Chemical Bonding

Holding atoms together

Chemical Reactivity

• Octet Rule – atoms lose or gain electrons to fill their outer s and p orbitals with 8 electrons to be stable

– Noble gases – stable (least reactive)

– Alkali Metals and Halogens – most reactive

– All other elements – varied reactivity

How reactive an element is depends on how close it is to having a full octet. The closer it is, the more reactive.

Valence Electrons and Ions

• Valence Electrons – the electrons in the outer most s and p orbitals (energy level)

• Ion – atom that gained or lost electron(s)

– Cation – ion with positive charge (lost e-)

– Anion – ion with negative charge (gained e-)Show them how to determine the number of valance electrons using the periodic table

The outer most s and p orbitals are the same as the outer most energy level, they are also sometimes called the valance shell

Metals vs. Nonmetals

• Metals form cations (+ ions) by losing electron– Cations are smaller than their parent atoms

More protons have an even stronger pull on the fewer electrons pulling the electron cloud tighter to the nucleus making the ion smaller than the parent atom

• Nonmetals form anions (- ions) by gaining electrons– Anions are larger than their parent atoms

more electrons than protons means that the extra electrons are very loosely held and can travel far from the nucleus making the electron cloud larger and the ion is larger than the parent atom

Stable Ions have Noble-Gas Electron Configurations

• Ca =

• Ca+2 =

• Ar =

• N =

• N-3 =

• Ne =

Ions & Parent Atoms• Ions and their parent atoms have different

chemical properties.

• Ions and atoms chemical properties depend on their number of valence electrons

• Since Na and Na+1 have a different number of valence electrons they have different propertiesNa burns when placed in water, but Na+ does not and is used in our bodies all the time

• Since Na and Na+1 have the same atomic number, they are the same elementSame number of protons which identifies the element

Now ask the K banana question!

Polyatomic ions – ion made of 2 or more atoms bonded together (pg 238)

Ionic Bonds• Strongest type of chemical bond

• First, electrons are transferred from one atom to another to form cations and anions

• Then, ions of opposite charge attract each other

Ionic Compounds • Any compound involving a cation and anion

– aka – salt – Conduct electricity when melted or dissolved

in water

Electricity can only be conducted when charged particles (like ions) are free to move around. They cannot move when they are in solid form

Ionic compounds don’t form moleculesCrystal Lattice – repeating pattern of ions (different

pattern for different compounds)

– Very high melting points

– Very hard & brittle

Naming Ionic Compounds

• Simple Ionic

• Ionic with transition metals

• Compounds containing polyatomic ions

Covalent Bonding

• Covalent bond – When atoms fill their outer energy level with 8 electrons by SHARING electrons

• Molecular orbital – area between two atoms where the electrons are being shared

Polar vs Nonpolar Covalent

• Nonpolar Covalent Bonds – the bonding electron pairs are shared equally

• The diatomic elements

• Polar Covalent Bonds – the bonding electrons are not shared equally, instead they are pulled closer to one atom

• Water

Dipole – molecule with partial positive and partial negative regions (polar covalent compounds)

Electronegativity – the ability of an atom in a chemical compound to pull shared electrons closer to itself

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Classifying Bond TypesThe difference in electronegativity values of two

bonded atoms determines what type of bond has formed between them

– less than 0.5 = nonpolar

– from 0.5 to 2.1 = polar

– greater than 2.1 = ionicchange numbers for Honors class

Covalent CompoundsMolecule – the smallest unit of a covalent compound

Most of the Time…

• Ionic bonds form when a metal bonds with a nonmetal

• Covalent bonds form when two nonmetals bond together

Intermolecular attractive forces

• Holds molecules together in solid and liquid states– Hydrogen bonding– Van der Waals forces– Dipole-dipole forces

• Much weaker than ionic & covalent bonds– Molecular compounds have much lower melting

points

Naming Covalent Compounds

• Binary compounds

• Acids

Metallic Bonding

• Atoms in a sea of electrons

Lewis StructuresRepresentation of the valence electrons around

an atom

Drawing Lewis Structures of Compounds

1. Draw individual Lewis Structure for each atom in compound

2. Count number of valence electrons

3. Arrange atoms: atom with most unpaired electrons in center, other atoms around it with their unpaired electrons facing central atom

4. Circle electrons that form bonds

5. Redraw structure replacing circled electron pairs with long dashes (chemical bonds)

6. Count number of electrons in structure, make sure it has the same number as step 2, make sure all atoms except hydrogen satisfy octet rule

Examples: K2S AlI3 C2H4 N2

Homework Problems

• Draw Lewis structure for each of the following compounds and write the number of valence electrons for each structure

1. H2 4. HCl 7. H2S 10. CH2Cl2

2. C2H6 5. SCl2 8. AsF3 11. SiH4

3. CHF36. C2H2 9. HCN 12. N2F2