Atomic Structure, Isotopes, and Atomic Mass: Study Notes

Analogy: nucleus size and electron mass

The nucleus is described as being the size of a chickpea, i.e., very small in comparison to the overall atom.
Electrons are said to have essentially no mass relative to the nucleus in this analogy.

Atomic number (Z): number of protons in the nucleus; defines the element.
Mass number (A): total number of protons and neutrons in the nucleus; varies among isotopes of the same element.
Neutron number (N): number of neutrons in the nucleus.
Fundamental relationships:
N = A - Z
- A = Z + N
Neutral atoms have equal numbers of protons and electrons; if ions, protons and electrons are not equal.
The element is defined by Z; changing Z changes the element, regardless of A.

Isotopes: atoms of the same element (same Z) with different mass numbers (A) due to different numbers of neutrons (N).
Every element has isotopes; the number of isotopes per element varies (two, three, seven, etc.).
Common example mention: carbon has isotopes such as carbon-12, carbon-13, carbon-14 (C-12, C-13, C-14).
The mass number A is a whole number (e.g., 12, 13, 14) and changes between isotopes; Z remains constant for isotopes of the same element.
Reading notation: an isotope is often written as
{}^{A}_{Z} ext{X}
where X is the element symbol.

In some slides, two numbers accompany an element: a blue number and a green number.
- The blue number represents the mass number A (not necessarily present on a gray/standard periodic table).
- The green number represents the atomic number Z (the number you find on the gray periodic table).
Reading the data this way helps identify A (mass number) and Z (atomic number).
The mass number A (not to be confused with atomic mass) is the value that changes with isotopes; Z is fixed for a given element.

Example element: zirconium (Zr)
- Atomic number: Z = 40
- Mass number given in a specific context: A = 91
- Neutron number: N = A - Z = 91 - 40 = 51
Critical reminder: the widely used formula for neutron number is N = A - Z, not Z - A.
In neutral zirconium (if neutral), protons = electrons = Z = 40
ight. If ionized, the electron count would differ from Z.

Mass number A is an integer and specific to a single isotope.
Atomic mass (often given in atomic mass units, amu) is a weighted average of the masses of all naturally occurring isotopes of the element.
Because isotopes have different abundances, the atomic mass is typically not an integer and is close to a weighted average of the isotope masses.
Example concept: Chlorine has an atomic mass around $$ar{m} \