Summary Atomic mass notes

Atomic mass notes
⚛️Atomic Mass (Also called Relative Atomic Mass, or Ar)


🧠 What Is Atomic Mass?

 Atomic mass is the average mass of all the naturally occurring isotopes of an
element.
 It is measured in atomic mass units (amu or u).
 It is a weighted average, not a simple average.

💡 1 amu is defined as 1/12 the mass of a carbon-12 atom.



🧪 Why Do We Use a Weighted Average?

Because elements exist as isotopes (same element, different masses), and not all isotopes
occur in equal amounts in nature.



🔣 Formula:

Atomic Mass (Ar)=(m1×p1)+(m2×p2)+⋯100\text{Atomic Mass (A}_r\text{)} = \
frac{(m_1 \times p_1) + (m_2 \times p_2) + \cdots}{100}

Where:

 m1,m2,…m_1, m_2, \dots = masses of isotopes
 p1,p2,…p_1, p_2, \dots = percentage abundances



📌 Key Points to Remember:

Term Definition
Isotope- Same protons, different neutrons
Relative atomic mass- Weighted average mass of an element’s isotopes
Unit of atomic mass- amu (atomic mass unit) or simply u
Found on Periodic Table Atomic mass = decimal number under the element symbol


🧠 Quick Check Questions: