The Atomic Scale
Lesson 3.2.1 & 3.2.2: Relative Atomic Mass (Ar) & Relative Molecular Mass (Mr)
🍕 The Carbon-12 Standard
Imagine you're weighing atoms. But atoms are impossibly tiny—you can't just put them on a normal scale!
So scientists chose a reference point: the Carbon-12 atom. Think of it like a "Dozen Egg Box" that weighs exactly 12 units.
1 Unit = 1/12th of a Carbon-12 atom
• Hydrogen weighs the same as 1 egg → Ar = 1
• Magnesium weighs the same as 24 eggs → Ar = 24
• Sulfur weighs the same as 32 eggs → Ar = 32
⚖️ The Atomic Balance
Select an element and see how many Carbon-12 "pizza slices" (1/12th portions) are needed to balance it!
Magnesium weighs the same as 24 slices of Carbon-12
Arrange these phrases in the correct order to define Relative Atomic Mass:
Calculate Mr of H2SO4
Relative Molecular Mass = Sum of all Ar values
EXAMINER TRAP WARNING!
When you see brackets in a formula like Ca(OH)2, the subscript outside multiplies EVERYTHING inside the bracket!
So Ca(OH)2 means: 1 Ca + 2 O + 2 H (not 1 Ca + 1 O + 2 H!)
Calculate the Relative Formula Mass of Calcium Hydroxide: Ca(OH)2
📋 Key Takeaways
Relative Atomic Mass (Ar)
The average mass of naturally occurring isotopes of an element compared to 1/12th of a Carbon-12 atom. Found on the Periodic Table.
Relative Molecular Mass (Mr)
The sum of the Ar values of all atoms in a molecule. For H2O: (2×1) + 16 = 18
The Bracket Trap
Ca(OH)2 means multiply EVERYTHING in brackets by 2! So it's 1Ca + 2O + 2H, NOT 1Ca + 1O + 2H.
Ionic Compounds
Technically called "Relative Formula Mass" for ionic compounds, but calculated exactly the same way as Mr.
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