Periodic Properties of the Elements Chapter 8 Periodic Properties of the Elements

Development of the periodic table Dobreiner: grouped elements into triads: 3 elements with similar properties (ex. Barium, calcium, strontium) Newlands: (about 50 years after Dobreiner) organized elements into octaves, similar to music Mendeleev: arranged elements in order of increasing MASS from left to right and placed elements with similar properties in the same columns

Mendeleev’s table was great. Undiscovered elements could be predicted Mendeleev’s table was great! Undiscovered elements could be predicted! (eka-aluminum, eka= one beyond) However, not every element fit this system… MOSELEY! Arranged by atomic number instead of mass. This resolved much of the issue with Mendeleev’s arrangement PERIODIC LAW! Stated that the elements could be arranged with predicting patterns of behavior, but not the WHY

Electron Configurations Quantum-mechanical theory describes the behavior of electrons in atoms. Electron Configuration: shows the particular orbitals that are occupied for that atom Ground State: lowest energy state Electron configuration considers four values: Energy level, sublevel, orbital, and spin

Electron spin and the Pauli Exclusion Principle Spin is a basic property of all electrons All electrons have the same amount of spin The orientation of the spin is quantized (spin up or spin down)(ms) Pauli Exclusion Principle: No two electrons in an atom can have the same four quantum numbers! Spin is represented by a +1/2 or a -1/2

Sublevel Energy: E(s orbital) < E(p orbital)< E(d orbital) <E(f orbital) ONLY WHEN THE 1s ORBITAL IS OCCUPIED! If 1s orbital is empty, all other orbitals are called degenerate: same energy

Penetration and Shielding Shielding: repulsive effect of one electron on another Think of lithium. What charge does lithium have? WHY???? 2 inner electrons SHIELD the total 3+ charge of the nucleus Effective nuclear charge (Zeff) = 1+

Penetration: the 2s orbital experiences more of the nuclear charge due to more penetration, SOOOO lower energy Because of penetration, sublevels of each energy level are NOT degenerate for multi-electron atoms In fourth and fifth energy levels, penetration becomes so important that it results in 4s being lower energy than 3d There can be variances in electron configuration, especially with TRANSITION METALS

Rules for electron configuration Pauli exclusion principle: no two electrons will have the same 4 quantum numbers Aufbau principle: aufbau = German for ‘build up’. Electrons will fill lowest energy first Hund’s rule: electrons fill orbitals singly first with parallel spins (think of kids on a bus)

Let’s try it! Pg 328, practice 8-1 and 8-2 Pg. 359, 41-50

Electron Configurations, valence electrons, and the periodic table Notice the periodic trend: As you move down a group, the number of outermost electrons remains the same. THIS IS THE KEY CONNECTION BETWEEN ATOMIC STRUCTURE AND ATOMIC BEHAVIOR! Valence Electrons: for main group electrons: outermost electrons in the principle energy level. For transition elements: we also count electrons in the outermost d electrons. Core electrons: all other electrons (electrons in complete principle energy levels)

The chemical properties of elements are largely determined by the number of valence electrons they contain. Why gain or lose? A full outermost energy level is relatively low energy. The element becomes inert. It cannot lower its energy any further by reacting with other elements.

Periodic trends in the size of atoms and effective nuclear charge

The volume of an atom is taken up mostly by its electrons The volume of an atom is taken up mostly by its electrons. But, we can’t know EXACTLY where these electrons are, so how do we determine size of atoms? Nonbonding atomic radius: determined by freezing a sample of an element and measuring the distance between the center of two adjacent atoms. The atoms are NOT bonded together, just very, very close to one another. Also called van der Waals radius

Another way to define the size of an atom is called the bonding atomic radius or covalent radius Nonmetals: one half the distance between two atoms bonded together Metals: one half the distance between two of thee atoms next to each other in a crystal of the metal

General Trend As you move down a group, radius increases As you move left to right, it generally decreases. WHY?

Atomic Radii and the transition elements The radius stays roughly consistent across a row. WHY???? Outermost electrons stay roughly consistent

Let’s try it! Pg 329, practice 8.3 Pg 332, practice 8.4 (both)

More Trends! Ionization energy: the energy required to remove an electron from an atom or ion in the gaseous state. Energy required to remove first electron? FIRST IONIZATION ENERGY, and so on. Trends: decrease as you move down a column/group. Increases left to right across a period. WHY????

Exceptions in Trends in First Ionization Energy Exceptions do occur! Boron has a smaller first ionization energy than Beryllium. WHY???? Jump to the p block In general, the trends are reliable and predictable

Electron Affinities and Metallic Character These characteristics also exhibit periodic trends! Electron Affinity: the energy change associated with gaining an electron Usually negative because an atom usually releases energy when it gains an electron There is not much of an observable trend down a group Becomes more negative as you move left t right across a group. WHY???

Metallic Character Conductivity, Malleability, ductility, shiny General Trend: As you move left to right across a row, metallic character decreases As you move down a column, metallic character increases

Chapter 8 TEST This really isn’t too Bad, SOOO 1. Have a 3 x 5 notecard if you want 2. Know the history of the development of the periodic table (Mendeleev, etc.) 3. Be able to complete full electron configurations, noble gas configurations, electron configuration diagrams 4. Be able to summarize (radii, ionization energy, metallic character, electron affinity) We will review/practice tomorrow and test on Wednesday, so COME WITH QUESTIONS!!!!

Homework Pg 345: practice 8.8 and more practice 8.8