Electron Configuration

Electron configurations tells us in which orbitals the electrons for an element are located. Three rules: electrons fill orbitals starting with lowest n and moving upwards;

The order is: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d etc.

Orbital Diagram

The electron configuration of an atom is a shorthand method of writing the location of electrons by sublevel. The sublevel is written followed by a superscript with the number of electrons in the sublevel. If the 2p sublevel contains 2 electrons, it is written 2p2

Writing Electron Configurations First, determine how many electrons are in the atom. Iron has 26 electrons. Arrange the energy sublevels according to increasing energy: 1s 2s 2p 3s 3p 4s 3d … Fill each sublevel with electrons until you have used all the electrons in the atom: Fe: 1s2 2s2 2p6 3s2 3p6 4s2 3d 6 The sum of the superscripts equals the atomic number of iron (26)

Electron Configurations and the Periodic Table The periodic table can be used as a guide for electron configurations. The period number is the value of n. Groups 1A and 2A have the s-orbital filled. Groups 3A - 8A have the p-orbital filled. Groups 3B - 2B have the d-orbital filled. The lanthanides and actinides have the f-orbital filled.

Blocks and Sublevels We can use the periodic table to predict which sublevel is being filled by a particular element.

Noble Gas Core Electron Configurations Recall, the electron configuration for Na is: Na: 1s2 2s2 2p6 3s1 We can abbreviate the electron configuration by indicating the innermost electrons with the symbol of the preceding noble gas. The preceding noble gas with an atomic number less than sodium is neon, Ne. We rewrite the electron configuration: Na: [Ne] 3s1

EXCEPTIONS: Chromium (24 electrons) EC = 1s2 2s2 2p6 3s23p64s23d4 is INCORRECT EC = 1s2 2s2 2p6 3s23p64s13d5 is CORRECT! The d-orbital becomes half-filled so, it changes to 4s13d5 instead. The next element, Manganese will have an EC of: 1s2 2s2 2p6 3s23p64s23d5

This exception also appears near the end of the d-orbital filling: Copper EC = 1s2 2s2 2p6 3s23p64s23d9 is INCORRECT EC = 1s2 2s2 2p6 3s23p64s13d10 is CORRECT Additional exceptions are Mo 5s14d5; Ag 5s14d10; Au 6s15d10 That is reasonable considering their position on the periodic chart.

Rules for drawing orbital-Box diagrams Similar to EC, except, you are now expected to draw out the orbitals and place the electrons to show proper spinning. Rules for drawing orbital-Box diagrams The Pauli Exclusion Principle- each orbital can hold 2e-, where those 2e- are always in opposite spin. 2) Hund's Rule – Electrons occupy all the orbitals of a given sublevel singly before pairing begins. Spins of electrons in different incomplete orbitals are parallel in the ground state.The most stable arrangement of electrons in the subshells is the one with the greatest number of parallel spins.

Suppose we want to draw the orbital-box diagram of carbon: Step 1: Draw the orbitals. Step 2: Fill the electrons in the 1st two s-orbitals, showing opposite spin. These electrons must fill the lower energy orbitals first before advancing to the next higher energy level (AUFBAU’s PRINCIPLE). Step 3: Fill the 2p sublevels one at a time (HUND’s RULE).

Orbital Diagrams Examples

IONS Lose electrons = become positively charged (Cation) VALENCE ELECTRONS The electrons in the outermost shell of the Bohr diagram. Lose electrons = become positively charged (Cation) Gain electrons = becomes negatively charged (Anion) Start with forming ions Chemquest Inquiry activity #18

ELECTRON CONFIGURATIONS OF IONS Electrons do not come out the same way as we put them in according to the Aufbau Principle. Electrons leave the outer most shell first. Let's look at V vs V2+ 23V 1s22s22p63s23p64s23d3 Try a few more anions and cations. Remember + means it has lost electrons, - means it has gained electrons. 23V2+ 1s22s22p63s23p63d3