Chapter 6.2 – Ionic and Covalent Bonding
Objectives
- Why do atoms form bonds?
- How do ionic bonds form?
- What do atoms joined by covalent bonds share?
- What gives metals their distinctive properties?
- How are polyatomic ions similar to other ions?
Chapter 6.2 – Ionic and Covalent Bonding
atoms bond when their valence electrons interact so that each atom has a
full or stable electron configuration
- each atom wants to fill its outer s and p sublevels
- 2 basic kinds of compounds
- each type of compound has a different kind of bond
Chapter 6.2 – Ionic and Covalent Bonding
1. ionic compounds
- formed from ions
- have ionic bonds – an attractive force between oppositely charged
ions, which form when electrons are transferred from one to another
- one atom loses one or more electrons and another atom or atoms gains
them
- the oppositely charged ions are then attracted to each other and form
an ionic bond
Chapter 6.2 – Ionic and Covalent Bonding
ionic compounds form strong networks
- because they do not form individual molecules, to
write the chemical formulas use the smallest ratio of
one ion to another, called the formula unit
NaCl 1:1
Na2O 2:1
AlBr3 1:3
- smallest ratio means they will not be divisible by
each other and get a whole number
- will never have an ionic compound with a 2:2, 6:3,
or 2:4 ratio
Chapter 6.2 – Ionic and Covalent Bonding
ionic bonds are very strong
- therefore ionic compounds are solid at room temperature and have
high melting and boiling points
- solid ionic compounds do not conduct electricity because their
electrons are held tightly by the ions
- when melted or dissolved in water, the ions can move and will conduct
electricity
Chapter 6.2 – Ionic and Covalent Bonding
- ionic compounds are neutral
- can tell from their formula units
NaCl Na+ Cl- 1:1 ratio
need 1 Na+ for every Cl- to be neutral
MgF2 Mg2+ F1:2 ratio
need 2 F- for every Mg2+ to be neutral
Na2S Na+ S2- 2:1 ratio
need 2 Na+ for every S2- to be neutral
Chapter 6.2 – Ionic and Covalent Bonding
Covalent compounds –
have covalent bonds – bonds formed when atoms share one or
more pairs of electrons
- electrons are shared so that both atoms have full outer energy levels
- also called molecules
- low melting and boiling points
- do no conduct electricity
- formed between nonmetals
Chapter 6.2 – Ionic and Covalent Bonding
- the lines in a structural formula show covalent bonds
- each line represents 2 electrons that are shared
- dots around atoms represent unshared electrons
- pairs of electrons stay together in structural formulas
Chapter 6.2 – Ionic and Covalent Bonding
atoms can share multiple electrons
- when this happens the bonds are stronger
- when two pairs of electrons are shared it is called a double bond
- when three pairs of electrons are shared it is called a triple bond
- when only one pair of electrons are shared it is called a single bond
Chapter 6.2 – Ionic and Covalent Bonding
atoms do not always share electrons equally
- when electrons are not shared equally the electrons spend more time
near one atom than the other
- this gives the atoms small charges and the bond is called polar
polar covalent bond –
bond where electrons are shared unequally and the atoms have
partial positive and negative charges
ex. H2O, HF, NH3, HCl
Chapter 6.2 – Ionic and Covalent Bonding
when atoms share electrons equally the bond is nonpolar
nonpolar covalent bond –
bond where electrons are shared equally and the atoms are not
partially charged
ex. H2, Cl2, N2, CH4, CO2
Chapter 6.2 – Ionic and Covalent Bonding
Metals form metallic bonds –
bonds between metal cations and the sea of electrons around them
- the nuclei form a closest packing structure
- the electrons flow around them and do not belong to any one atom
- there is a sea of freely moving electrons
- this allows metals to flex into sheets or wires without breaking
- very strong bonds
- high melting and boiling points
- conducts electricity well
Chapter 6.2 – Ionic and Covalent Bonding
polyatomic ion –
a group of covalently bonded atoms that have a charge
- form ionic bonds and ionic compounds with other atoms
- acts as a single ion in a compound
- polyatomic ions can be positive or negative
- polyatomic ions can combine with other ions or other polyatomic ions
Chapter 6.2 – Ionic and Covalent Bonding
- when more than one of a specific polyatomic ion appears in a
compounds name put parentheses around
(NH4)2S
Al2(SO4)3
Fe3(PO4)2
- the charge of a polyatomic ion applies to the whole ion
- many polyatomic ions have oxygen in them
- polyatomic ions with many oxygen atoms end with –ate
- polyatomic ions with few oxygen atoms end with -ite