The word isomer comes from the Greek words iso, meaning "equal," and meros, meaning "part" or "share." The parts of an isomer are the atoms within the compound. Listing all the types and numbers of atoms in a compound yields the molecular formula. Showing how the atoms connect within a compound gives the structural formula. Chemists named compounds consisting of the same molecular formula but different structural formula isomers. Drawing an isomer of a compound is the process of rearranging the places where atoms are bonded in a structure. It is similar to stacking building blocks in different arrangements by following rules.
Visualizing isomers as three-dimensional objects in space may be difficult for some individuals. Ball and stick models or computer programs are available to help people understand the structures of different isomers.
Sometimes when asked to draw an isomer a molecular formula is already given, so counting and identifying is unnecessary. If a molecular formula is already given, skip Step 1. If a structure of a compound is given, do not skip Step 1 and consider the structure to be one of the possible isomers when examining final isomers for mirrored or flipped versions.
If a compound has more than two types of atoms that require different numbers of bonds, continue adding from most to fewest required bonds. If two atoms require the same number of bonds it is acceptable to add in any order.
There are many exceptions to the general column rule for how many bonds an atom of an element may make. The numbers provided in step 2 are guidelines but not solid rules and should only be considered for common elements used in beginner isomer drawing such as C, H, O, N, etc. Students must study orbitals and valence shells to understand exactly how many bonds each element can make. Elements should be researched individually for the number of possible bonds that could be made.
In a branched isomer, it is easy to believe a mirror image of an isomer is a different isomer. If an isomer would have the same structure when reflected in a mirror or flipped any direction then it is the same structure and not a different isomer. Keep track of different isomers by numbering the atoms and monitoring if it could be the same shape as another by flipping or mirroring.
Advanced isomers could contain ring shapes and other structural designs that should not be considered until after straight chain and branched isomers have been mastered. Different rules may apply for elements in ring shapes.
Identify and count all the atoms to be drawn in the isomers. This will yield a molecular formula. Any isomers drawn will contain the same number of each type of atom found in the original molecular formula of the compound. A common example of a molecular formula is C4H10, meaning that there are four carbon atoms and 10 hydrogen atoms in the compound.
Refer to a periodic table of elements to determine how many bonds one atom of an element can make. Generally, each column may make a certain number of bonds. Elements in the first column such as H can make one bond. Elements in the second column can make two bonds. Column 13 can make three bonds. Column 14 can make four bonds. Column 15 can make three bonds. Column 16 can make two bonds. Column 17 can make one bond.
Note how many bonds each type of atom in the compound may make. Each atom in an isomer must make the same number of bonds that it made in another isomer. For example, for C4H10, carbon is in the 14th column, so it will make four bonds, and hydrogen is in the first column, so it will make one bond.
Take the element that requires more bonds to be made and draw an evenly spaced row of those atoms. In the example C4H10, the carbon is the element requiring more bonds, so the row would just have the letter C repeated four times.
Connect each atom in the row from left to right with a single line. The C4H10 example would have a row that looked like C-C-C-C.
Number the atoms from left to right. This will ensure that the correct number of atoms from the molecular formula are used. It will also aid in identifying the structure of the isomer. The C4H10 example would have the C on the left side labeled as 1. The C directly right of it would be 2. The C directly right of 2 would be labeled as 3 and the C on the far right end would be labeled as 4.
Count each line between the drawn atoms as one bond. The C4H10 example would have 3 bonds in the structure C-C-C-C.
Determine if each atom has made the maximum number of bonds according to the notes made from the periodic table of elements. Count the number of bonds that are represented by lines connecting each of the atoms in the row. The C4H10 example uses carbon, which requires four bonds. The first C has one line connecting it to the second C, so it has one bond. The first C does not have the maximum number of bonds. The second C has one line connecting it to the first C and one line connecting it to the third C, so it has two bonds. The second C does not have the maximum number of bonds, either. The number of bonds must be counted for each atom to prevent you from drawing incorrect isomers.
Start adding the atoms of the element that require the next fewest number of bonds to the previously created row of connected atoms. Each atom will have to be connected to another atom with a line that counts as one bond. In the C4H10 example, the atom that requires the next fewest number of bonds is hydrogen. Each C in the example would have one H drawn near it with a line connecting the C and the H. These atoms can be drawn above, below or to the side of each atom in the previously drawn chain.
Determine again if each atom has made the maximum number of bonds according to the notes from the periodic table of elements. The C4H10 example would have the first C connected to the second C and to an H. The first C would have two lines and thus have only two bonds. The second C would be connected to the first C and the third C and an H. The second C would have three lines and thus three bonds. The second C does not have the maximum number of bonds. Each atom must be examined separately to see if it has the maximum number of bonds. Hydrogen only makes one bond, so each H atom drawn with one line connecting to a C atom has the maximum number of bonds.
Continue adding atoms to the previously drawn chain until each atom has the maximum number of bonds allowed. The C4H10 example would have the first C connected to three H atoms and the second C. The second C would connect to the first C, the third C and two H atoms. The third C would connect to the second C, the fourth C and two H atoms. The fourth C would connect to the third C and three H atoms.
Count the number of each type of atom in the drawn isomer to determine if it matches the original molecular formula. The C4H10 example would have four C atoms in a row and 10 H atoms surrounding the row. If the number in the molecular formula matches the original count and each atom has made the maximum number of bonds, then the first isomer is complete. The four C atoms in a row cause this type of isomer to be called a straight chain isomer. A straight chain is one example of a shape or structure that an isomer can take.
Start drawing a second isomer in a new location by following the same process as Steps 1-6. The second isomer will be an example of a branched structure instead of a straight chain.
Erase the last atom on the right side of the chain. This atom will connect to a different atom than it did in the previous isomer. The C4H10 example would have three C atoms in a row.
Locate the second atom in the row and draw the last atom connecting to it. This is considered a branch because the structure no longer forms a straight chain. The C4H10 example would have the fourth C connecting to the second C instead of the third C.
Determine if each atom has the maximum number of bonds according to the notes made from the periodic table. The C4H10 example would have the first C connected to the second C by one line so it would have only one bond. The first C does not have the maximum number of bonds. The second C would be connected to the first C, the third C and the fourth C so it would have three bonds. The second C would not have the maximum number of bonds. Each atom must be determined separately to see if it has the maximum number of bonds.
Add the atoms of the element requiring the next fewest number of bonds in the same process as in Steps 9-11. The C4H10 example would have the first C connected to the second C and three H atoms. The second C would be connected to the first C, the third C, the fourth C and one H atom. The third C would be connected to the second C and three H atoms. The fourth C would be connected to the second C and three H atoms.
Count the numbers of each type of atom and the bonds. If the compound contains the same number of each type of atom as the original molecular formula and each atom has made the maximum number of bonds, the second isomer is complete. The C4H10 example would have two complete isomers, a straight chain and a branched structure.
Repeat Steps 13-18 to create new isomers by choosing different locations to branch atoms. The lengths of branches may also change by the number of atoms located in the branch. The C4H10 example has only two isomers, so it is considered complete.
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