About this Video

• Video Title: Periodic trends and Coulomb's law | Atomic structure and properties | AP Chemistry | Khan Academy
• Channel: Khan Academy
• Speakers: Instructor
• Duration: 10:43

Introduction

This video uses Coulomb's Law to explain periodic trends in atomic properties. The instructor connects the attractive and repulsive forces between protons, core electrons, and valence electrons to explain trends in atomic and ionic radii, ionization energy, electron affinity, and electronegativity.

Key Takeaways

• Coulomb's Law and Atomic Structure: The magnitude of the force between charged particles is proportional to the product of their charges divided by the square of the distance between them. In atoms, this explains the relationship between the nucleus (protons), core electrons, and valence electrons.
• Effective Nuclear Charge (Z-effective): Z-effective is approximated by the atomic number minus the number of core electrons. This value influences the strength of attraction between the nucleus and valence electrons.
• Atomic Radius Trends: Atomic radius decreases across a period (left to right) due to increasing Z-effective and increases down a group (top to bottom) due to the addition of electron shells.
• Ionization Energy Trends: Ionization energy increases across a period and decreases down a group. High ionization energy indicates a strong attraction between the nucleus and valence electrons, making it difficult to remove an electron.
• Electron Affinity Trends: Electron affinity is the energy released when an electron is added to a neutral atom. High electron affinity is found in the upper right of the periodic table (excluding noble gases).
• Electronegativity Trends: Electronegativity is related to electron affinity and describes an atom's ability to attract shared electrons in a bond. It generally follows the same trend as electron affinity.

3. Explain the relationship between electron affinity and electronegativity.

Electron affinity measures the energy change when a neutral atom gains an electron. Electronegativity describes an atom's ability to attract shared electrons in a covalent bond. They are closely related because atoms with high electron affinities (meaning they release a lot of energy upon gaining an electron) will also tend to strongly attract shared electrons in a bond, resulting in high electronegativity. Both properties reflect the atom's ability to attract electrons, but electron affinity focuses on a single atom gaining an electron, while electronegativity considers the attraction of shared electrons in a bond between atoms.

4. Why do noble gases have high ionization energies but generally low electron affinities?

Noble gases have high ionization energies because their electron shells are completely filled. Removing an electron would disrupt this stable configuration, requiring a significant amount of energy. They have low electron affinities because adding an electron would require placing it in a higher energy level, outside the completely filled shells, creating an unstable configuration. Adding an electron is not energetically favorable and wouldn't significantly increase stability, unlike in the halogens (one electron short of a filled shell) where the energy released is significant.