Study Notes: Atoms, Ions, and Isotopes

Atoms, Ions, and Isotopes

Atomic Structure and Notation

Understanding the structure of atoms is fundamental in general chemistry. Atoms consist of protons, neutrons, and electrons, and their arrangement determines the identity and properties of each element.

• Atomic Number (Z): The number of protons in the nucleus of an atom. It defines the element.

• Mass Number (A): The total number of protons and neutrons in the nucleus.

• Isotopes: Atoms of the same element (same Z) with different numbers of neutrons (different A).

• Ion: An atom or molecule with a net electric charge due to the loss or gain of electrons.

Example: The notation 23Na indicates a sodium atom with a mass number of 23.

Determining Subatomic Particles

To fully describe an atom or ion, you must be able to determine the number of protons, neutrons, and electrons it contains.

• Number of Protons: Equal to the atomic number (Z).

• Number of Neutrons: Calculated as mass number (A) minus atomic number (Z): $\text{Number of neutrons} = A - Z$

• Number of Electrons: For a neutral atom, equal to the number of protons. For ions, adjust for the charge: $\text{Number of electrons} = Z - \text{charge}$ (for cations) $\text{Number of electrons} = Z + |\text{charge}|$ (for anions)

Example: A 35Cl- ion has 17 protons, 18 neutrons, and 18 electrons.

Isotopic Notation and Calculations

Isotopic notation provides a concise way to represent specific isotopes and ions.

• General Format: AZXcharge, where X is the element symbol.

• Example: 24Mg2+ represents a magnesium ion with 12 protons, 12 neutrons, and 10 electrons.

Tabular Comparison of Atoms and Ions

The following table summarizes the number of protons, neutrons, and electrons for selected atoms and ions:

Symbol	Number of Protons	Number of Neutrons	Number of Electrons
23Na	11	12	11
24Mg2+	12	12	10
35Cl-	17	18	18
56Fe2+	26	30	24
79Br-	35	44	36

Summary

• Atoms are defined by their number of protons (atomic number).

• Isotopes have the same number of protons but different numbers of neutrons.

• Ions are atoms or molecules with a net charge due to electron gain or loss.

• Use isotopic notation to concisely represent specific atoms and ions.