Section 3: Periodic Trends

Nuclear Charge Na (11 p+) Al (13 p+) Cl (17 p+) Nuclear charge is the pull that an electron “feels” from the nucleus. Increases across a period because the number of protons increases (positive charge increases).

Shielding The decrease in attraction between the nucleus and outer electrons as more shells of electrons are added. Constant across a period because the # of energy levels is constant. Increases down a group because each period adds an energy level.

Atomic Radius The distance from the nucleus of an atom to the outer edge of its electron cloud Measured as half of the distance between two adjacent nuclei in molecules or crystal units.

decreases across a period Atomic Radius decreases across a period -due to greater nuclear charge (more protons) -due to more shielding (more energy levels) increases down a group Note: covalent or metallic radius of bonded atoms vs. van der Waals radius of nonbonded atoms (Noble Gases)

Atomic Radius

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

Ionic Radius Anions Cations negatively charged atoms formed when nonmetals gain electrons larger than neutral atom more e- repulsion & less nuclear charge Cations positively charged atoms formed when metals lose electrons smaller than neutral atom outer shell is lost

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

Cation Formation Na atom 1 valence electron 11p Valence e- lost in ion formation Result: a smaller sodium cation, Na+

Anion Formation A chloride ion is produced. It is larger than the original atom. Chlorine atom with 7 valence e- One e- is added to the outer shell. 17p There is increased repulsion between electrons and the e- cloud expands.

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: Ca2+, K+, Al3+, S2-, Cl-, Te2-

Quick Quiz! Atoms get ___ down a group due to ___. bigger, greater nuclear charge (protons) smaller, less nuclear charge (protons) bigger, more shielding (levels) smaller, less shielding (levels)

Quick Quiz. Atomic radii ___ across a period due to ___. decrease, greater nuclear charge (protons) decrease, less nuclear charge (protons) increase, more shielding (levels) increase, less shielding (levels)

WHY? Quick Quiz. most shielding (levels) least nuclear charge (protons) Which of the following has the largest radius? Na Mg K Ca

Quick Quiz. Metals tend to ________ electrons to form _________ cations. gain, positive gain, negative lose, negative lose, positive