After today, you will be able to… Explain how atomic radius is found Explain the trend for atomic size across periods and down groups Explain what shielding is, and describe the role it plays in atomic size

Unit 4: The Periodic Trends Trends with Atomic Radius (Size)

Trends with Atomic Size An atom does not have a fixed radius. The radius of an atom can only be found by measuring the distance between the nuclei of two touching atoms, and then halving that distance.

Atomic Radius Atomic Radius: ½ of the distance between the nuclei of two like atoms

Atomic Size (cont.) Group trend: () atomic size will increase as you move down a group due to a greater number of occupied energy levels

Period trend: () atomic size decreases from left to right The elements have the same number of occupied energy levels, but a greater number of protons (increased nuclear charge)

Shielding Effect The period trend is less pronounced for periods further down the P.T. due to the shielding effect. The inner electrons “shield” or block the attraction from the nucleus. So there is still a increase in size for the lower periods but it is less significant.

Atomic Radii for Selected Atoms:

Summary: Atomic Size Decreases Increases Easy way to remember: THE SNOWMAN Decreases Increases Someone kicked down my snowman! 

Do now: Complete and Turn in “Exit Ticket” Then: Begin WS 2 (Homework)