Question

In going across a row of the periodic table, protons and electrons are added and ionization energy generally increases. In going down a column of the periodic table, protons and electrons are also being added but ionization energy generally decreases. Explain.

Short answer

In summary, ionization energy generally increases across a row in the periodic table due to the increase in nuclear charge with a roughly constant shielding effect, resulting in stronger attraction of electrons towards the nucleus. Meanwhile, ionization energy decreases down a column as the increase in nuclear charge is offset by increasing distance between the outermost electron and nucleus and increased electron shielding effect. This leads to weaker effective nuclear charge and less energy required to remove an electron.

Step-by-step solution

The factors affecting Ionization Energy

Ionization energy is the energy required to remove an electron from a gaseous atom or ion, hence making the atom into a positive ion. There are three main factors affecting ionization energy: 1. The nuclear charge (the number of protons in the nucleus) 2. The electron shielding effect (the repulsion between electrons in different shells) 3. The distance between the outermost electron and the nucleus Now, let's analyze the trends in ionization energy across the periodic table using these factors.

Ionization Energy Across a Row

As we move across a row in the periodic table (left to right), the number of protons in the nucleus increases. This results in a stronger nuclear charge, which means the electrons are more attracted to the nucleus. Since the electrons are more attracted to the nucleus, more energy is required to remove the electrons. Consequently, the ionization energy increases across a row. Additionally, the shielding effect remains roughly constant across a row, as electrons are being added to the same energy level, and the increase in nuclear charge outweighs any slight increase in shielding. Together, these factors lead to an increase in ionization energy as we move across a row in the periodic table.

Ionization Energy Down a Column

As we move down a column in the periodic table (top to bottom), the number of protons does increase, which would imply a stronger nuclear charge. However, the distance between the outermost electron and the nucleus also increases due to the addition of electron shells. This additional distance weakens the effective nuclear charge experienced by the outermost electrons. Additionally, the electron shielding effect increases as more inner electron shells are filled, adding more repulsion between electrons. Both the weaker effective nuclear charge due to increasing distance and the increased shielding effect result in a decrease in ionization energy as we move down a column in the periodic table.

Key concepts

Periodic Table Trends

Understanding the periodic table trends is crucial for grasping the changes in ionization energy. As we explore the table, we notice a pattern: moving from left to right across a period, the ionization energy typically increases, while descending down a group, it generally decreases. This behavior can be linked to the effective nuclear charge that the outermost electrons experience.
Across a Period: Adding protons to the nucleus alters the nuclear charge, making it more difficult for electrons to be removed and therefore increasing the ionization energy. Electrons are added to the same energy level, so the screening or shielding by inner electrons doesn't change considerably, allowing for the nuclear charge to be the dominating factor.
Down a Group: With each step down a group, the additional electron shells increase the distance between the outermost electrons and the nucleus. This larger distance and the increase in electron shielding weaken the effective nuclear charge. Even though protons are added, their effect diminishes due to these factors, causing a decrease in ionization energy.

• Ionization energy increases across a period
• Ionization energy decreases down a group

Nuclear Charge

The nuclear charge is fundamentally the total positive charge of the nucleus and is directly proportional to the number of protons present. It influences how tightly an atom holds its electrons, playing a pivotal role in determining the atom's ionization energy.
As you add protons to the nucleus, the nuclear charge naturally intensifies. Stronger nuclear charge results in a greater pull on the electrons, increasing the energy required to remove them – thus, higher ionization energy. This effect is clearly observed when comparing elements across a period.

Impact on Periodic Trends

In a period, despite a constant increase in nuclear charge, the ionization energy doesn't increase linearly. This is due to other competing factors such as electron-electron repulsion and the overall structure of the electron cloud. Yet, the underlying trend of increasing ionization energy correlates to the rising nuclear charge from left to right across a period.

Electron Shielding Effect

The electron shielding effect arises from the repulsion between electrons in different shells of an atom. Electrons in inner shells 'shield' outer electrons from the full impact of the nucleus's positive charge, effectively reducing the nuclear charge felt by those outer electrons.
This repulsion counteracts the nucleus's pull on more distant electrons, making them easier to remove and lowering the ionization energy. As you travel down a group and more electron shells are added, the shielding effect becomes stronger due to the increased number of inner electrons.

Variables Affecting Shielding

More electron shells result in a stronger shielding effect. Across a period, the effect is less pronounced because electrons are added to the same shell. This distinction helps explain why the shielding effect does not impede the upward trend in ionization energy across a period but does contribute to the downward trend as you move down a group.

Effective Nuclear Charge

Effective nuclear charge (ENC) is the net positive charge experienced by an electron in a multi-electron atom. The ENC takes into account both the shielding effect from other electrons and the actual nuclear charge of the atom.
It can be considered as the strength of the pull an electron feels from the nucleus, factoring in the electron-electron repulsion from inner electrons. ENC is crucial for understanding ionization energy trends because it provides insight into the actual force holding an electron within the atom.

Mathematically Estimating ENC

While calculating ENC exactly is complex, it can be approximated using Slater's rules or other empirical equations. This concept helps to rationalize why ionization energy increases across a period — because ENC increases — and why it decreases down a group, where ENC is mitigated by the stronger electron shielding effect of the added inner electron shells.