Section 3: Periodic Trends

Nuclear Charge Nuclear charge is the pull that an electron “feels” from the nucleus. The closer an electron is to the nucleus, the more pull it feels. As nuclear charge increases, the electron cloud is pulled in tighter. Increases across a period.

Shielding As more energy levels are added to atoms, the inner layers of electrons shield the outer electrons from the nucleus. Outer electrons are less tightly held. Constant across a period. Increases down a group.

Atomic Radius Definition: Half of the distance between nuclei in covalently bonded diatomic molecule

Electronic Structure of Atoms Atomic Radius -due to more shielding (more energy levels) increases down a group -due to greater nuclear charge (more protons) decreases across a period

Atomic Radius

Example of Atomic Radius Trend Arrange the following elements in order of increasing atomic radii: Cs, F, K, Cl

Electronic Structure of Atoms Cations from metals:  e – ’s lost  positive (+) charge  smaller  outer shell lost Anions from nonmetals:  e – ’s gained  negative (–) charge  larger  less nuclear charge Ionic Radius

Ions Here is a simple way to remember which is the cation and which the anion: This is a cat-ion. This is Ann Ion. He’s a “plussy” cat! She’s unhappy and negative. +

Cation Formation 11p + Na atom 1 valence electron Valence e- lost in ion formation Nuclear charge on remaining electrons increases. Remaining e- are pulled in closer to the nucleus. Ionic size decreases. Result: a smaller sodium cation, Na +

Anion Formation 17p + Chlorine atom with 7 valence e- One e- is added to the outer shell. Nuclear charge is reduced and the e- cloud expands. A chloride ion is produced. It is larger than the original atom.

Ionic Radius Notice that the anions are larger than the cations!

Example of Ionic Radius Trend Arrange the following ions in order of increasing ionic radii: Ca 2+, K +, Al 3+, S 2-, Cl -, Te 2-

Electronic Structure of Atoms 1. Atoms get ___ down a group due to ___. A. bigger, greater nuclear charge (protons) B. smaller, less nuclear charge (protons) C. bigger, more shielding (levels) D. smaller, less shielding (levels) Quick Quiz!

Electronic Structure of Atoms 2. Atomic radii ___ across a period due to ___. A. decrease, greater nuclear charge (protons) B. decrease, less nuclear charge (protons) C. increase, more shielding (levels) D. increase, less shielding (levels) Quick Quiz.

Electronic Structure of Atoms 3.Which of the following has the largest radius? A. Na B. Mg C. K D. Ca Quick Quiz. WHY? most shielding (levels) least nuclear charge (protons)

Electronic Structure of Atoms 4.Metals tend to ________ electrons to form _________ cations. A. gain, positive B. gain, negative C. lose, negative D. lose, positive Quick Quiz.