CME-260: Chapter 2

Electron

A negatively charged subatomic particle that occupies orbitals around the nucleus.

Proton

A positively charged subatomic particle located in the nucleus.

Neutron

A neutral subatomic particle located in the nucleus.

Nucleus

The dense, positively charged center of an atom containing protons and neutrons.

Atomic Number

The number of protons in the nucleus; uniquely identifies an element.

Isotope

Different forms of the same element that vary in the number of neutrons.

Atomic Mass Unit (amu)

A unit of mass equal to 1/12 the mass of a carbon-12 atom; approximately the mass of one proton or neutron.

Atomic Mass

The weighted average mass of all isotopes of an element.

Atomic Radius

A measure of the size of an atom, typically defined by the extent of its outermost electron density.

Avogrado’s Number

The number of particles (6.023 × 10²³) in one mole of a substance.

Gram-atom

Avogadro’s number of atoms of an element.

Mole

Avogadro’s number of molecules (or formula units) of a substance.

Group (periodic table)

A column of elements with similar chemical properties due to the same number of valence electrons.

Periodic Table

An arrangement of elements by atomic number that groups elements with similar properties.

Electron Orbital

A region of space around the nucleus where an electron is most likely to be found.

Energy level

A fixed binding energy between an electron and the nucleus.

Energy well

A plot of bonding energy versus distance showing the stable bonded state.

Electron density

A measure of how likely it is to find an electron at a given location around an atom or molecule.

Delocalized electron

An electron not associated with a single atom but shared among many atoms.

Hybridization

Redistribution of electrons into new orbitals to produce symmetrical bonding geometries.

Ion

An atom that has gained or lost electrons and carries an electric charge.

Cation

A positively charged ion formed by loss of electrons.

Anion

A negatively charged ion formed by gain of electrons.

Ionic bond

A bond formed by electron transfer from one atom to another and the electrostatic attraction between resulting ions.

Ionic radius

The effective radius of an ion after gaining or losing electrons.

Coulombic attraction

The electrostatic force of attraction between oppositely charged ions.

Repulsive force

A force arising from interactions between similarly charged nuclei at short distances.

Bond length

The equilibrium distance between two bonded atoms where net force is zero.

Bonding force

The net force resulting from attractive and repulsive interactions between atoms.

Bonding energy

The energy required to separate bonded atoms to an infinite distance.

Bond Angle

The angle between bonds formed by an atom due to directional electron sharing.

Coordination number

The number of nearest neighboring atoms or ions surrounding a reference atom.

Radius ratio

The ratio of the smaller ion radius to the larger ion radius; determines ionic packing and coordination number.

Hard-sphere model

A bonding model that treats atoms as rigid spheres touching at their radii.

Soft-sphere model

A bonding model that allows overlapping electron clouds for more accurate bond lengths.

Covalent bond

A directional bond formed by the sharing of valence electrons between adjacent atoms.

Valence electron

An electron in the outermost orbital that participates in bonding.

Double bond

A covalent bond involving two shared pairs of electrons.

Primary bond

A strong bond formed by electron transfer or sharing (ionic, covalent, metallic).

Secondary bond

A weak bond arising from electrostatic attraction between dipoles without electron transfer.

Polymeric molecule

A long-chain molecule composed of repeating structural units (mers).

Metallic bond

A nondirectional bond formed by sharing a cloud of delocalized valence electrons.

Energy trough

The minimum of the bonding energy curve corresponding to stable bonding.

Electronegativity

An atom’s ability to attract electrons to itself.

Dipole

A separation of positive and negative charge within a molecule.

Dipole moment

The product of charge magnitude and separation distance between charges.

Polar molecule

A molecule with permanent charge separation due to unequal electron sharing.

Van der Waals bond

A weak secondary bond caused by attraction between electric dipoles.

Hydrogen bond

A strong dipole-dipole interaction involving hydrogen bonded to highly electronegative atoms (F, O, N).

Melting point

The temperature at which sufficient thermal energy breaks cohesive bonds and causes a solid to become liquid.

Valence

The combining capacity of an atom based on the number of valence electrons.