Chapter 2 Notes: Atoms, Ions, and Molecules

Nucleus

The positively charged center of an atom, containing nearly all of the atom’s mass, and consisting of protons and neutrons.

Protons

Positively charged subatomic particles found in the nucleus.

Neutrons

Electrically neutral subatomic particles found in the nucleus.

Atomic mass units (u)

Units used to express the relative masses of atoms and subatomic particles, where 1 ext{ u} = 1 ext{ Dalton (Da)}.

Isotope Symbol (Nuclide Symbol)

A notation ^{A}_Z X where A is the mass number (protons + neutrons), Z is the atomic number (number of protons), and X is the element’s symbol.

Isotopes

Atoms of the same element (same Z) with different mass numbers (A) due to different numbers of neutrons, leading to different nuclei masses and slightly different physical properties.

Carbon-14 Dating

A method used to determine the age of organic material by measuring the remaining C-14, effective up to about 50,000 ext{ years}.

Average Atomic Mass

The weighted average of the masses of all naturally occurring isotopes of an element, calculated by the formula ar{M} = \sum\left( \text{fractional abundance} \times m \right).

Periods

Horizontal rows in the periodic table.

Groups

Vertical columns in the periodic table, where elements tend to have similar physical and chemical properties.

Ionic Compounds

Compounds consisting of charged particles (ions) formed by the transfer of electrons between atoms, held together by electrostatic forces.

Cations

Ions with a positive charge.

Anions

Ions with a negative charge.

Formula Unit

The smallest electrically neutral unit of an ionic compound.

Oxoanions

Polyatomic anions that include oxygen with another element, named by changing the ending of the element's name to -ite or -ate.

Polyatomic Ions

Ions composed of two or more atoms acting as a unit (e.g., sulfate, nitrate, ammonium).

Molecular Compounds

Compounds formed by covalent bonds between nonmetals, where electrons are shared.

Covalent Bonds

Bonds created by sharing one or more pairs of electrons between nonmetal atoms.

Molecular Formula

Shows the exact number and type of atoms in one molecule (e.g., glucose: ext{C}{6} ext{H}{12} ext{O}_6).

Empirical Formula

Shows the smallest whole-number ratio of elements in a compound (e.g., for glucose, CH2O).

Binary Acids

Acids formed from hydrogen and a halide, named using the rule: Hydro- + the base name of the halogen + -ic + acid (e.g., HBr → hydrobromic acid).

Oxoacids

Acids related to oxoanions; if the oxoanion ends in -ate, the acid ends in -ic; if ending is -ite, the acid ends in -ous (e.g., NO3- → nitric acid (HNO3)).

Stock System

A naming convention for ionic compounds that uses Roman numerals in parentheses to indicate the charge on the cation (e.g., Fe2+ → iron(II)).