Molecular structure and bonding To understand the formation and structure of molecular compounds, first one has to learn, recognize, use, count, take into account: • the periodic table with groups and periods, • the number of electrons and valence electrons (i.e. count electrons), (2 (K), 8 (L) = 2 + 6, 18 (M) = 2 + 6 + 10, 32 (N) = 2 + 6 + 10 + 14, etc.), • the electronic structure (ground state, exited state), • the need of 2 electrons for a bond, •atomic orbitals and quantum states (with quantum numbers n, m, l, s), • the energy levels of the different shells, subshells, orbitals, • the maximum number of shells, subshells, orbitals, and electrons, • the numbers and/or maximum numbers of bonds, an atom likes to or can build or have, • the Lewis concept/structure.
Schematic representation of atomic shells (n) and subshells/orbitals (s, p, d, f). The orbitals (or quantum states) are defined by the quantum numbers n, m, l, s.
K to Kr Na, Mg, Al, Si, P, S, Cl, Ar Li, Be, B, C, N, O, F, Ne
H, He
Orbital split for n > 1 !
Energy increases with increasing n, and is high between the shells (except d and f). n → size, l → shape, m → orientation, s → spin of e; Pauli principle (not all 4 quantum numbers can be equal), Hund‘s rule (spin maximizing).
Energies of the atomic shells K, L, M, N and the subshells/orbitals s, p, d, f
← Start of 3d elements (Sc)
Atomic number Z
Energy increases with increasing n, and is high between the shells (except d and f). n → size, l → shape, m → orientation, s → spin of e, Pauli principle (not all 4 quantum numbers can be equal), Hund‘s rule (spin maximizing).
It is useful to learn also the maximum number of shells (n), subshells/orbitals (s, p, d, f), and electrons an atom can have. Shell
n
Orbital
Nunber of orbitals
Number of electrons in orbital shell (2n2)
The next step is to find the numbers and/or maximum numbers of bonds, an atom likes to or can have, e.g. in hydrogen compounds of group 4 to 8 elements. Main group
Electron * configuration Number of possible bonds Existing H compouns Lewis formula
/4*
none
use – instead of :
* Promotion/exitation of an 1s electron of C, Si to 1p needs a lot of energy (460 kJ/mol for C), but is compensated by the formation of 2 additional bonds (CH4 instead of CH2). For promotion to a d level, the energy is to high.
Electron configuration and number of bonds for elements of 2. period Atom or ion
Electron configuration
Number of bonds
Number * of electrons
Examples
Elements of 2. period can build not more than 4 bonds, because there are only 4 orbitals available → octett rule. Consider ground states (Pauli, Hund), exited states. * Number of electrons in bonds
Electron configuration and number of bonds for elements of 3. period Atom or ion
Electron configuration Number Number * Examples of bonds of electrons
For n > 2 one can have more than 4 bonds, because there are empty low-level d orbitals avaiable. For elements of 3. period, the maximum number of bonds is 7. * Number of electrons in bonds
Highest number of bonds is equivalent to the main group number. Element
Group
P S Cl
5 6 7
Some examples: SiF62-, PF5, SF6 do exist CF62-, NF5, OF6 do not exist
Number of bonds 3, 5 2, 4, 6 1, 3, 5, 7
Examples: Lewis structure of N2, NH3, NO2-, NO3-, S2, SO2, SO3, SO32. Those which are not necessarily resonance structures/hybrids are marked with an asterisk. → Octett rule, hypervalence, formal charges
- are used instead of : * No other resonance structure
