# Molecular structure and bonding

Molecular structure and bonding To understand the formation and structure of molecular compounds, first one has to learn, recognize, use, count, take ...

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

* 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