Unit 9 : Atomic Theory and Periodicity

1. Unit 9: Atomic Theory and Periodicity Section 2: Quantum Mechanical Theory

2. Problems with the Bohr Model • Suggests that orbiting electrons move at a specific radius, like a planet does around the sun • We later find out that oribtals are actually electron clouds, and differ in size, shape, and orientation • We use the quantum model to find the probability that an electron resides at a particular location • The denser the electron cloud, the higher the probability of finding an electron in that region

3. Quantum Numbers • Specifies the properties of atomic orbitals and the properties of electrons in orbitals • Each electron can be assigned a set of four quantum numbers • These numbers are the electron’s address

4. The First Quantum Number • Principal quantum number (n) • Describes the energy level • Are positive integers beginning with 1, 2, 3, etc. • The first energy level is closest to the nucleus, and each one after that moves farther away from the nucleus and increases in energy • More than one electron can have the same energy level • The total number of orbitals that exist is equal to n2 • In energy level 1, there is only 1 orbital (12) • In energy level 2, there are 4 orbitals (22)

5. The Second Quantum Number • Angular momentum quantum number (l) • Describes the shape of the orbital as s, p, d, or f • s, p, d, and f are called energy sublevels • Has values from 0 to 3 • s = 0 • p = 1 • d = 2 • f = 3 • At this level, we don’t go into much detail with the “f” sublevel

6. The Second Quantum Number • s sublevel has one orbital, an s orbital • A capacity of two electrons • Makes up columns 1 and 2 on the p.t. • p sublevel has three orbitals, x, y, and z • A capacity of six electrons • Makes up columns 13 to 18 on the p.t. • d sublevel has five orbitals • A capacity of ten electrons • Makes up columns 3 to 12 on the p.t. • f sublevel has seven orbitals • A capacity of 14 electrons • Makes up the lanthanides and actinides on the p.t.

7. The Third Quantum Number • Magnetic quantum number (m) • Determines the orientation of an orbital around the nucleus • Has values ranging from –l through 0 to +l

8. The Fourth Quantum Number • Spin quantum number • Electrons are said to have either a +½ spin or a -½ spin • Also known as a clockwise and counterclockwise spin • Within an orbital, the first electron has a positive spin and the second electron has a negative spin • A single orbital can hold a maximum of two electrons, which must have opposite spin states

9. Quantum Number Relationships in Atomic Structure

10. Electron Configuration • A way of describing each of an element’s electrons • Written using the following steps: • Find out the # of electrons for the element • Start with the 1s part • 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, etc. • For each orbital, insert the maximum # of electrons in the exponent position • Continue until each element’s electrons has been described • The superscripts should add up to the element’s atomic # (indicates # of electrons)

11. Electron Configuration Examples • The element sodium has 11 electrons • 1s22s22p63s1 • The sum of the superscript #s equal 11 • 2+2+6+1 = 11 • The coefficients indicate the energy level and row # on the p.t. • The alphabet’s superscripts indicate the location and column # on the p.t. • The element silver has 47 electrons • 1s22s22p63s23p64s23d104p65s24d9