8.4-8.6 Electron Configuration, The Explanatory Power of the Quantum Mechanical Model, and a Few Periodic Trends

Valence vs. Core Electrons The key connection between the macroscopic world (an elements chemical properties) and the atomic world (an atom’s electronic structure) lies in these outer most electrons. The reason that elements within the same group have similar chemical properties is because they have the same number of valence electrons. Valence electrons, those in the outer most principle energy level, are those that are most important in chemical bonding because they are the most loosely held. Core electrons are the electrons in lower complete principle energy levels.

Let’s Try a Practice Problem Write an electron configuration for phosphorus. Identify the valence electrons and core electrons. P 1s22s22p63s23p3 or [Ne] 3s23p3 Phosphorus has 5 valence electrons and 10 core electrons.

Orbital Blocks on the Periodic Table The number of columns (groups) in a block corresponds to the maximum number of electrons that can occupy the particular sublevel of that block. s block: groups 1-2 (1A-2A) p block: groups 13-18 (3A-8A) d block: groups 3-12 transition metals (Bs) f block: inner transition metals (below the main table)

Let’s Try a Practice Problem Use the periodic table to write the electron configuration of bismuth (Bi). Bi [Xe] 6s24f145d106p3

Exceptions to aufbau’s priciple The patterns that we have discussed allow us to accurately predict the electron configurations for most of the elements in the periodic table. However there are a few elements that have been proven, based on spectroscopy, to have a more stable electron configuration that is irregular. Memorization of the few exceptions is beyond the scope of the AP exam!

The Explanatory Power of the Quantum-Mechanical Model The number of valence electrons that an element contains determines the chemical properties of the element. Even though we do not explore the quantum mechanical model quantitatively in this curriculum, mathematical computation has proven that the valence electrons of noble gases have particularly low PE, so they are unreactive. Elements with electron configurations close to noble gases are the most reactive because they only need to lose or gain 1e- to achieve stability.

Periodic Trends: Atomic Size Atomic radius: the covalent radius of an atom is ½ the distance between two bonded atoms of the same non-metal. The approximate bond length between two covalently bonded atoms is the sum of the two atoms radii. As we move down a group (or family) on the periodic table, atomic radius increases. As we move across a period on the periodic table, atomic radius decreases.

Reasons for the Trends in Atomic Radius Atomic radius increases down a group because as we move down a group, we enter higher principle energy levels (valence electrons are further from the nucleus). Atomic radius decreases across a period because as we move from left to right across a period, the elements experience a effective nuclear charge (Zeff becomes more positive) resulting in a stronger attraction of the valence electrons to the nucleus, and a smaller atomic radii.

Let’s Try a Practice Problem! Which electrons experience a greatest effective nuclear charge? (a) the valence electrons in Mg (b) the valence electrons in Al (c) the valence electrons in S (c) The valence electrons in S will experience a greater nuclear charge because they lie further to the right of the same period.

Let’s Try Another! On the basis of periodic trends, chose the larger atom in each pair (if possible). Sn or I Ge or Po Cr or W F or Se (a) Sn is further to the left of the same period experiencing a less effective nuclear charge. (b) Can’t tell based on trend alone (c) W is further down the same group meaning that it’s valence electrons are in a higher energy level. (d) Se is both further down and to the left of the periodic table, indicating a larger radius.

Mention Ionization Energy for quiz!!!

8.4-8.6 Pg. 376-377 #’s 62, 64, 92 Study For Quiz On 8.1-8.6