Chemical Bonds
Chemical bonds are the forces that hold atoms together in compounds. They are formed because atoms are not happy with the number of electrons that they have. Only the noble gases (column 8A) are content with the number of electrons. They have the optimum number of electrons and don't like to form chemical bonds. The desire of atoms to gain or lose electrons to get a noble gas number of electrons is what leads to chemical bonding.
(It is quite common for chemists to personify the atoms and molecule with which they work! Saying an atom wants an another electron is akin to saying a ball wants to roll down a hill. Later will we examine the energetic and thermodynamic bases of this personification.)
There are three types of chemical bonds:
- Covalent Bonds - atoms are held together by sharing electrons
- Electrostatic (Ionic) Bonds - cations and anions are held together by electrostatic attractions
- Metallic Bonds - occurs in metals (similar to covalent bonds)
Later, you will be able to determine just how ionic or covalent a bond will be, but for now here are some guidelines to follow:
- Bonds amongst non-metal atoms are covalent. (For example, a P-S bond is a covalent bond.)
- Bonds between a non-metal and a metal are ionic (For example, a Na+Cl- bond is ionic.)
- Bonds amongst metal atoms are metallic
- Metalloid--Non-metal bonds are usually covalent
- Metalloid--Metal bonds are usually ionic
Covalent Bonding
In covalent bonding atoms share electrons.
Take for example the H2 molecule. Each hydrogen atom says, "I only need one more electron to be like a noble gas (helium) ." Since each hydrogen has only one electron, when two hydrogens get together they can share their electrons.
So each hydrogen atom now sees 2 electrons when it is covalently bonded to another hydrogen atom. Pure hydrogen exists as H2 molecules. The same is true for all of the halogens in column 7A:
- Pure chlorine exists as Cl2
- Pure bromine exists as Br2
- Pure iodine exists as I2
Chemists often use the symbol "-" to represent a bond. For example, H-H is a "hydrogen molecule" and Cl-Cl is a "chlorine molecule." The line in between the two atoms means that they are sharing two electrons between them. Let's take oxygen as another example. Oxygen atoms like to combine to form O2. In this case, each oxygen atom wants 2 more electrons, so when the two oxygen atoms get together they share a total of 4 electrons. We write O2 as:
Chemists call this a double bond. By forming a double bond between them, each oxygen atom can then see as many electrons as a Ne atom has.
Now let's look at nitrogen. It also likes to combine to form a diatomic molecule, in this case N2. Each nitrogen atom, however, wants 3 electrons, so two nitrogen atoms share a total of 6 electrons.
We call this a triple bond.
Of course, you can form molecules from more than one type of atom. Let's look at water. H2O consists of two hydrogen atoms sharing their electrons with one oxygen atom.
Another example is hydrogen peroxide, H2O2.
Think about hydrogen peroxide and decide on your own if all of the atoms are happy with the number of electons around them.
Here is one final example. Carbon atoms want to share 4 electrons, so it is very happy if it can get together with 4 hydrogens to form methane, CH4.
In this example, carbon is sharing 4 electrons with 4 hydrogens and each hydrogen is sharing one electron with carbon.
Structural and Empirical Formulas
Structural Formula
To avoid confusion, chemists often write the structural formula when identifying a molecule. The structural formula tells you how many of each type of atom are in a molecule and also how they are connected. For example, here is the structural formula of ethanol.
Chemical Formula
You will also see the term chemical formula. The chemical formula tells you how many of each type of atom are in a molecule. For example, the chemical formula for ethanol is
C2H6O.
Notice that this is less information than the structural formula (but more compact). You must be careful not to confuse substances that have the same chemical formula. For example, ethanol and dimethyl ether have the same chemicial formula (i.e. C2H6O).
Their chemical formulas are identical, but their structural formulas and their physiological effects are markedly different.
Empirical Formulas
An empirical formula (simplest formula) tells us the simplest whole number ratio of atoms in a molecule. When identifying an unknown pure substance, chemists will often start by performing experiments to determine the empirical formula of the substance.
For example, hydrogen peroxide's chemical formula is H2O2, but its empirical formula is HO.
The chemical formula for glucose is C6H12O6, but its empirical formula is CH2O, and its structural formula is
Now, let's try some sample quiz questions on Empirical Formulas:
Molecular Cations and Anions
Molecules can also lose or gain electrons to become cations or anions. For example, the NO3 molecule will gain an electron to form the nitrate anion.
If you count up all of the electrons you'll find that all of the atoms feel like neon.
Here is the ammonium ion, an example of a molecular cation.
The ammonium ion has given up an electron to become a cation.
Ionic Bonds
Ionic bonds are generally formed when you bring atoms which really want to lose electrons together with atoms which really want to gain electrons.
The Na+ cation and the Cl- anion are held together by electrostatic or ionic bonds. There is no sharing in ionic bonding. The anion takes the electron for itself and the cation is happy to get rid of its electron. The ions in ionic compounds are arranged in three-dimensional structures. There are no discrete molecules of NaCl. We can only write an empirical formula of NaCl.
Here are some other examples.
| NH4Cl : | here NH4+ | = cation |
| here Cl- | = anion | |
| BaCl2 : | here Ba2+ | = cation |
| here Cl- | = anion |
All substances are electrically neutral. We can use this fact to obtain the chemical formula of an ionic compound.
| Ba2+ | and | SO42- | form | BaSO4 |
| Na+ | and | S2- | form | Na2S |
Notice that in Na2S, two sodium cations were needed to balance the -2 charge of S2-, making things electrically neutral.