Science league topic 6: writing Lewis dot formula

1. Science league topic 6: writing Lewis dot formula

2. Valence Electrons • the electrons in all the subshells with the highest principal energy shell are called the valence electrons • electrons in lower energy shells are called core electrons • chemists have observed that one of the most important factors in the way an atom behaves, both chemically and physically, is the number of valence electrons Tro, Chemistry: A Molecular Approach

3. Predicting Valence Electrons • The Roman numeral in the American convention indicates the number of valence electrons. • Group IA elements have 1 valence electron • Group VA elements have 5 valence electrons • When using the IUPAC designations for group numbers, the last digit indicates the number of valence electrons. • Group 14 elements have 4 valence electrons • Group 2 elements have 2 valence electrons

4. Lewis Electron Dot Formulas • An electron dot formula of an elements shows the symbol of the element surrounded by its valence electrons. • We use one dot for each valence electron. • Consider phosphorous, P, which has 5 valence electrons. Here is the method for writing the electron dot formula.

5. The octet of electron configuration • If an atom, or an ion contain the 8e- in the outermost shell, it is called octet configuration. • All the atoms want to have the noble gas configuration by forming ions or covalent bonds with others. • For period 2, the full shell of the outermost electrons are 8 e- (2S22p6). • Only C, N, O and F obey octet rule restrictly.

6. Lewis Structures Lewis structures are representations of molecules showing all electrons, bonding and nonbonding. One single bond “―” represent a pair of e-.

7. F F •• •• •• • • •• F F •• • • • • H H O •• •• •• •• •• •• •• •• •• F F •• H H •• O •• •• •• Single Covalent Bonds • two atoms share a pair of electrons • 2 electrons • one atom may have more than one single bond Tro, Chemistry: A Molecular Approach

8. •• •• • • • • O O •• •• •• •• O •• •• •• •• O Double Covalent Bond • two atoms sharing two pairs of electrons • 4 electrons Tro, Chemistry: A Molecular Approach

9. •• •• • • • • N N • • N N •• •• •• •• •• Triple Covalent Bond • two atoms sharing 3 pairs of electrons • 6 electrons Tro, Chemistry: A Molecular Approach

10. Find the sum of valence electrons of all atoms in the polyatomic ion or molecule. If it is an anion, add one electron for each negative charge. If it is a cation, subtract one electron for each positive charge. PCl3 Writing Lewis Structures 5 + 3(7) = 26

11. Writing Lewis Structures • The central atom is the least electronegative element that isn’t hydrogen. Connect the outer atoms to it by single bonds. Keep track of the electrons: 26  6 = 20

12. Writing Lewis Structures • Fill the octets of the outer atoms. Keep track of the electrons: 26  6 = 20  18 = 2

13. Writing Lewis Structures • Fill the octet of the central atom. Keep track of the electrons: 26  6 = 20  18 = 2  2 = 0

14. Writing Lewis Structures • If you run out of electrons before the central atom has an octet… …form multiple bonds until it does.

15. Writing Lewis Structures • Then assign formal charges. • For each atom, count the electrons in lone pairs and half the electrons it shares with other atoms. • Subtract that from the number of valence electrons for that atom: The difference is its formal charge.

16. Writing Lewis Structures • The best Lewis structure… • …is the one with the fewest charges. • …puts a negative charge on the most electronegative atom.

17. Writing Lewis Structures of Molecules HNO3 • Write skeletal structure • Make least electronegative atom central. In oxyanion and oxyacid, the key element is always central with O linked to it. • N is central • H always terminal • in oxyacid, H outside attached to O’s N = 5 H = 1 O3 = 3∙6 = 18 Total = 24 e- • Count valence electrons • sum the valence electrons for each atom • add 1 electron for each − charge • subtract 1 electron for each + charge

18. Writing Lewis Structures of Molecules HNO3 • Attach central atom to the surrounding atoms with pairs of electrons and subtract from the total Electrons Start 24 Used 8 Left 16

19. Writing Lewis Structures of Molecules HNO3 • Complete octets, outside-in • H is already complete with 2 • 1 bond and re-count electrons N = 5 H = 1 O3 = 3∙6 = 18 Total = 24 e- Electrons Start 24 Used 8 Left 16 Electrons Start 16 Used 16 Left 0 Tro, Chemistry: A Molecular Approach

20. Writing Lewis Structures of Molecules HNO3 • If all octets complete, give extra electrons to central atom. • elements with d orbitals can have more than 8 electrons • Period 3 and below • If central atom does not have octet, bring in electrons from outside atoms to share • follow common bonding patterns if possible Tro, Chemistry: A Molecular Approach

21. CO2 SeOF2 NO2-1 H3PO4 SO3-2 P2H4 Practice - Lewis Structures Tro, Chemistry: A Molecular Approach

22. CO2 SeOF2 NO2-1 H3PO4 SO3-2 P2H4 : : :O::C::O: Practice - Lewis Structures 16 e- 32 e- 26 e- 26 e- 18 e- 14 e-

23. Homework • Page 400: 53, 54, 59 a, b, c, e and f.