5.2 – NOTES Organizing the Periodic Table

II. Classification of the Elements A. Organizing the Elements by Electron Configuration 1. Valence electrons outermost s and p electrons 2. Valence electrons and period energy level of an element’s valence e- indicates the period where the element can be found on the PT; 3. Valence electrons and group number representative Element’s group # and # of valence e- are related - exception – He only has 2 even though in group 8A atoms in the same group have similar properties b/c they have same # of valence electrons 1A has 1 so it must be an s1

B. The s, p, d, f block elements 1. s block elements 2. p block elements 3. d block elements 4. f block elements

III. Periodic Trends Vertical and horizontal trends on the periodic table allow us to make predictions about behavior.

Effective Nuclear Charge, Zeff:   Zeff = Z – S, where Z = nuclear charge (at. no.) S = shielding (e- in inner shells) What do the valence e- feel from the nucleus?

Within a group (family) of elements, Zeff is the same. Zeff for family 1 is 1, 2 is 2, 3 is 3, etc. Consider Li and Cs – have the same Zeff, but do they have the same force of attraction? NO! Li: 1s22s1 Zeff: 3-2 = 1 Cs: [Xe]6s1 Zeff: 55-54 = 1 Did shielding keep pace with nuclear charge? Yes, both increased. Li and Cs have the same Zeff, but do they have the same force of attraction for the electron? No!

Within a series (period) of elements, Zeff is increasing. Consider Li and F – Li: 1s22s1 Zeff: 3-2 = 1 F: 1s22s22p5 Zeff: 9-2 = 7 Li and F are in the same period and have the same shielding. Zeff is greater for F than Li. How does this impact the atom? Fluorine’s nucleus has a greater ability to attract electrons.

Atomic Radius B. Atomic Radius size/volume of e- cloud; measured by ½ the distance b/t adjacent nuclei 1. Within a family, the trend is Increasing down the family

Atomic Radius Two reasons for this trend: 1) as you go down a family, an energy level is being added to the electron cloud. Li – 2s1 Na – 3s1 2.) Shielding Effect – as energy levels are being added, the number of inner core electrons increases and the valence electrons get farther away from the nucleus. Li – 1s22s1 Na – 1s22s22p63s1 (The nucleus exerts a force on the electrons called the nuclear pull.) The more inner core electrons present, the more the nuclear pull will be absorbed by the inner core electrons so the attraction between the nucleus and the valence electrons goes down. If the valence electrons are not feeling the nuclear pull, then they will not be pulled as tight to the nucleus. Result: the electron cloud gets larger.

Atomic Radius 2. Across a period, the trend is decrease from left to right Explain why: increasing effective nuclear charge (Zeff), so the nuclear pull is getting stronger; with increased nuclear pull the electrons will be pulled tightly to the nucleus. The largest atom is Fr ; The smallest atom is He

Ionic Radius C. Ionic Radius “Ion” means an atom has lost/gained electrons; written with a charge (superscript) 1. Octet rule atoms tend to gain, lose, or share electrons in order to acquire a full set of 8 valence electrons (noble gas configuration); Na – [Ne]3s1 to get 8, Na could lose 1 e- to become like Ne or gain 7 e- to become like Ar; easier to lose 1 than gain 7 so Na+ has configuration of Ne Cl – [Ne]3s23p5 to get 8, Cl could lose 7 e- to become like Ne or gain 1 e- to become like Ar; easier to gain 1 than lose 7 so Cl- has configuration of Ar

Ionic Radius 2. Positive ions (cations) form when an atom loses electrons metals lose electrons positive ions are smaller than the atoms they originate from the electron(s) lost is/are valence e- and once removed, the nuclear pull increases and the electrons will be pulled tightly to the nucleus. 3. Negative ions (anions) form when an atom gains electrons nonmetals lose electrons negative ions are larger than the atoms they originate from increased # of e- increases electrostatic repulsion forcing the e- farther apart Every other trend on the periodic table is inversely related to the trend of atomic radius.