Atomic Number and Mass Number Flashcards

Fundamentals of the Atomic Number

• Definition of Atomic Number ($Z$): The atomic number, denoted by the symbol $Z$, is a fundamental property of a chemical element. It specifically indicates the total number of protons residing within the nucleus of an atom of that element.

• Relationship to Electrons: In the neutral state of an atom—where the atom carries no net electrical charge—the atomic number ($Z$) is also equal to the number of electrons orbiting the nucleus.

• Elemental Identity: The atomic number is a fixed value for every specific element. This means that every atom of a particular element will always have the same number of protons, which serves as the element's unique identifier.

Atomic Mass and Subatomic Variability

• The Role of Neutrons: While the number of protons (atomic number) is constant for an element, the number of neutrons in the nucleus may vary between different atoms of the same element.

• Composition of Atomic Mass: The atomic mass is comprised of the combined total of two types of subatomic particles found in the nucleus:

  • Protons

  • Neutrons

• The Mass Number Formula: Although not explicitly written as an equation in the text, the transcript defines the mass number through the following relationship:     $\text{Mass Number} = \text{Number of Protons} + \text{Number of Neutrons}$

Symbolic Representation and Notation

• Standard Chemical Notation: Chemical symbols use a specific arrangement of superscripts and subscripts to provide information about the atom's composition at a glance.

• Case Study: Lithium ($Li$):

  • Symbol: The chemical symbol for Lithium is represented as $\text{Li}$.

  • Mass Number (Superscript): The number written as a superscript to the left of the symbol represents the mass number (the sum of protons and neutrons). For Lithium, this value is $7$.

  • Atomic Number (Subscript): The number written as a subscript to the left of the symbol represents the atomic number (the number of protons). For Lithium, this value is $3$.

  • Visual Representation Summary: In the example provided (Figure 1), the element is shown as ${}^{7}_{3}Li$, where $7$ is the mass number and $3$ is the atomic number.

Source and Methodology

• Reference Source: This information is derived from the Labster Theory pages on Atomic Number.

• Resource Location: Available at https://theory.labster.com/atomic-number/.

• Access Date: The document reflects data accessed on June 18, 2026.