Structural, Condensed & Molecular Formulae ഘടനാവാക്യം · കണ്ടൻസ്ഡ് ഫോർമുല · തന്മാത്രാസൂത്രം
The textbook writes every hydrocarbon in three ways: as a structural formula that shows every bond, as a shorter condensed formula, and as a molecular formula that only counts the atoms. It also shows ball-and-stick models. On this page you learn to read and write all of them, to draw a structure so that every carbon has exactly four bonds, and to spot a number pattern. You need that last skill to find the general formulas on pages 4 and 5.
- Page 1: carbon has valency 4; hydrogen has valency 1; single, double and triple bonds count 1, 2 and 3.
- If you can already turn CH₃–CH₂–CH₃ into C₃H₈, draw its structure with all eight hydrogen atoms, and continue 4, 6, 8, 10, … with a rule, do the labs and the quiz, then go to page 3.
1. Three ways to write the same molecule
Look at ethane, the gas in Fig 8.4 of your textbook. It has two carbon atoms joined by a single bond, and each carbon carries three hydrogen atoms.
- The structural formula (ഘടനാവാക്യം) draws every atom and every bond as a line. It shows exactly which atom is joined to which.
- The condensed formula (കണ്ടൻസ്ഡ് ഫോർമുല) is a shortcut: each carbon is written together with its own hydrogen atoms, and only the bonds between carbons are shown: CH₃–CH₃.
- The molecular formula (തന്മാത്രാസൂത്രം) only counts the atoms of each element in one molecule: C₂H₆.
- A ball and stick model (ബോൾ & സ്റ്റിക്ക് മാതൃക) is a 3-D picture: balls for atoms (dark for carbon, white for hydrogen) and sticks for bonds.
2. Subscripts and counting atoms
In a formula, a small number written low after a symbol is a subscript. It tells how many atoms of the element just before it are in one molecule. CH₃ means one C and three H. C₃H₈ means three C and eight H. A symbol with no subscript means one atom: in CH₄ there is one carbon. A big number written in front of a formula counts whole molecules: 2CH₄ is two molecules of methane.
To get the molecular formula from a condensed formula, add up the atoms group by group: CH₃–CH₂–CH₂–CH₃ has 4 carbon atoms and 3 + 2 + 2 + 3 = 10 hydrogen atoms, so it is C₄H₁₀. The bond signs (–, =, ≡) are not atoms; they are not counted.
3. Drawing a structure so that every carbon has four bonds
- Write the carbon atoms in a row and join neighbours with single bonds.
- If the molecule has a double or a triple bond, make that one C–C bond double (=) or triple (≡).
- Go along the chain and count the bonds each carbon already has. Add hydrogen atoms until every carbon has exactly four: an end carbon with single bonds gets 3 H, a middle one 2 H, and fewer next to a double or triple bond.
- Count all the atoms to get the molecular formula. Check: every carbon 4 bonds, every hydrogen 1 bond.
You can also tap a C–C bond in the picture to move the double or triple bond there. A carbon can never get more than 4 bonds, so one carbon atom on its own (methane) can only have single bonds.
Build chains with only single bonds and watch the number of hydrogen atoms as you add carbons: 4, 6, 8, 10, … Each extra carbon brings exactly 2 more hydrogen atoms (it is a –CH₂– unit put into the chain). A double bond always means 2 hydrogen atoms fewer, and a triple bond 4 fewer. Section 5 turns this into a rule.
4. Reading a ball-and-stick model
A ball-and-stick model shows the real shape of a molecule in 3-D. Count the dark balls to get the number of carbon atoms and the white balls for the hydrogen atoms; that gives the molecular formula. The sticks are the bonds: a double bond is two sticks and a triple bond three. In methane the four hydrogen atoms point to the corners of a tetrahedron (a pyramid with a triangular base), not to a flat cross as the structural formula seems to show. The structural formula shows which atoms are joined, not the true shape.
Real atoms are not coloured balls and bonds are not sticks; the model only shows which atoms are joined and in what directions. You can make one at home with clay or beads and broomstick pieces (an extended activity on page 11).
5. Spotting a number pattern
On pages 4 and 5 you will put hydrocarbons in a table and look for a rule that gives the number of hydrogen atoms from the number of carbon atoms. Here is the skill you need. Look at the numbers 4, 6, 8, 10, … They go up by 2 each time, so the rule contains “2 × n”. For the 1st number, 2 × 1 = 2, but the number is 4, which is 2 more. Check the 2nd: 2 × 2 + 2 = 6 ✔; the 3rd: 2 × 3 + 2 = 8 ✔. So the rule is 2n + 2, “twice the position, plus 2”. With the rule you can find any number without writing the whole list: the 20th is 2 × 20 + 2 = 42. A rule written with a letter like this is a general formula (പൊതുവാക്യം).
- Structural formula (ഘടനാവാക്യം): every atom and every bond drawn. Condensed formula (കണ്ടൻസ്ഡ് ഫോർമുല): each carbon with its hydrogens, CH₃–CH₂–CH₃. Molecular formula (തന്മാത്രാസൂത്രം): only the totals, C₃H₈.
- A subscript counts the atoms of the symbol just before it; bond signs are not counted.
- Draw carbons first, then the C–C bonds, then add H until every carbon has exactly 4 bonds.
- Ball and stick model (ബോൾ & സ്റ്റിക്ക് മാതൃക): dark balls carbon, white balls hydrogen, sticks bonds; it shows the 3-D shape.
- A number pattern can be written as a rule such as 2n + 2: a general formula (പൊതുവാക്യം).
- Writing C₃H₈ as C₈H₃ or CH₃CH₂CH₃ as C₃H₃ ✗ – add the hydrogen atoms of every group: 3 + 2 + 3 = 8.
- Counting the bond signs (–, =) as atoms ✗.
- Forgetting the hydrogen atoms at the two ends of a chain ✗ – an end carbon with single bonds carries 3 H.
- Thinking the rule of 4, 6, 8, 10 is “n + 2” because it adds 2 ✗ – adding 2 each time means 2n + something: 2n + 2.