Valence and lewis dot structure

Chemistry

1. Valence Electrons

2. What is a Valence Electron? The electrons that are in the highest (outermost) energy level. That level is also called the valence shell Of the atom. Electrons not in the outermost shell are called core electrons.

3. Bohr Atomic Structures Electron Configuration of Na: 1s 2 2s 2 2p 6 3s 1 The first energy level contains 2 electrons. (s orbital…1s 2 ) The second level contains 8 electrons. (2s and 2p orbitals…2s 2 2p 6 ) How many electrons do you see in the outermost level? 3s 1 … 1electron! This is the Valence number. Sodium has 1 Valence electron.

4. Valence Electrons How many valence electrons are in the following atoms? Ne O Na Cl

5. Valence Electrons How many valence electrons are in the following atoms? 1s 2 2s 2 2p 6 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 4 [Kr]5s 2 4d 10 5p 2 [Ne]3s 2 3p 3

6. Valence electron trends As you go down a group the number of electrons in the valence shell stays the same 2) As you go across a period (left to right) the # of e - in the valence shell of the main group elements increases one by one - This ignores the transition metals

7. 9 Lewis dot Structures for the atoms of groups 1-2 and 13-18 of The periodic table The Main group elements Notice the e - trends in the Groups and Periods

8. The number of valence electrons in an atom determines : The properties of the atom The way that atom will bond chemically As a rule, the fewer electrons in the valence shell, the more reactive the element is . When an atom has eight electrons in the valence shell, it is stable .

9. Noble Gas Stability Noble gases are usually unreactive This is because they have full valence shells An element with a full valence shell is a happy element  For two atoms to join together atoms must gain, lose or share electrons Elements with full valence shells do not easily gain or lose electrons

10. Noble Gas Stability Atoms want to gain stability Atoms will try to gain or lose electrons to have a full valence shell Metals try to lose electrons Non-Metals try to gain electrons

11. Oxidation Numbers An oxidation number represents the number of electrons an atom will typically gain or lose to achieve the octet . For example: The oxidation number of Sodium is +1 The oxidation number of Oxygen is - 2 Some Atoms can have more than one oxidation number

12. Electron Dot Structure Electron-dot structure consists of the element’s symbol representing the nucleus, surrounded by dots representing the element’s valence electrons.

13. Lewis Theory G.N. Lewis (1875-1946) recognized valence (outmost) electrons fundamental to bonding electron transfer resulting in ionic bonds sharing electrons resulting in covalent bonds atoms tend to acquire a noble-gas electronic configurations

14. Basic Chemical Bonding Lewis Dot Structure Lewis wrote in a memorandum dated March 28, 1902

15. Lewis Structures Lewis structures show ONLY valence electrons. Examples : Sodium (Na) is in group 1A, so we know it has 1 valence electron. Group 1A is known as the alkali metals Beryllium (Be) is in group 2A, so we know it has 2 valence electrons. Group 2A is known as the alkaline earth metals

16. Lewis Structures Lewis structures for oxygen Oxygen is in group 6A, so it has 6 valence electrons.

17. How to Draw Lewis Structures Look at the element you want to draw the structure for and see what group it is in. D raw dots around the symbol according to how many valence electrons it has. Example: Draw a Lewis Structure for calcium We see calcium ( Ca ) is in group 2A, so it has 2 valence electrons Ca

18. Formation of Cations A positively charged ion Na  Na + + e -

19. Mg  Mg 2+ + 2e -

20. Formation of Anions Negatively charged particles Cl + 1e -  Cl -

21. What is the Lewis electron dot diagram for each ion? Ca 2 + O 2−

22. Solution Having lost its two original valence electrons, the Lewis electron dot diagram is just Ca2+. Ca2+ The O2− ion has gained two electrons in its valence shell, so its Lewis electron dot diagram is as follows:

23. Questions?

24. Thank you for listening! 

25. ASSESSMENT

26. Questions What is a valence electron? Draw Na atom and show the valence electrons. How many valence electrons do the following elements have: Si, C, O, Ar , K What does inert mean? How many valence electrons do atoms like to have? What do “groups” on the periodic table have in common?

27. Questions 1. Draw the Lewis electron dot diagram for each element. bromine gallium 2. Draw the Lewis electron dot diagram for each ion. Mg 2+ S 2−

28. Questions 1. Draw the Lewis electron dot diagram for each element. bromine gallium 2. Draw the Lewis electron dot diagram for each ion. Mg 2+ S 2−