Lesson 2: Periodic Trends in Atomic Properties
This lesson dives into the core of “periodicity.” It explores how various atomic properties change predictably across a period and down a group.
- Atomic Radius: This is the distance from the center of the nucleus to the outermost electron shell. Students will learn the trends: atomic radius decreases across a period due to increased effective nuclear charge and increases down a group as a new electron shell is added.
- Ionic Radius: This concept is an extension of atomic radius. Students will differentiate between the radius of a cation (smaller than the parent atom) and an anion (larger than the parent atom). The trend for ionic radius is similar to that of atomic radius: it decreases across a period for both isoelectronic ions and isoelectronic series.
- Ionization Enthalpy (): This is the energy required to remove the most loosely bound electron from a gaseous atom. The trend is that ionization enthalpy increases across a period because of the increasing effective nuclear charge, which holds the electrons more tightly. It decreases down a group as the outermost electron is farther from the nucleus and shielded by inner electrons. Students also need to understand the concepts of first, second, and successive ionization enthalpies.
- Electron Gain Enthalpy (): This is the energy change that occurs when an electron is added to a gaseous atom. The trend is generally that electron gain enthalpy becomes more negative across a period (more energy is released) as the effective nuclear charge increases, making the atom more attractive to an incoming electron. It becomes less negative down a group. Students must be aware of the exceptions, such as the low electron gain enthalpy of noble gases and the unexpected values for Group 15 elements.
- Electronegativity: This is the ability of an atom in a chemical bond to attract shared electrons. It is not a measurable quantity like the others but is a relative concept. Electronegativity increases across a period and decreases down a group. Students should know the Pauling scale and understand its applications in predicting bond polarity.