Chemistry Basics: SI units

SI units

The metric system is a set of standardized weights and measures that have been widely adopted across the world because of its methodical naming system that divides measurements into units of ten.  The modern form of the metric system is called the Systéme International (International System) or SI for short.  Science has adopted SI because of its simplicity, and out of a need to have a standard way of measuring things that could be understood by scientists all over the world.

The SI system consists of one name for every type of measurement and then the unit is modified by a prefix that adjusts the size of the named unit by factors of ten if the measurement is large or small.

SI prefix multiplying by Scientific notation
peta (P) 1 000 000 000 000 000 1015
tera (T) 1 000 000 000 000 1012
giga (G) 1 000 000 000 109
mega (M) 1 000 000 106
kilo (k) 1 000 103
base unit 1 1
centi (c) 0.01 10-2
milli (m) 0.001 10-3
micro (µ) 0.000 001 10-6
nano (n) 0.000 000 001 10-9
pico (p) 0.000 000 000 001 10-12

 

Physical quantities

There are seven SI units that relate directly to physical quantities in the world, called base units. The units are:

  1. the ampere (A),
  2. the candela (cd),
  3. the kelvin (K),
  4. the gram (g),
  5. the meter (m),
  6. the mole (mol), and
  7. the second (s).

However, in chemistry, we are mostly interested in five base units: mass(gram), length(meter), time(second), temperature (kelvin), and amount(mole).

The gram is the base unit of mass, which measures the amount of matter in an object.  It’s important to differentiate weight from mass. weight is the force exhibited by the object on the surface due to gravity, while mass is mass and it is constant.  An object in space will be weightless but it will still have mass.  The kilogram is roughly equivalent to 2.2 imperial pounds.

The SI unit of length is the meter.  The modern definition of a meter is the distance that light travels in .000000003336 seconds.  It is equal to about 39.3 inches. In chemistry, the length of a meter is usually much too large to be useful so chemists use another unit called the Angstrom(Å) which is 10-10m. The Angstrom is convenient when measuring things that are very small, such as the distance between two atoms connected by a chemical bond or the wavelength of light (which is also commonly expressed in nanometers (1nm=10-9m)).  Although the Angstrom is not technically a SI unit, it is SI compatible, and is defined as one ten billionth of a meter, or 0.1nm

The SI unit for time is the second.  These are the same unit any stopwatch would give you.  The SI definition of a second is “the duration of 9192631770 periods of radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.”

The SI unit for temperature is kelvin.  The significance of the Kelvin scale is that the 0 K is postulated to be the absolute zero at which all, even atomic motion, stops. The scale-wise it equals the Celsius scale, however, it is offset by 273.15 degrees.

The last base unit commonly used in chemistry the mole. The mole is especially important, but is slightly different than the other units covered so far because it counts the number of objects rather than measuring properties of objects.  A mole is defined as the number, called the Avogadro’s number (NA), 6.0225*1023 of “things” which is 602,250,000,000,000,000,000,000! The mole is a convenient way to count atoms because they are very small so you need a very large number of to manipulate them on a human scale.  Moles of atoms, and molecules are usually associated with different weights so that scientists can weigh a pure substance and know how many atoms are in that substance.  In other words, the mole is the “ratio” unit that relates the atomic scale quantities to measurable “human scale” quantities.  The art of using chemical equations, moles, and masses to determine the amount of raw materials needed to form an amount of products is called Stoichiometry, and it will get its own tutorial.

Derived Units

There are various SI units that are mathematically derived from the seven base units.  Here we will list the units for volume (cubic meter), force (Newton), energy (joule)

The SI unit of volume is a cubic meter (m3) which is the volume contained in a box with length, width, and depth of one meter.  In chemistry, we commonly use liters (L) and milliliters (mL).  A liter is a much smaller measure of volume than a cubic meter, it takes about 3.8 liters to make a gallon.  Liters are SI compatible and are equal to the volume of a cube with length, width and depth of 10 centimeters.  What that means is that one cubic meter equals 1000 liters, and one milliliter equals to one cubic centimeter.   This is useful to know because it allows for convenient unit conversion.

The newton (N) is a unit of force that is required to accelerate a mass of one kilogram by one meter per second per second (N=Kg*m/s2).  This unit is derived from Isaac Newton’s second law of motion that states that “force is equal to mass times acceleration squared.”

The joule (J) is a measurement of energy that is equal to the work done when one newton of force acts on an object through the distance of one meter.  J=kg*m2/s2.  Because the joule is somewhat difficult to understand intuitively there is a second measurement of energy that chemists often use called the calorie.  The calorie is the amount of energy required to raise the temperature of 1 gram of water by 1 degree Celsius.  Although the calorie is defined in metric terms it is not a part of the SI system and doesn’t convert nicely into joules.  One calorie is equal to about 4.184 joules.

By using the prefixes, base units, and derived units presented in this tutorial you should now be able to understand and communicate all the common measurements that chemists take.  Complete the practice problems that are associated with this tutorial to see how well you can use the metric system.