Question

(a) What is meant by the term effective nuclear charge? (b) How does the effective nuclear charge experienced by the valence electrons of an atom vary going from left to right across a period of the periodic table?

Short answer

The effective nuclear charge (Z_eff) is the net positive charge experienced by an electron in an atom, accounting for electron-electron repulsion using the formula \(Z_{eff} = Z - S\), where Z is the atomic number and S is the electron shielding constant. As we move from left to right across a period in the periodic table, the effective nuclear charge generally increases due to a higher number of protons in the nucleus and a relatively constant electron shielding.

Step-by-step solution

Definition of effective nuclear charge

Effective nuclear charge is a concept used to describe the net positive charge experienced by an electron in an atom. Since electrons are negatively charged and attracted to positively charged protons in the nucleus, their energy levels and behavior are significantly influenced by the positive charge of the nucleus. However, because electrons are also repelled by other negatively charged electrons in the atom, this repulsion reduces the attraction between electrons and the nucleus. Therefore, the effective nuclear charge is the net positive charge experienced by an electron after accounting for the repulsive forces from other electrons. Mathematically, the effective nuclear charge (Z_eff) is given by the formula: \(Z_{eff} = Z - S\) where Z is the atomic number (total number of protons in the nucleus) and S is the electron shielding constant, which accounts for the repulsion between electrons.

Variation of effective nuclear charge across a period

As we move from left to right across a period in the periodic table, the atomic number (Z) increases, and the electrons in the outermost shell (valence electrons) remain in the same shell or energy level. As a result, the number of protons in the nucleus increases, resulting in a stronger overall positive charge in the nucleus. At the same time, there is not a significantly larger increase in electron shielding (S) since the additional electrons are added to the same energy level without adding new inner electron shells. Consequently, the overall repulsive force experienced by the valence electrons is not drastically affected, and therefore, the effective nuclear charge increases. In conclusion: - The effective nuclear charge (Z_eff) is the net positive charge experienced by an electron in an atom after accounting for electron-electron repulsion. - As we move from left to right across a period in the periodic table, the effective nuclear charge experienced by the valence electrons generally increases. This is due to a higher number of protons in the nucleus and not having a significant increase in electron shielding.

Key concepts

Electron Shielding

Electron shielding is a fundamental concept in chemistry that plays a vital role in determining the behavior of electrons in an atom. At its core, electron shielding refers to the phenomenon where inner shell electrons block the full positive charge of the nucleus from the outer shell electrons.
The more inner electrons present, the more they can mask or shield the full nuclear charge from reaching the valence electrons. This affects how tightly the valence electrons are held by the nucleus, impacting the atom's size and its chemical properties.

When you learn about electron shielding, it's helpful to know:

• Electrons in the same energy level do not shield each other effectively.
• Inner electrons shield outer electrons from the nucleus's full charge more effectively.
• Electron shielding impacts ionization energy and atomic size.

Understanding this concept is essential, as it directly influences an atom's effective nuclear charge, which is key for explaining periodic trends.

Periodic Table Trends

The periodic table is an organized way to display elements based on their atomic number. It's a powerful tool to understand various trends in chemical and physical properties of elements.
One of the most important trends is the pattern of effective nuclear charge as you move across a period. As you go from left to right across a period, each element has an additional proton added to its nucleus.

Here's a summary of what you observe:

• Atomic number increases from left to right, increasing the positive charge in the nucleus.
• Valence electrons are added to the same energy level, maintaining relatively constant shielding.
• The effective nuclear charge increases as you move across a period, leading to a smaller atomic size.

These trends help in predicting the reactivity and other properties of elements and are crucial for understanding the behavior of elements in chemical reactions.

Valence Electrons

Valence electrons are the electrons in the outermost shell of an atom. They are pivotal in determining how atoms interact and bond with each other.
These electrons are directly involved in chemical bonding because they are the most "exposed" electrons of an atom to the external environment and thus are often the first to interact in chemical reactions.

Key points about valence electrons include:

• They determine the chemical properties of elements.
• Valence electrons dictate an element's position in a group on the periodic table.
• Elements in the same group have the same number of valence electrons, contributing to similar chemical behaviors.

Understanding valence electrons is foundational for predicting how an element will react with others and for drawing Lewis structures in chemistry.

Atomic Number

The atomic number of an element is the number of protons in the nucleus of its atoms. It is a unique identifier for each element and determines the element's place on the periodic table.
The atomic number provides insight into the element's properties since it dictates the electron configuration and therefore the distribution of electrons in an atom.

Important insights about the atomic number include:

• An element's identity is defined by its atomic number.
• As the atomic number increases across a period, the effective nuclear charge experienced by electrons increases.
• The atomic number helps predict the element's position and the specific trends it might exhibit on the periodic table.

A clear understanding of the atomic number is fundamental in exploring and grasping complex concepts like effective nuclear charge and periodic trends.