Molecules, ions and naming compounds Atoms combine together to form compounds either by forming ions or molecules

Towards the noble gas configuration  Noble gases are unreactive – they have filled shells  Shells of reactive elements are unfilled  Achieve noble gas configuration by gaining or losing electrons  Metals lose electrons – form positive ions  Nonmetals gain electrons – form negative ions

Predicting ion charges  s and p block elements are easy:  charge = group number for cations  charge = -(8 – group number) for anions

Less predictable for transition metals  Occurrence of variable ionic charge  Cr 2+, Cr 3+, Cr 4+, Cr 6+ etc.  4s electrons are lost first and then the 3d  Desirable configurations coincide with empty, half-filled or filled 3d orbitals  Fe 2+ ([Ar]3d 6 ) is less stable than Fe 3+ ([Ar]3d 5 )

Covalent Bonding Chemical socialism

Bonding between nonmetals: ionic bonding is not an option  Ionic bonds meet requirements of elements in compounds of metals and nonmetals to obtain noble gas configurations  In the vast ocean of compounds involving nonmetals exclusively (all organic compounds) the avenue of electron transfer is not open, since all members tend to form negative ions  Solution: electron sharing

Still electrostatics  Balancing forces:  Attractive forces between nucleus and electrons of different atoms  Repulsive forces between nuclei and between electrons  As the atoms approach, electrons shift from approximate spherical distribution to being localized between the atoms

Chemical formulas  Chemical formula  Description of the atoms in the compound  Molecular formula  For covalent compounds the exact number of each atom in the molecule  Empirical (simplest) formula  The smallest description of the atoms in the compound. The normal way of describing ionic compounds (but not molecular ones)  Structural formula  Shows more detailed arrangements of atoms in the molecule. One molecular formula can yield several structural formulae (isomers)