F. Schifano, Department of Science Bayonne High School, Bayonne NJ

Describe electrons in detailWrite electron configurationsUse electron configurations

 The valence electrons of an element are the outermost electrons.  Only s and p electrons in an atom’s highest energy level count as valence electrons.

Write the configuration Identify the highest energy level Add up s and p electrons at that level only

 Each side represents a sub-orbital.  s-orbitals get both electrons first.  Each p sub-orbital gets one e - before any p- sub-orbital gets two.  This is called Hund’s Rule.

Determine valence # Write chemical symbol Draw dots to represent the valence e

 Determine the number of valence electrons in each of the following elements. Then draw their dot diagrams:  Ca  F  Br  Sr  N  O

 Formation of Covalent Bonds Formation of Covalent Bonds  When molecules get close to each other, the valence electrons are attracted to the nucleus of the other atom.

 If trading or sharing electrons would make both atoms more stable, this temporary attraction becomes a full-fledged chemical bond.

 If two elements have the same number of valence electrons, they will react in a very similar way.  They need similar changes in their electron configuration to become stable.

 Which ones should have similar properties and react in similar ways?  Ca  F  Br  Sr  N  O

 The most stable configuration of electrons most atoms can have is valence = 8, also known as the octet configuration.  In chemical reactions, most atoms just take the simplest path to valence = 8.

 Neon, argon, krypton, radon, and xenon are all called noble gases.  They already have valence=8. They don’t benefit from reacting, so they don’t react!

 The periodic table is designed in just such a way that elements with the same valence fall into the same vertical column.

Alkali Metals Group I s 1 Lose 1 e - Alkaline Earth Metals Group 2 s 2 Lose 2 e - Halogens Group 17 s 2 p 5 Gain 1 e - Noble Gases Group 18 s 2 p 6 No Reaction

 Hydrogen becomes stable at valence = 0 or at valence = 2.  It can give away an electron, becoming valence =0 (H+) or it can gain an electron, becoming valence =2 (H-).  Helium is already stable at valence =2.  Like the other noble gases, helium is already stable and doesn’t react.  Boron is stable at valence =6.

 An element’s position on the periodic table tells you the last electron that filled its orbitals:  Period (horizontal) = energy level  Block = orbital shape  Box number = #e - in the orbital.  Aufbau principle tells you all the rest!

 Sulfur is in the fourth box of the third row, in the p-block.  Its last electron is 3p 4.  Aufbau principle says everything under 3p 4 must be filled, so:  1s 2 2s 2 2p 6 3s 2 3p 4.

 If an element is in the d-block its last orbital will really be one energy level down from the row it’s in.  Last electron in chromium (Cr) = 3d 4, NOT 4d 4  If an element is in the f-block its last orbital will really be two energy levels down from the row it’s in.  Last electron in plutonium (Pu) = 5f 5, NOT 6f 5

 Longer configurations are a chore.  Noble gases can be used as starting points for longer configurations.

Find target element on the Periodic Table Choose a noble gas core Write only electrons between core and target

Cr =1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 4 Ar = 1s 2 2s 2 2p 6 3s 2 3p 6 ____________________________ So we can write the configuration of Cr as [Ar] plus the difference: [Ar] 4s 2 3d 4

 What are valence electrons?