Please enable JavaScript.
Coggle requires JavaScript to display documents.
Covalent Structures (Molecular Geometry (Linear (In a linear model, atoms…
Covalent Structures
-
-
Lewis Structure
Lewis (electron dot) structures show all the valence electrons in a covalently bonded species. When the Lewis structure of an ion is written, the entire structure is placed in brackets, and the charge is written as a superscript on the upper right, outside the brackets.
Electron pair is represented as two dots or by two crosses. 
The total number of electrons represented in a Lewis structure is equal to the sum of the numbers of valence electrons on each individual atom. Non-valence electrons are not represented in Lewis structures.
Once the total number of available electrons has been determined, electrons must be placed into the structure. They should be placed initially as lone pairs: one pair of dots for each pair of electrons available. Lone pairs should initially be placed on outer atoms (other than hydrogen) until each outer atom has eight electrons in bonding pairs and lone pairs; extra lone pairs may then be placed on the central atom. When in doubt, lone pairs should be placed on more electronegative atoms first.
In terms of Lewis structures, formal charge is used in the description, comparison, and assessment of likely topological and resonance structures by determining the apparent electronic charge of each atom within, based upon its electron dot structure, assuming exclusive covalency or non-polar bonding. In general, the formal charge of an atom can be calculated using the following formula, assuming non-standard definitions for the markup used: C = N - U - B/2
where:
- C is the formal charge.
- N represents the number of valence electrons in a free atom of the element.
- U represents the number of unshared electrons on the atom.
- B represents the total number of electrons in bonds the atom has with another.
Octet Rule
The "octet rule" refers to the tendency of atoms to gain a valence shell with a total of eight electrons.
Some atoms, like Be and B, might form stable compounds with incomplete octets of electrons.
Resonance Structures
-
The ozone molecule is represented by two resonance structures. In reality the two terminal oxygen atoms are equivalent and the hybrid structure is drawn on the right with a charge of −1⁄2 on both oxygen atoms and partial double bonds with a full and dashed line and bond order 1 1⁄2.
C6H6- BENZENE
Shapes of species are determined by the repulsion electrons pairs according to the valence shell electron pair repulsion (VSEPR) theory.
Molecular Geometry
The molecular geometry can be determined by various spectroscopic methods and diffraction methods. IR, microwave and Raman spectroscopy can give information about the molecule geometry from the details of the vibrational and rotational absorbance detected by these techniques. X-ray crystallography, neutron diffraction and electron diffraction can give molecular structure for crystalline solids based on the distance between nuclei and concentration of electron density.
Molecular geometry is the three-dimensional arrangement of the atoms that constitute a molecule. It influences several properties of a substance including its reactivity, polarity, phase of matter, color, magnetism and biological activity. The angles between bonds that an atom forms depend only weakly on the rest of molecule.
Linear
In a linear model, atoms are connected in a straight line. The bond angles are set at 180°. For example, carbon dioxide and nitric oxide have a linear molecular shape.
Trigonal Planar
Molecules with the trigonal planar shape are somewhat triangular and in one plane (flat). Consequently, the bond angles are set at 120°. For example, boron trifluoride.
Bent
Bent or angular molecules have a non-linear shape. For example, water (H2O), which has an angle of about 105°. A water molecule has two pairs of bonded electrons and two unshared lone pairs.
Tetrahedral
Tetra- signifies four, and -hedral relates to a face of a solid, so "tetrahedral" literally means "having four faces". This shape is found when there are four bonds all on one central atom, with no extra unshared electron pairs. In accordance with the VSEPR (valence-shell electron pair repulsion theory), the bond angles between the electron bonds are arccos(−1/3) = 109.47°. For example, methane (CH4) is a tetrahedral molecule.
Trigonal Pyramidal
A trigonal pyramidal molecule has a pyramid-like shape with a triangular base. Unlike the linear and trigonal planar shapes but similar to the tetrahedral orientation, pyramidal shapes require three dimensions in order to fully separate the electrons. Here, there are only three pairs of bonded electrons, leaving one unshared lone pair. Lone pair – bond pair repulsions change the bond angle from the tetrahedral angle to a slightly lower value. For example, ammonia (NH3).
Octahedral
Octa- signifies eight, and -hedral relates to a face of a solid, so "octahedral" means "having eight faces". The bond angle is 90 degrees. For example, sulfur hexafluoride (SF6) is an octahedral molecule.
-
-
-
-
Molecular Polarity
The polarity of molecules is distinct from the polarity of individual bonds; a non-polar molecule may have polar bonds.
Deduction of Molecular Polarity