Atomic Structure and Symbolism

Core Concepts & Principles

An atom consists of a tiny, dense nucleus containing positively charged protons and uncharged neutrons, surrounded by negatively charged electrons occupying a much larger volume. Atoms are extremely small: atomic diameters are $\sim 10^{-10}\text{ m}$, whereas nuclear diameters are $\sim 10^{-15}\text{ m}$ (about 100,000 times smaller). Element symbols such as C, Na, and Hg represent elements or individual atoms. Only the first letter is capitalized: Co means cobalt; CO means carbon monoxide. Some symbols come from older names (Na from natrium, Fe from ferrum). IUPAC approves element names; three-letter symbols are temporary names used before approval.

Subatomic Particles & Mass Units

• Proton: Mass $1.0073\text{ amu}$ (or $1.00727\text{ u}$), charge $1+$ ($1.602 \times 10^{-19}\text{ C}$).
• Neutron: Mass $1.0087\text{ amu}$ (or $1.00866\text{ u}$), charge $0$.
• Electron: Mass $0.00055\text{ amu}$, charge $1-$ ($-1.602 \times 10^{-19}\text{ C}$).
• Mass Units: The unified atomic mass unit ($u$), dalton ($Da$), and atomic mass unit ($amu$) are equivalent units defined as exactly $\frac{1}{12}$ of the mass of a carbon-12 atom ($1.6605 \times 10^{-24}\text{ g}$). The elementary charge has magnitude $e \approx 1.602 \times 10^{-19}\text{ C}$.

Atomic Number, Mass Number, and Isotopes

• Atomic Number ($Z$): Number of protons in the nucleus; defines the element's identity and equals the electron count in neutral atoms.
• Mass Number ($A$): Total number of protons and neutrons ($A = \text{protons} + \text{neutrons}$). The number of neutrons is given by $A - Z$.
• Isotopes & Naming: Atoms of the same element with identical atomic numbers but different neutron counts. Isotopes are named by "element-mass number" (e.g., carbon-12, magnesium-24) or designated using nuclide symbols where the mass number is a left superscript, atomic number is a left subscript, and ion charge is a right superscript (e.g., $^{24}_{12}\text{Mg}^{2+}$).

Hydrogen-2 is also called deuterium (D); hydrogen-3 is tritium (T). Mass number counts particles and has no units; an isotope's measured mass is in amu and need not be a whole number. The small difference between the mass of an atom and the sum of its separate particles is the mass defect (covered in chapter 20).

Ions

Electrically charged atoms or molecules with unequal numbers of protons and electrons ($\text{charge} = \text{protons} - \text{electrons}$). Cations are positively charged (lost electrons); anions are negatively charged (gained electrons).

Atomic Mass & Mass Spectrometry

The tabulated atomic mass of an element is the weighted average of its naturally occurring isotopes. A mass spectrometer vaporizes and ionizes a sample, using electric or magnetic fields to separate ions by their mass-to-charge ratio ($m/z$).

Problem-Solving Routines & Methods

A platinum ion with $A=195$ and 74 electrons has $Z=78$: 78 protons, $195-78=117$ neutrons, and charge $78-74=4+$, written $^{195}_{78}\text{Pt}^{4+}$.

Practice & Concept Checks

Key Terms & Vocabulary