1.
Electron Configuration, The Explanatory Power of the Quantum Mechanical Model, and Periodic Trends

2. Electron Configurations, Valence Electrons, and the Periodic Table
The periodic table is arranged by grouping elements with similar chemical properties- connection between electron configuration and chemical properties
As we move right across a period, the orbitals fill in the correct order
With each subsequent row, the highest principal quantum number increases by 1
As you move down group the number of electrons in outermost principal energy level (highest n value) stays the same

4. Why?
Valence electrons - the electrons in the atom’s outermost energy level, important for chemical bonding
This explains why elements in a group have the same chemical properties (they have the same number of valence electrons)
Core Electrons - all other electrons in atom, those in complete principle energy levels, and those in complete d and f sublevels
Example) Silicon’s electron configuration is 1s^2 2s^2 2p^6 3s^2 3p^2
So it has 4 valence electrons (those in the n=3 level) and 10 core electrons

5. Practice: Write the Electron Configuration for Ge
Practice: Write the Electron Configuration for Ge. Identify Valence/ Core Electrons
1)Determine the total number of electrons from Ge’s atomic number (32) and distribute them into proper orbitals
2)Since germanium is a main-group element, its’ valence electrons are those in the outermost principal energy level. n= 1,2, and 3 are full and the n=4 level is outermost. So, the n=4 electrons are valence electrons and the rest are core electrons
1s^2 2s^2 2p^6 3s^2 3p^6 3d^10 4s^2 4p^2

6. Orbital Blocks in the Periodic Table
The periodic table can be divided into blocks representing the filling of particular sublevels
First 2 groups on left represent the s block
6 groups on right side represent the p block
The transition elements represent the d block
The inner transition elements represent the f block
The number of columns in a block corresponds to the max number of electrons that can occupy the particular sublevel of that block
s block has 2 columns (1 s orbital holding 2 e-)
p block has 6 columns (3 p orbitals with 2 e- each)
d block has 10 columns (5 d orbitals with 2 e- each)
F block has 14 columns (7 f orbitals with 2 e- each)
The number of valence e- for any main group is equal to its lettered group number (except for helium)

7. Orbital Blocks (cont.)
The row number (period) in the periodic table is equal to the n value of the highest principal energy level
Example: chlorine is period 3, group 7A which means it has 7 valence electrons and the highest principal energy level is the n=3 level

8. Writing an Electron Configuration for an Element from its Position in the Periodic Table
Example: Cl
The inner configuration is that of the noble gas that precedes it , Ne
The outer configuration is obtained by the tracing the elements between Ne and Cl and assigning electrons to appropriate orbitals
Remember, the highest n value is obtained by the period value (n=3 for chlorine)
So, we add two 3s electrons as we trace across the s block and five 3p electrons as we trace across the p block to Cl which is in the 5th column of the p block:
[Ne] 3s^2 3p^5

9. Practice: Use the periodic table to write the electron configuration for Se
The Atomic number of Se is 34
The noble gas that precedes Se is Ar so the inner electron configuration is [Ar]
Se is in row 4 so add two 4s electrons as you trace across the s block (n = row number)
Add ten 3d electrons (n = row number - 1)
Add four 4p electrons as you trace across p block to Se (because Se is in the 4th column of the p block) (n= row number)
Answer: Se[Ar] 4s^2 3d^10 4p^4

10. The Transition and Inner Transition Elements
The electron configurations of the transition elements (d block) and inner transition elements (f block) show different trends than those of main-group elements
The principal quantum number of the d orbitals that fill across each row in the transition series is equal to the row number minus 1
The principal quantum number of f orbitals that fill across each row in the inner transition series is equal to the row number minus 2

11. The Explanatory Power of the Quantum-Mechanical Model
Chemical properties of elements are largely determined by number of valence electrons they contain
Since elements within a group have same number of valence electrons, they have similar properties
Noble gases are very stable and unreactive because their electrons cannot lower their energy by reacting with other atoms or molecules
Elements with electron configurations closest to noble gases (7 or 1 valence electrons) are highly reactive

12. Atomic Radius
Nonbonding atomic radius/ van der Waals radius- the distance between 2 nonbonding atoms that are in direct contact
Bonding atomic radius/ covalent radius- defined directly as:
Nonmetals- ½ the distance between 2 of the atoms bonded together
Metals- ½ the distance between 2 of the atoms next to each other in the crystal of the metal
Atomic radius- refers to set of average bonding radii determined from a large number of elements and compounds
The aprox. bond length of any 2 covalently bonded elements is the sum of their atomic radii
As we move down a group, atomic radius increases
As we move to the right accross a period, atomic radius decreases
Why? Because as n increases, the electrons occupy larger orbitals, resulting in larger atoms

13. Effective Nuclear Charge
According to Coulomb’s law, the attraction between a nucleus and an electron increases with increasing magnitude of nuclear charge
We define the average or net charge experienced by an electron as the effective nuclear charge
Zeff = Z - S
Zeff = effective nuclear charge
Z = actual nuclear charge
S = charge screened by other electrons

14. Effective Nuclear Charge (continued)
Core electrons efficiently shield electrons in the outermost principal energy level from nuclear charge, but outermost electrons do not efficiently shield one another from nuclear charge
As we move right across a period, the effective nuclear charge experienced by the electrons in the outermost principal energy level increases, resulting in a stronger attraction between the outermost electrons and the nuclear, and smaller atomic radii

15. Atomic Radii and the Transition Elements
The radii of transition elements stay roughly constant across each period
Why? Because the highest n value is nearly constant across a row of transition elements
As another proton is added to the nucleus of each successive element, another electron is added as well, but the electron goes into an n(highest) - 1 orbital
The number of outermost electrons stays constant and they experience a roughly constant effective nuclear charge, keep the radius approximately constant

16. Practice: Based on periodic trends, chose the larger atom in each pair (if possible) and explain your choices
N or F
C or Ge
N or Al
Al or Ge
N; F is right of N across a period, radius decreases as you move right
B. Ge; Ge is below C in a group, radius increases as you move down group
C. Al; move down a group (inc.) and left across period (inc.)