8. The Periodic Table and Periodic Trends

The script begins with an introduction to MIT OpenCourseWare, highlighting its mission to provide free, high-quality educational resources, with a call to support the initiative. The lecturer, Catherine Drennan, then delves into a chemistry lesson about electron configurations, explaining the significance of the 4s-3d energy switch when electrons are lost or gained, and how this affects the electron configuration of ions compared to neutral atoms. She emphasizes the importance of understanding these concepts for exam preparation and provides guidance on using old exams, noting that the material covered in the current lecture is typically on the first exam.

The script transitions into a discussion about the periodic table, likening it to an artist's paintbox for chemists, containing all the elements that constitute everything in the world. The lecturer expresses excitement about teaching trends within the periodic table and hints at the importance of understanding electron configurations for exam success. A song by They Might Be Giants, 'Meet the Elements,' is introduced to illustrate the diverse properties and applications of elements, from their use in explosives to their necessity in the human body. The song is used as a fun and informative tool to emphasize the importance of learning about elements and their properties.

This section of the script focuses on ionization energy, defined as the minimum energy required to remove an electron from an atom. The lecturer explains how ionization energy is inversely related to binding energy and how it varies with different electron configurations within an atom. The concept of effective nuclear charge (Z effective) and its impact on ionization energy is discussed, with examples provided to illustrate the differences in ionization energy for various elements, such as boron, beryllium, and helium. The trends in ionization energy across periods and down groups in the periodic table are highlighted, with the underlying reasons for these trends being explained.

The script continues with a deeper exploration of ionization energy, emphasizing the differences in energy required to remove electrons from various orbitals within the same element, such as boron. The concept of electron shielding is introduced to explain why ionization energy varies between different ionic states of the same element, with a specific focus on the difference between neutral boron and boron plus. The lecturer uses a clicker question to engage students in understanding these concepts and provides an explanation for the observed trends in ionization energy.

The lecturer discusses the general trends of ionization energy across the periodic table, noting the increase in ionization energy as you move from left to right across a period and the decrease as you move down a group. Anomalies or 'glitches' in these trends are also examined, such as the unexpected ionization energy of beryllium compared to boron and the slight increase for nitrogen compared to oxygen. The reasons behind these anomalies are explained, providing a deeper understanding of the periodic trends.

This part of the script introduces the method of photoelectron spectroscopy (PES) as a means to measure ionization energies. The process involves using energy to excite an electron from an atom, such as neon, and then measuring the kinetic energy of the ejected electron. The relationship between incident energy, ionization energy, and kinetic energy is explained, and an example calculation is provided to demonstrate how ionization energies can be determined from the measured velocities of ejected electrons.

The script concludes with an introduction to electron affinity, the ability of an atom to gain an electron and form a negatively charged ion. The concept is illustrated using chlorine as an example, which has a high electron affinity, indicating its eagerness to gain an electron and become more stable. The trends in electron affinity are discussed, with an increase in affinity as you move across a period and a decrease as you move down a group, culminating in the negative electron affinities of noble gases, which are stable and reluctant to gain additional electrons.

In the final paragraph, the lecturer wraps up the discussion on electron affinity and hints at continuing the topic of electronegativity in the next lecture. The importance of understanding these concepts for the exams is reiterated, and the session ends with a clicker question to reinforce the understanding of electron affinities, particularly regarding noble gases.