Atomic Structure, Isotopes, and Chemical Formulas

Vocabulary flashcards covering core topics in introductory chemistry including atomic subatomic properties, isotope notations, electron energy shells, radioactive half-life, and chemical formula atom counting rules.

Periodic Table Mass

The average mass of all the isotopes of a given element.

Protons

Subatomic particles located in the nucleus with a positive charge and a mass of $1$.

Neutrons

Subatomic particles located in the nucleus with a neutral charge and a mass of $1$.

Electrons

Subatomic particles with a negative charge and a mass of $0$.

Atomic Number ($Z$)

The number of protons contained in the nucleus of an atom.

Mass Number ($A$)

The total number of protons plus neutrons in an atom ($A = Z + N$).

Neutron Number ($N$)

The number of neutrons in an atom, determined by subtracting the atomic number from the mass number ($N = A - Z$).

Isotope

An element that has a different number of neutrons, which changes its mass number, while keeping the same number of protons.

Symbol Notation

An isotope writing convention where the mass number ($A$) is placed at the upper left and the atomic number ($Z$) at the lower left of the chemical symbol ($X$), formatted as ${}^{A}_{Z}\text{X}$.

Hyphen Notation

A shorthand notation for isotopes where the element name or chemical symbol is followed by a hyphen and its mass number, written as \text{Name}-$A$ or $X-A$.

Electron Shell Capacities

The maximum number of electrons per shell level: 1st shell = $2$, 2nd shell = $8$, 3rd shell = $8$, and 4th shell = $18$.

Neutral Atom Rule

In a neutral atom, the number of protons equals the number of electrons.

Ion Formation

The process of altering an atom's net charge by adding electrons to make it more negative (e.g., adding $1$ electron creates a $1-$ charge) or removing electrons to make it less negative (e.g., removing $1$ electron creates a $1+$ charge).

Radioactive Half-Life

The time required for half the radioactive nuclei in any given sample to undergo radioactive decay.

Half-Life Decay Pattern

The remaining portion of a radioactive sample over time: one-half remains after $1$ half-life, one-fourth ($\frac{1}{4}$) after $2$ half-lives, and one-eighth ($\frac{1}{8}$) after $3$ half-lives.

Uranium-235 Half-Life Example

A radioisotope with a half-life of $2.5\,\text{years}$; a starting mass of $100\,\text{g}$ decays to $50\,\text{g}$ after $1$ half-life ($2.5\,\text{years}$) and $25\,\text{g}$ after $2$ half-lives ($5\,\text{years}$).

Implicit Subscript Rule

The rule in counting atoms stating that if no number is written to the right of an element symbol, the number of atoms is assumed to be $1$.

State Symbol (s)

A notation in chemical equations indicating that a substance is in the solid state.

State Symbol (l)

A notation in chemical equations indicating that a substance is in the liquid state (e.g., \text{H}2\text{O}{(l)}, \text{Hg}_{(l)}).

State Symbol (g)

A notation in chemical equations indicating that a substance is in the gas state.

State Symbol (aq)

A notation in chemical equations indicating that a substance is aqueous (dissolved in water).

Polyatomic Group Subscript Rule

The atom-counting rule where a subscript outside parentheses multiplies every subscript inside; for example, \text{Fe}_2(\text{SO}_4)_3 contains $2 \times \text{Fe}$, $3 \times \text{S}$, and $12 \times \text{O}$.

Carbon Isotopes

The three isotopes of carbon: Carbon-12 (${}^{12}_{6}\text{C}$ with $6$ neutrons), Carbon-13 (${}^{13}_{6}\text{C}$ with $7$ neutrons), and Carbon-14 (${}^{14}_{6}\text{C}$ with $8$ neutrons).

Silicon Atom Example (${}^{28}_{14}\text{Si}$)

A neutral silicon atom containing $14$ protons, $14$ electrons, and $14$ neutrons ($28 - 14 = 14$).

Gold-200 Notation Example (${}^{200}_{79}\text{Au}$)

A gold isotope with a mass of $200$, containing $79$ protons, $79$ electrons, and $121$ neutrons ($200 - 79 = 121$).