3.2.6 Define valence electrons. Identify the valence electrons in orbital diagrams and electron configurations. Draw dot diagrams for elements
Diagrams contain a lot of useful information in a compact format. What does the football play diagram below tell us? The play diagram below describes the lineup of each player on the team and describes how they will move when the ball is snapped. What does the heart diagram below tell us? The heart diagram shows us the different parts of the heart. Diagams of electrons give similar information about where certain electrons are. We can mark these electrons and indicate what happens to them when an element reacts.
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When elements react with each other only their valence electrons interact. This means that it is important to understand how many valance electrons an atom of an element contains. Valence electrons are defined as the electrons that are located in the s and p sub-levels of the highest energy level of an atom. To determine the number of valence electrons, it is helpful to write out the electron configuration. For example the oxygen has an electron configuration of 1s22s22p4. The highest energy level is the 2nd energy level and there are 2 electrons in the s sub-level and 4 electrons in the p sub-level, therefore, oxygen has 6 valence electrons. If we look at bromine, there is an noble gas configuration of1s22s22p63s23p64s23d104p5 and bromine has 7 valence electrons (note that the d electrons are NOT included in the number of valence electrons).
Figure:the electron configurations of cobalt and bromine. The Valence electrons are underlined in red.
Electron Dot Diagrams
The valence electrons of an atom are the electrons located in the s and p sub-level of the highest energy level. Valence electrons are primarily responsible for the chemical properties of elements. The number of valenceelectrons can be easily determined from the electron configuration. Elements from the second period are shown in the following table.
TABLE:Several examples from the second period
In each case, the valence electrons are those in the second energy level. As one proceeds left to right across a period, the number of valence electrons increases by one. In the s block, Group 1 elements have one valence electron, while Group 2 elements have two valence electrons. In thep block, the number of valence electrons is equal to the group number minus ten. Group 13 has three valence electrons, Group 14 has four, up through Group 18 with eight.The eight valence electrons, a full outersandpsublevel, give the noble gases their special stability.
Figure (above):Bohr diagrams showing the number of electrons in elements 1-18. Valence electrons are in red.
When examining chemical bonding, it is necessary to keep track of the valence electrons of each atom. Electron dot diagrams are diagrams in which the valence electrons of an atom are shown as dots distributed around the element’s symbol. A beryllium atom, with two valence electrons, would have the electron dot diagram below.
The dots for a given atom are distributed evenly around the symbol before they are paired. The Table belowshows the electron dot diagrams for the entire second period.
TABLE (below): Electron Dot Diagrams for the Second Period Elements
Electron dot diagrams would be the same for each element in the representative element groups. Most transition elements have two valence electrons, though some have a different number of valence electrons.
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• Valence electrons are located in the s and p sub-level of the highest energy level of an atoms or ion
• Electron dot diagrams show the valence electrons for an atom.
• The dot diagrams are the same for each element in the representative element groups.
ReviewQuestionsWhat are valence electrons?How many valence electrons does magnesium have?How many valence electrons does nitrogen have?Draw the dot diagram of oxygen and Sulfur. Why do oxygen and sulfur have the same electron dot diagram?What are valence electrons primarily responsible for?Why are both 4s and 4p electrons included but the 3d electrons are not included as valence electrons forbromine?Calcium would have the same electron dot diagram as which element pictured in the table?Would you expect all the group 8 elements to have the same electron dot diagram as neon?