Section 5.2— Drawing Molecules

1. Section 5.2—Drawing Molecules • Objectives: • use the periodic table to identify the number of valence electrons in an atom • draw Lewis Dot structures for ionic and covalent compounds • introduce the idea of isomers

2. Drawing Molecules on Paper • Lewis Structures (or Dot Structures) are one way we draw compounds on paper • Since paper is 2-D and compounds aren’t, it’s not a perfect way to represent how atoms bond…but it’s a good way to begin to visualize compounds

3. Drawing Ionic Compounds

4. How do we determine the number of valence electrons in an atom? The number found with the “A” in the group number IS the number of valence electrons. 1 2 3 4 5 6 7 8

5. Remember: • Put electrons on each side of the element’s symbol. • Put one in each spot before doubling up!

6. Practice: Oxygen Oxygen is in group VIA or 6A. There are 6 valence electrons. Example: Draw the Lewis Structure for an oxygen atom O

7. To Draw Ionic Compounds • Draw each atom • Transfer electrons from metal atoms to non-metal atoms, keeping track of their new charge

8. Practice: KCl Potassium has 1 electron Chlorine has 7 electrons Example: Draw the Lewis Structure for KCl Cl K

9. Example: Draw the Lewis Structure for KCl -1 +1 K Cl

10. Add more atoms if needed Example: Draw the Lewis Structure the ionic compound of Barium fluoride

11. Barium has 2 electrons Fluorine has 7 electrons Example: Draw the Lewis Structure the ionic compound of Barium fluoride Ba F The fluorine is full, but the Barium isn’t!

12. Another fluorine atom is needed! Example: Draw the Lewis Structure the ionic compound of Barium fluoride Ba F F

13. Now all have full valence shells and the charges are balanced, just as when you learned to write in BaF2 in Chapter 2! Example: Draw the Lewis Structure the ionic compound of Barium fluoride -1 +2 Ba F -1 F

14. A note about Ionic Dot Structures • The atoms are not sharing the electrons—make sure you clearly draw the atoms separate!

15. Practice Problems Draw Lewis Dot Structures for compounds that form between the following atoms. What are the formulas for the compounds? • Al and Cl • Ba and O • Na and P

16. Drawing Covalent Compounds

17. Arrange the Atoms • Atoms found in the middle of covalent compounds are the atoms with the LEAST electron affinity. • Hydrogen & Halogens (F, Cl, Br, I) can only bond with one other atom—they can’t go in the middle of a molecules • Always put them around the outside • In general, write out the atoms in the same order as they appear in the chemical formula

18. Next: • Use the periodic table to decide how many electrons are around each atom • Write the electrons around each atom

19. Practice: CH4 Carbon is in the middle – the H’s are around it. Carbon has 4 electrons Each hydrogen has 1 Example: Draw the Lewis Structure for CH4 H H H C H

20. Any electron that is being shared between two atoms gets to be counted by both atoms! • All atoms are “full” with 8 valence electrons (except H—it can only hold 2) • Each pair of electrons shared by two atoms forms a bond • A bond can also be represented by a single dash H C H H H

21. Sometimes . . . • Sometimes, you don’t end up having a full valence shells for all atoms in the compound.

22. Practice: CH2O • C has to go in the middle – the O and 2 H’s are around it. • C has 4 electrons, oxygen has 6 and each H has 1. Example: Draw the Lewis Structure for CH2O H O H C

23. Note: the carbon and oxygen only have 7 each • However, they each have a single, unshared electron. • They could share those with each other! H Example: Draw the Lewis Structure for CH2O O H C

24. Now the carbon and oxygen both have 8 valence electrons! (And the H’s have 2!) H Example: Draw the Lewis Structure for CH2O O H C

25. Double Bonds • Double bonds are when 2 pairs of electrons are shared between the same two atoms Double Bond

26. Lone Pairs • Lone pairs are a pair of electrons not shared—only one atom “counts” them Lone pair

27. And when a double bond isn’t enough… • Sometimes forming a double bond still isn’t enough to have all the valence shells full

28. Practice: C2H2 • Both C’s go in the middle and 1 H goes on each side. • The C’s have 4 electrons each; the H’s have 1. Example: Draw the Lewis Structure for C2H2 H C H C

29. Each carbon atom only has 7 electrons…not a full set of 8! • But they each have an un-paired electron left! Example: Draw the Lewis Structure for C2H2 H C H C

30. Each carbon atom only has 7 electrons…not a full set of 8! • But they each have an un-paired electron left! Example: Draw the Lewis Structure for C2H2 H C H C Now they each have 8 electrons!

31. H C H C Triple Bonds • A triple bond occurs when two atoms share 3 pairs of electrons Triple Bond

32. Shorter bonds (atoms closer together) Stronger bonds (takes more energy to break) Properties of multiple bonds Single Bond Double Bond Triple Bond

33. Practice Problems Draw Lewis dot structures for: • H2O • O2 • HCN

34. Special Notes

35. Polyatomic Ions • They are a group of atoms bonded together that have an overall charge • Polyatomic ions have a covalent bond within themselves… • But an ionic bond with other ions

36. H C C C H H H H H C C C H H Isomers • For many compounds, there is more than one correct way to draw a Lewis Dot Structure Chemical Formula: C3H4 Contains 2 sets of double bonds between carbons Both structures have full valence shells! Chemical Formula: C3H4 Contains 1 triple bond and 1 single bond between carbons

37. H C C C H H H H H C C C H H Both are “correct” • The chemical formula alone does not give you enough information to differentiate between the two structures Chemical Formula: C3H4

38. Isomers • Isomers: Structures with the same chemical formula but different chemical structure • Atoms can be bonded differently (multiple versus single bonds) or in a different order) but have the same overall chemical formula in isomeric structures.