Atomic Mass and Isotopes - Chemistry

1. Fundamental Concepts

Atomic Mass: The mass of a single atom, measured in atomic mass units (amu; 1 amu = 1/12 the mass of a carbon-12 atom). It equals the sum of protons and neutrons in the atom’s nucleus (electrons contribute negligible mass).
Isotopes: Atoms of the same element with the same number of protons (same atomic number) but different numbers of neutrons (different mass numbers). Isotopes of an element have identical chemical properties but different physical properties (e.g., mass, radioactivity).

2. Key Concepts

Mass Number (A): The total number of protons + neutrons in an isotope’s nucleus (written as a superscript before the element symbol: e.g., ¹²C).
Atomic Number (Z): The number of protons (written as a subscript before the element symbol: e.g., ₆C). For any isotope, Z is fixed for the element; A varies.
Isotope Identity: Defined by the element name/symbol + mass number (e.g., carbon-12, uranium-235).
Atomic Mass vs. Mass Number: Mass number is an integer (count of protons/neutrons); atomic mass is a precise value (e.g., carbon-12 has a mass number of 12 and an atomic mass of exactly 12.0000 amu; carbon-13 has a mass number of 13 and an atomic mass of 13.0034 amu).

3. Examples

Easy

Identify the number of protons, neutrons, and mass number for the isotope ¹⁶O (oxygen-16).
- Atomic number of oxygen = 8 → 8 protons.
- Mass number = 16 → neutrons = 16 – 8 = 8.

Medium

An isotope of magnesium has 12 protons and 14 neutrons. What is its mass number, and how is it named/symbolized?
- Mass number = 12 + 14 = 26.
- Name: magnesium-26; Symbol: ²⁶₁₂Mg.

Hard

An unknown isotope has an atomic number of 17, a mass number of 37, and a atomic mass of 36.9659 amu. Classify it (element name + isotope label), and calculate the number of neutrons.
- Atomic number 17 = chlorine → isotope: chlorine-37 (³⁷₁₇Cl).
- Neutrons = 37 – 17 = 20.

4. Problem-Solving Techniques

Relate Protons, Neutrons, and Mass Number: Use the formula:

Neutrons = Mass Number (A) – Atomic Number (Z)

(Atomic number = protons = electrons in neutral atoms).

Isotope Symbolization: Follow the format: ᴬ_Z Element Symbol (superscript = mass number, subscript = atomic number).

Isotope Naming: Use element name + mass number (e.g., “hydrogen-3” for tritium).

Distinguish Atomic Mass vs. Mass Number: Mass number is a whole number (protons + neutrons); atomic mass is a measured value (often close to the mass number but not identical, except for carbon-12).

Verify Element Identity: Atomic number (protons) uniquely identifies the element—never use neutrons or mass number alone to name an element.