Relative Atomic Mass (RAM)
Definition of RAM: Relative Atomic Mass (RAM) tells us how heavy an atom is when we compare it to something called the carbon-12 atom. Scientists chose carbon-12 as a standard because it is stable and found easily. So, RAM means how many times heavier or lighter an atom is compared to 1/12 of a carbon-12 atom.
Relative, not absolute: RAM does not tell us the exact weight of an atom in grams or kilograms. It only shows us how heavy the atom is in comparison to carbon-12. It’s like saying, “This apple is twice as heavy as that apple,” without knowing the real weight of either.
No units: RAM doesn’t come with any units like grams or kilograms. This is because it’s a comparison or ratio, not a direct measurement. So, when we write RAM, we just use numbers like 1, 12, or 35.5.
Carbon-12 standard: Scientists all over the world agree to use carbon-12 as the base for comparing atomic masses. That means all RAM values are linked back to the mass of carbon-12.
Value of carbon-12: The atom of carbon-12 is given a special RAM value of exactly 12. This number never changes and is used to help compare all other atoms.
Comparison method: To find the RAM of any atom, we compare its mass to the mass of carbon-12. This helps us know how much heavier or lighter it is.
RAM calculation: We calculate RAM by taking the average mass of one atom of an element and dividing it by 1/12 of the mass of a carbon-12 atom. This gives us a number that shows how the atom’s mass relates to carbon-12.
Ratio expression: Another way to think about RAM is as a ratio. That means RAM = (Average mass of an atom) ÷ (1/12 of the carbon-12 mass). This helps us understand RAM as a comparison, not a real weight.
Isotope consideration: Many elements have atoms with different numbers of neutrons, called isotopes. RAM takes into account all the isotopes of an element and their amounts (abundance). So, it’s an average, not just from one kind of atom.
Chlorine example: Chlorine has two types of atoms, chlorine-35 and chlorine-37. The RAM of chlorine is about 35.5 because there are more chlorine-35 atoms than chlorine-37. So, we use a weighted average to get this value.
Periodic table usage: When we look at the periodic table, each element has a number under its symbol. That number is the RAM and it helps us in calculations in chemistry.
Symbol placement: If you look at an element’s box on the periodic table, you will usually see the RAM written below the symbol (like below ‘O’ for oxygen).
Element examples: Some common RAM values are: hydrogen = 1, carbon = 12, and oxygen = 16. These numbers help us when we calculate the mass of molecules or compounds.
Relative Molecular Mass (RMM)
Definition of RMM: Relative Molecular Mass (RMM) is the total of all the RAM values of each atom in a molecule. It tells us how heavy a molecule is compared to 1/12 the mass of a carbon-12 atom.
Relative to carbon-12: Just like RAM, RMM also compares a molecule’s mass to 1/12 of a carbon-12 atom. That makes it easy to relate atoms and molecules in the same way.
How to calculate: To find RMM, we just add together the RAM of every atom in a molecule. For example, for water, we add up the RAM of 2 hydrogen atoms and 1 oxygen atom.
Water example: In a water molecule (H₂O), we have 2 hydrogen atoms (RAM = 1 each) and 1 oxygen atom (RAM = 16). So, RMM = 2×1 + 16 = 18.
Use of RMM: RMM is used only for molecules, not for things like salt (which is ionic). It tells us how heavy one molecule is.
Molecular definition: A molecule is formed when two or more atoms stick together by chemical bonds. These atoms can be the same or different.
Molecular examples: Some common molecules are: water (H₂O), carbon dioxide (CO₂), and oxygen (O₂). Each of these has atoms joined together by chemical bonds.
Relative Formula Mass (RFM)
Definition of RFM: Relative Formula Mass (RFM) is like RMM, but for ionic compounds. It adds up the RAMs of all the atoms in one formula unit of an ionic compound.
Used for ions: RFM is used for compounds made of ions (charged particles). These compounds are called ionic compounds.
Calculation method: To find the RFM, we add the RAMs of every atom shown in the compound’s chemical formula.
NaCl example: Sodium chloride (NaCl) has one sodium atom (RAM = 23) and one chlorine atom (RAM = 35.5). So, RFM = 23 + 35.5 = 58.5.
RFM use case: RFM is used because ionic compounds do not exist as separate molecules. Instead, they form large crystal structures.
Ionic compound structure: Ionic compounds are made up of positive ions (called cations) and negative ions (called anions) that are held together by strong forces.
Ionic examples: Some common ionic compounds are: sodium chloride (NaCl), magnesium oxide (MgO), and copper(II) sulfate (CuSO₄).
Key Relationships
Carbon-12 reference: RAM, RMM, and RFM all use carbon-12 as their base for comparison. This keeps all mass values consistent.
No unit consistency: RAM, RMM, and RFM do not have units because they are ratios. This makes them easy to compare and use in chemical equations.
RMM vs. RFM: RMM and RFM both require us to add up the RAMs of atoms. The difference is that RMM is for molecules, while RFM is for ionic compounds.
Term usage: We use the term RMM when we talk about molecules like water, and RFM when we talk about ionic compounds like table salt (NaCl).