[H2 Chemistry] 2023 Topic 1 Atomic Structure & Physical Periodicity 2

The lecturer introduces the second lecture on atomic structure, which is shorter in duration and covers fewer sections than the first. The focus is on section 4, discussing periodic trends in atomic and ionic radii, and the general trends in atomic properties across a period and down a group. The lecture aims to explain these trends in terms of nuclear charge, shielding effects, and effective nuclear charge. The importance of understanding these concepts for future reference and revision is emphasized.

This paragraph delves into the specifics of atomic and ionic radii, explaining the difference between covalent and Van der Waals radii, and how these measurements are used as proxies for atomic size. The lecturer discusses the challenges of measuring atomic radii, especially for gases like helium and neon, and introduces the concept of metallic radius. Additionally, the paragraph highlights the importance of reliable sources for atomic and ionic radii data, recommending webelements.com for comprehensive and trustworthy information.

The lecturer presents the general trends in atomic radius, explaining that the atomic radius increases down a group due to the addition of principal quantum shells and the relative distance of valence electrons from the nucleus. Despite an increase in nuclear charge, the effective nuclear charge remains relatively invariant due to the balancing increase in shielding effect. The paragraph also discusses the decrease in atomic radius across a period, attributing this to the increasing nuclear charge and relatively constant shielding effect, which results in a stronger attraction of the outermost electrons towards the nucleus.

This section discusses the trends in ionic radius, comparing anions and cations, and explaining why anions are generally larger due to having an additional principal quantum shell. The paragraph also covers the decreasing trend in ionic radius across a period, which is attributed to the increasing effective nuclear charge as nuclear charge increases and the screening constant remains relatively constant. The introduction of ionization energy as a new topic is highlighted, with an emphasis on understanding its definition and its endothermic nature.

The lecturer explains the concept of successive ionization energies, illustrating how they increase for the same element as more electrons are removed. The relationship between effective nuclear charge, nuclear charge, and screening constant is discussed to explain this trend. The paragraph also addresses the ability to deduce the electronic configuration of elements from their successive ionization energies, using magnesium as an example to show how the energy required to remove electrons varies based on their orbital and the associated screening effects.

This section focuses on the practical application of ionization energy data, showing how to identify elements and deduce their group numbers based on the patterns and jumps in their successive ionization energies. The importance of recognizing the largest jumps in ionization energy as indicative of the outermost electrons is emphasized. The paragraph provides examples of how to apply this knowledge to determine the group number of an element and highlights the relevance of data booklets in such analyses.

The lecturer explores the factors that influence ionization energies, such as the increase in nuclear charge across a period and the relatively constant shielding effect, which results in an overall increase in effective nuclear charge and a general outward trend in ionization energies. Exceptions to this trend, such as dips between certain groups, are explained by the increased shielding effect when moving from an s to a p orbital, and the first instance of electron pairing, which causes an increase in inter-electronic repulsion and a dip in ionization energy.

This paragraph delves into the unique behavior of transition metals, where the atomic radius and first ionization energy remain relatively invariant despite an increase in nuclear charge. The addition of electrons to the inner 3d subshell contributes to the shielding effect, which counterbalances the increase in nuclear charge, maintaining the effective nuclear charge and the energy required to remove the outermost electrons.

The final paragraph presents exercises that test the application of learned concepts to identify elements with the largest atomic radii and those with specific first ionization energies. The讲师 uses data from the periodic table and ionization energy trends to determine the correct elements for given scenarios, emphasizing the importance of understanding periodic trends and the structure of the periodic table for such analyses.

The lecturer concludes with a preview of the upcoming topic on chemical bonding, highlighting its fundamental importance in chemistry. An introduction to electronegativity is provided, explaining it as a measure of an atom's ability to attract electrons in a chemical bond. The implications of electronegativity differences on intermolecular forces are discussed, with a teaser for more in-depth coverage in future lectures.