influenced by the strong nuclear charge of its two protons.
• With no inner electron layers, these electrons experience no shielding, meaning they remain very close and tightly held to the nucleus.
• This lack of electron shielding ensures that Helium's electrons require a significant amount of energy to be removed.

Its small atomic size and high ionization energy contribute to its stability and non-reactivity, which are typical traits of noble gases.