Molecular Compounds

1. Molecular Compounds

2. Bonding Review - Compounds • Which statement(s) is(are) true about compounds? A) They have to contain at least 2 different kinds of elements (like Na, S) B) The combination is of a specific ratio of atoms. (as in Na2S) C) The combination of atoms in a compound can be predicted knowing # of valence electrons.(Na has 1, S has 6)

3. Answer • All 3 are correct. • A compound is composed of 2 or more elements bonded together in a certain ratio. • The ratio IS able to be figured out knowing how many valence electrons each type of atom has.

4. Element types and bonding MgCl2 - Mg is a metal, Cl is a non-metal (ionic) SO2 - S and O are both nonmetals, molecular compound (using covalent bonds to hold atoms together).

5. Practice • A) LiBr • B) SBr2 • C) Li2SO4 • A) ionic • B) molecular • C) ionic (polyatomic SO4 is an ion silly)

6. Molecular Compounds • Substances consisting of atoms that are covalently bonded • Covalent bonds • Made through electron sharing among atoms • Electrons are not transferred • Sharing allows substance to achieve stability (Noble gas configuration) • Sharing forms molecules

7. Molecules • Are held together by the attraction of electrons of one atom and the nucleus of a second atom • A single bond forms from a single pair of shared electrons • Two pairs of electrons form a double bond • Paired electrons have opposite spins and occupy less space than a pair of electrons surrounding only one atom • Their bonds are flexible, somewhat like springs

8. Ionic compounds Covalent compounds

9. Covalent Compound Properties • Lower melting and boiling points vs. ionic • Molecules have no charge, hence do not conduct heat or electricity in any state. • Melting molecular compounds • Does NOT separate the clusters of atoms within a molecule, • Separates just molecules from each other

10. Naming Binary Molecular Compounds • Molecular compound names include prefixes that indicate the number of atoms in the molecule • They are composed of nonmetal elements

11. To name molecular compounds: • Write the name of the least electronegative element with a prefix to indicate the number of atoms of the element that are present • Write the name of the most electronegative element second with a prefix to indicate the number of atoms of the element that are present • Suffix –ide is added to the name of the last element

12. More rules . . . • The prefix mono- is NOT written with the first word of a compound’s name • Some prefixes are sometimes shortened to make a name easier to say

14. Examples H2O is dihydrogen monoxide NH3 N2H4 phosphorus pentachloride nitrogen monoxide carbon tetrachloride

15. What is an Acid? • An acid is a molecular substance that dissolves in water to produce hydrogen ions • They are molecular compounds that separate into a cation and an anion in water (ionic tendency)

16. Naming Acids • Binary acids: • The name begins with the prefix hydro- • The name is derived from the anion • The suffix –ide should be changed to –ic • Polyatomic acids: • Do not use prefix hydro- • The suffix –ate should be changed to –ic • The suffix –ite should be changed to –ous

17. Examples • H2Se is hydroselenic acid • HCl • HNO3 • H2SO4 • nitric acid • phosphoric acid • oxalic acid • hydroiodic acid

18. What are Lewis Dot Structures? • Use the chemical symbol to represent the nucleus and inner energy levels • Uses dots to represent valence electrons • Types of bonds: • single bonds share 1 pair of electrons (also known as sigma) • double bonds share 2 pairs of electrons (also known as pi) • triple bonds share 3 pairs of electrons (one sigma and two pi bonds)

19. Rules for Drawing Lewis Dot Structures • See handout

20. Molecular Shapes • VSEPR: Valence Shell Electron Pair Repulsion theory– system for predicting molecular shape based on the idea that pairs of electrons orient themselves as far apart as possible • Can only really be used with simple molecules

21. Terms • Structural formula: indicates the spatial arrangement of atoms and bonds within a molecule • Ligand: an atom attached to the central atom • Unshared pairs: pairs of electrons that are not involved in covalent bonding, but instead belong exclusively to central atom

23. Linear Bent Trigonal pyramidal Trigonal Planar

24. T-shaped Tetrahedral see-saw Square planar

25. Trigonal bipyramidal Square pyramidal Octahedral http://chemlab.truman.edu/CHEM121Labs/MolecularModeling1.htm

26. Polarity • Electrons are not always shared equally in molecules • Creates a partial charge within the molecule • Atoms with uneven electronegativities share electrons unequally • The greater the difference, the greater the polarity • Polar – “having opposite ends”; one atom attracts electrons more strongly than the others • Nonpolar – doesn’t have opposite ends; electrons shared equally among bonding atoms

27. Examples CO2: Is a symmetrical molecule; therefore it is nonpolar H2O: H = 2.20, O = 3.44 3.44 – 2.20 = 1.24 Water molecules are asymmetrical, so the molecule is polar covalent, with the electrons concentrating around the O atom (higher electronegativity)

28. Differences in Electronegativity

29. Diatomic Molecules • A diatomic molecule is a molecule formed from two identical atoms • The atoms join together because they are more stable that way than if they exist as single atoms • Remember HOFBrINCl • H2, O2, F2, Br2, I2, N2, and Cl2