Bonding

Unit Overview Types of bonds Why/how bonds form Lewis structures (again) Writing chemical formulas Naming compounds

Chemical Bonds link between atoms due to mutual attraction of nuclei for e-

Chemical Bonds Why bond??? Bonding can result in lower potential energy (this is usually associated with a release of energy) Lower energy = greater stability (greatest stability = completed energy level/octet)

Chemical Bonds Octet rule – in forming compounds, each atom, by gaining/losing/sharing e- achieves an octet of e- in its highest occupied energy level

Chemical Bonds Bonds are classified by how the valence e- are distributed around nuclei of combined atoms

Types of Chemical Bonds Ionic bond – results from electrostatic attraction between positive and negative ions (usually done when metal bonds w/ nonmetal)

Types of Chemical Bonds Ionic bond Created by transfer of e- Active metals readily give up their e-, usually to a nonmetal atom Oppositely charged ions are formed by this process of transferring e- Cation: + charge Anion: - charged ion

Types of Chemical Bonds Ionic bond Forming ionic bonds: e- are transferred from atom with lower EN value to one with higher value How can you determine if bond is ionic? See if atoms are active metal & non-metal or look up EN values to get difference. (if EN difference is 1.7  4.0, ionic bond)

Types of Chemical Bonds Ionic bond Types of ions Monoatomic – single element with charge Polyatomic – 2 or more elements with charge

Types of Chemical Bonds Ionic bond Ionic compound – composed of positive & negative ions combined so that the positive & negative charges are equal in number (Ex. NaCl instead of Na2Cl or NaCl2)

Properties… Ionic Compounds Held together tightly (attraction of charges) High MP High BP Hard & brittle crystalline solids Dissolve in water (generally) Carry a current (very well) in water

Representing Ionic Compounds Empirical formula or Formula unit – indicates lowest whole number ratio of cations to anions in any sample of an ionic compound (ex. NaF = 1 Na+ ion + 1 F- ion) This “unit” doesn’t represent something that can be isolated, it is only the smallest possible ratio to make a neutral electrical charge # of ions in one formula unit depends on the charges of the ions to be combined (ex. B + F, Na + Cl, K + O, etc)

Representing Ionic Compounds Just a thought… Can you use the periodic table to determine the charge of an ion? Yes, look @ how many e- are in the outer shell (valence e-).

Representing Ionic Compounds Determining formula units by the crisscross & “accounting” methods Ca + Br K + P Al + O Ca + O Al3+ + CN1- Mg2+ + PO43-

Representing Ionic Compounds Lewis structures Diagrams that show valence e- as dots, the inner e- and nucleus are included in the letter symbol for the element being represented

Representing Ionic Compounds Lewis Structures Group 1 Group 2 Group 13 Group 14 Group 15 Group 16 Group 17 Group 18

Representing Ionic Compounds Lewis Structures Structures of individual elements may be joined to form compounds pairs of dots (or a dash) between symbols represent bonds (or electron pairs), dots adjacent only to one symbol are unshared e-

Representing Ionic Compounds Lewis Structures Structures of individual elements may be joined to form compounds Lone (unshared) pair of e- – pair of e- that is not involved in bonding, but instead belongs exclusively to one atom

Representing Ionic Compounds How to represent ionic bonds with Lewis diagrams KI BaCl2 LiBr Rb2O CaO Na3P MgH2

Types of Chemical Bonds Metallic bond – force of attraction that holds metals together; consists of the attraction of free-floating valence e- for positively charged metal ions

Types of Chemical Bonds Covalent (molecular) bond – results from the sharing of e- between two atoms (usually done w/ nonmetal atoms)

Covalent Bonds The e- are not always equally shared (like tug of war) Bonds between 2 unlike atoms are never completely covalent - can represent these bonds by showing partial charges on respective atoms Non-polar covalent – e- are shared equally ( which only happens between two identical atoms) Polar covalent – e- are not equally shared (due to differences in EN)

Covalent Bonds Atoms that are bonded covalently form stable particles called molecules Ex. CO2, H2O2, SO2, O2 7 diatomic molecules to know - H2, N2, O2, F2, Cl2, I2, Br2 first 5 are gas @ room temp, I2 is a solid, & Br2 is a liquid

Covalent Bonds Molecular compound – chemical compound whose simplest formulas are molecules

Properties… Molecular Compounds Most are not tightly held Most have low MP (due to weak attractions) Most have low BP Usually soft, amorphous solids Some dissolve in water Do not carry current well in water

Representing Molecular Compounds Molecular formula – shows the types and numbers of atoms combined in a single molecule of a compound Ex. CO2, H2O2, SO3, O2 Chemical formula – shorthand representation of the composition of a substance using atomic symbols and numerical subscripts

Representing Molecular Compounds Structural formula – indicates kind, number, arrangement, and bonds of the atoms in a molecule (or polyatomic ion) Single bond – one pair of shared e- between two atoms Double bond – two pairs of shared e- between two atoms Triple bond – three pairs of shared e- between two atoms

Representing Molecular Compounds Structural formula – indicates kind, number, arrangement, and bonds of the atoms in a molecule Ex.

Representing Molecular Compounds Lewis Structures Ex. F2, NH3, H2O, CH4, O2, CO2, N2, CH2O, C2H2, PI3 (must draw structures)

Representing Molecular Compounds Lewis structures Octet exceptions: Less than full octet Ex: Boron compounds such as BF3

Representing Molecular Compounds Lewis structures Octet exceptions: More than full octet – atoms beyond 2nd period, most often S & P (extra e- go to 3d level) Ex: SF4

Representing Molecular Compounds Lewis Structures Octet exceptions: Molecules w/odd # of e- - not full octet, short-lived & unstable Ex: NO, SO32-, NH41+