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Atomic Orbital Filling Order
and Electron Configurations

There are 3 main rules for filling atomic orbitals
1. Aufbau principle - Electrons get filled in order from lowest to highest energy levels. Each atom has seven possible energy levels (cleverly labeled 1 through 7). One is the lowest, seven is the highest. They go by period (rows) in the periodic table. The possible number of electrons for an energy level is found by using the equation 2n2 (where n = an energy level, 1-7).

Each energy level is further divided into sublevels
Each energy level is further divided into sublevels. There are four basic types of sublevels which are arranged by groups. s -- The “s” sublevel has one orbital. p -- The “p” sublevel has three orbitals. d -- The “d” has 5 orbitals. f -- The “f” sublevel has 7 orbitals.

Increasing energy 7p 6d 5f 7s 6p 5d 6s 4f 5p 4d 5s 4p 3d 4s 3p
Orbital diagram Aufbau is German for “building up” We must follow this orbital energy diagram! Notice that the 4s orbital will fill before the 3d because it is lower in energy!

2. Pauli exclusion principle
- Only two electrons can fit in each orbital, one with an “up” spin, the other with a “down” spin -This means that each “s” has 2 electrons, “p” has 6 (3 orbitals with 2 e- each), “d” has 10, and “f” has 14. To show the different direction of spin, a pair in the same orbital is written as:

3. Hund’s rule When filling a sublevel with multiple orbitals (p, d, or f), each orbital gets one electron (the “up” spin) before any get two.

Orbital Diagram form Sodium
Energy Sodium has 11 electrons 4s 3p 3s 2p 2s 1s

Electron Configuration
An atom’s electron configuration is the position of its electrons around the nucleus. It reflects the orbital filling diagram. The electron configuration of Sodium is 1s22s22p63s1 -- This goes in order on the periodic table from the first level to last electron the atom has. The number in front of the letter tells the current energy level (1-7). The letter tells you the sublevel. The superscript after the letter tells you how many electrons have been filled in that sublevel.

Writing Electron Configuration of Sodium
Energy 4s 3p 3s 2p 2s 1s2 2s2 2p6 3s1 1s

Electron Configurations - Ions
If you know how many electrons an element or ion has, you can write its electron configuration or draw its orbital filling diagram. Even for an ion! Example - Na has 11 electrons, Na+1 (the ion) has 10 electrons. Ions will form so that it ends with a completely filled main energy level.

Writing Electron Configuration of Sodium Ion
Energy Na+ 4s Means it lost 1 electron 3p 3s 2p 2s 1s2 2s2 2p6 3s1 1s Na+ electron configuration 1s22s22p6

Adding Electrons = (-) Ions
If it needs one to three more electrons to complete the main energy level, then it will tend to gain those electrons and become a (-) charged ion. If it needs to gain 1 electron, it is a (-1) charged ion. If it needs to gain 2 electrons, then it will become a (-2) charge ion. Etc……

Removing Electrons = + Ions
If the atom has 3 electrons or less in a main energy level, then it will lose those electrons to become a (+) charge ion. If it loses 1 electron  +1 ion, if it loses 2 electrons  +2 ion, etc.

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