just one electron, and its Lewis structure looks like this:
The letter H lets us know that it’s a hydrogen atom, and the one dot represents its one electron.
Moving on to the Lewis structure for a molecule, let’s look at the Lewis structure for F2:
Here the two Fs let us know there are two fluorine atoms. The line connecting them shows that they are bonded with a single bond (containing two electrons). Each has six more electrons surrounding it, and these electrons are nonbonding.
And finally for a molecule with more than one bond, CH2O:
This molecule is called formaldehyde. The Lewis structure shows us that the carbon is involved in a single bond (sharing two electrons) with each hydrogen atom, and a double bond (sharing four electrons) with the oxygen atom. The oxygen atom also has four nonbonding electrons.
What is a “stable octet”?
The term “stable octet” describes the fact that many atoms in molecules are most stable when the valence shell contains effectively eight electrons. This counts both non-bonding electrons and electrons in chemical bonds between atoms. Molecules tend to be most stable when the valence shells of each atom in the molecule contain eight electrons. In the Lewis structures for F2 and CH2O (see the previous question), we see that the fluorine, carbon, and oxygen atoms are each surrounded by eight electrons. We get this total by adding both the nonbonding and bonding electrons. Since hydrogen atoms are in the first row and have just a single orbital in their valence shell, they only need two electrons (a single bond) to fulfill their analogue of a stable octet.
What is electronegativity?
Electronegativity is a property that describes the tendency of an atom to attract electrons in a chemical bond. The most electronegative atoms are those which “pull” hardest on the electron density they share in a bond with another atom. There is more than one scale and definition for electronegativity, and our description here follows that given by Linus Pauling, which is the most commonly used scale in chemistry courses. Electronegativity can most readily be described in terms of the number of protons in the nucleus of the atom and the distance to which its valence electron cloud extends away from the nucleus. As a general trend, the most electronegative atoms are those with the shortest distance between the valence electrons and the nucleus. Electronegativity isn’t a physical quantity that can be directly measured, but several scales have been developed that derive values for this property based on other measurable physical quantities.
What is polarity and how is it related to molecular structure?
Polarity is related to the symmetry of the arrangement of electron density in a molecule. Polar molecules are those which possess a net dipole moment, which means that the electron density is not symmetrically distributed in all directions. Nonpolar molecules have the electron density distributed in such a way that there is no net dipole moment. Typically this doesn’t mean that nonpolar molecules have their electron density distributed evenly over every part of the molecule, but rather that the dipole moments created by an unequal sharing of electrons in each individual bond cancel each other out, so that there is no net direction in which an asymmetry of electron density exists.
What is the charge of a molecule?
The overall charge of a molecule is determined by the number of protons and electrons in the whole molecule. If there are more protons than electrons, the molecule will possess an overall positive charge. If there are more electrons than protons, the molecule will similarly possess an overall negative charge. A molecule with the same number of electrons and protons is neutral and has no net charge.
How are formal charges different?
Formal charges are given for individual atoms within molecules. These are determined by dividing the electrons in every bond equally between the atoms that share them, regardless of the elements involved. Textbooks typically follow this somewhat obtuse statement with an equation (which always helps, right?) like this:
Formal Charge = Group Number – Nonbonding Electrons – ½ Bonding Electrons
Let’s work through this with an example, starting with carbon monoxide:
Carbon is in Group 4 of the periodic table; it has two nonbonding electrons (the two dots shown), and since there are three bonds to oxygen, there are six bonding electrons. So the formal charge is 4 – 2 – ½ (6), or –1. Oxygen is in Group 6 and has the same number of nonbonding and bonding electrons as carbon does in this example. The formal charge on oxygen is therefore 6 – 2 – ½ (6), or +1. Carbon monoxide has no net (or total) charge (because 1 + – 1 = 0), but the individual atoms do have formal charges.
What is Coulomb’s law?
Coulomb’s Law tells us the force experienced by a pair of separated charges. It’s a fundamental equation in the study of electrostatics, which is a broad area of physics concerned with the interactions between stationary charges. The equation for this force can be written:
where charges q1 and q2 are separated by a distance r12 and have a “unit of charge” defined by:
in which z is the charge in Coulomb’s and ε0 is the permittivity of free space, a fundamental physical constant.
The key features of Coulomb’s Law are that it predicts an attractive force between particles of opposite charge and that this force decreases with the square of the distance between the particles. For chemistry, it’s relevant to point out that the force between charges falls off rather slowly with the distance between them, so where charges are present in relatively dense materials (like liquids and solids), they have a significant effect on their environment.
What is a dielectric constant?
The dielectric constant of a material characterizes the extent to which it insulates against the flow of charge or against the effects of an electric field. Materials with a high dielectric constant screen the effects of charges within the material, while materials with a low dielectric constant allow the effects of a charge to be felt more strongly. In solutions containing ions, the dielectric constant of the solution will determine the extent to which the other molecules in the solution feel the effects of the charges present. The lowest possible dielectric constant exists in a vacuum in which there is no material present to screen the charge of a field.
The top graphic (A) illustrates the pi-orbital formation from two p-orbitals; the bottom graphic (B) illustrates the formation of sigma- and pi-molecular orbitals from two sp2
