Glossary: The quantum theory

azimuthal quantum number Symbol:
A quantum number that labels the subshells of an atom. Sometimes called the orbital angular momentum quantum number, this quantum number dictates orbital shape. can take on values from 0 to n-1 within a shell with principal quantum number n.

atomic number
The number of protons in an atomic nucleus.

atomic radius
one half the distance between nuclei of atoms of the same element, when the atoms are bound by a single covalent bond or are in a metallic crystal. The radius of atoms obtained from covalent bond lengths is called the covalent radius; the radius from interatomic distances in metallic crystals is called the metallic radius.

Aufbau construction
An approximate procedure for writing the ground state electronic configuration of atoms by building the configuration one electron at a time while counting across blocks of the periodic table.

block
A region of the periodic table that corresponds to the type of subshell (s, p, d, or f) being filled during the Aufbau construction of electron configurations.

constructive interference
When the peaks and troughs of two interfering* waves match, the amplitudes add to give the resultant wave a higher amplitude.

core electrons
d block
degenerate (orbitals)
A set of orbitals are said to be degenerate if they all have the same energy.

constructive interference
When the peaks of one wave match the troughs of another, the waves interfere* destructively. The amplitudes* of the interfering waves cancel to give the resultant wave a lower amplitude.

diamagnetism
Diamagnetic materials are very weakly repelled by magnetic fields. The atoms or molecules of diamagnetic materials contain no unpaired spins. Contrast with paramagnetism.

diffraction
The ability of a wave to bend around the edges of obstacles or holes. The effect is most noticeable when the obstacle or hole is comparable to the size of the wavelength*.

effusion
Gas molecules in a container escape from tiny pinholes into a vacuum with the same average velocity they have inside the container. They also move in straight-line trajectories through the pinhole.

electromagnetic wave
A wave that involves perpendicular oscillations in the electric and magnetic fields, moving at a speed of 2.99792458×108 m/s in a vacuum away from the source. gamma rays*, x-rays*, ultraviolet light*, visible light*, infrared radiation*, and radio waves are all electromagnetic waves.

electron configuration
A list showing how many electrons are in each orbital or subshell. There are several notations. The subshell notation lists subshells in order of increasing energy, with the number of electrons in each subshell indicated as a superscript. For example, 1s2 2s2 2p3 means "2 electrons in the 1s subshell, 2 electrons in the 2s subshell, and 3 electrons in the 2p subshell.

effective nuclear charge
the nuclear charge experienced by an electron when other electrons are shielding the nucleus.

frequency
The number of cycles of a wave that move past a fixed observation point per second. The SI unit of frequency is the Hertz.

gamma rays
A very high energy form of electromagnetic radiation, typically with wavelengths of less than 3 pm. Gamma rays are produced by certain nuclear decay processes, and are used to sterilize food.

ground state
The lowest energy state for an atom or molecule. When an atom is in its ground state, its electrons fill the lowest energy orbitals completely before they begin to occupy higher energy orbitals, and they fill subshells in accordance with Hund's rule (usually!)

group
A vertical column on the periodic table. Elements that belong to the same gr oup usually show chemical similarities, although the element at the top of the group is usually atypical.

Hertz Symbol: Hz
The SI unit of frequency, equal to one cycle of the wave per second (s-1).

Hund's rule
Subshells fill so that the number of unpaired spins is maximized. Spread them out and line them up! Hund's rule is sometimes called "the rule of maximum multiplicity".

infrared radiation
Electromagnetic radiation* with wavelength* longer than visible light but shorter than that of microwaves*. Infrared radiation is produced by hot objects; absorption of infrared radiation causes chemical bonds to vibrate.

interference
The amplitudes* of waves* moving into the same region of space add to produce a single resultant wave. The resultang wave can have higher or lower amplitude than the component waves. See constructive interference* and destructive interference*.

ionic radius
the radii of anions and cations in crystalline ionic compounds.

ionization energy (Units: eV, kJ/mol)
energy needed to remove an electron from a gaseous atom or ion. Sometimes called ionization potential.

isoelectronic
isoelectronic atoms or ions have identical electron configurations. For example, F-, Ne, and Na+ are isoelectronic. Since members of isoelectronic series have the same electron configuration, they will also have the same number of electrons.

lanthanides
Elements 57-70 are called lanthanides. Electrons added during the Aufbau construction of lanthanide atoms go into the 4f subshell.

lanthanide contraction
an effect that causes sixth period elements with filled 4f subshells to be smaller than otherwise expected. The intervention of the lanthanides increases the effective nuclear charge, which offsets the size increase expected from filling the n=6 valence shell. As a consequence, sixth period transition metals are about the same size as their fifth period counterparts.

main group elements
Elements of the s and p blocks.

momentum
Momentum is a property that measures the tendency of a moving object to keep moving in the same direction. Increasing the speed of an object increases its momentum, and a heavy object will have more momentum than a lighter one moving at the same speed. For a particle with mass m and velocity v, the momentum of the particle is mv.

microwaves
Electromagnetic radiation* with wavelength* between 3 mm and 30 cm.

p block

paramagnetism
Paramagnetic materials are attracted to a magnetic field due to the presence of least one unpaired electron in their atoms or molecules. Contrast with diamagnetism.

Pauli principle
No two electrons in an atom can have the same set of 4 quantum numbers. Because the n, , and m quantum numbers address a particular orbital, and because the ms quantum number has only two possible values, the Pauli principle says that a maximum of two electrons can occupy an atomic orbital- and these electrons must have opposite spins.

penetration
Electrons in penetrating orbitals can reach the nucleus. The n and l quantum numbers determine how well an orbital penetrates. Lower n and lower l values mean better penetration. A low n value means the orbital is small. A low l value means the orbital has fewer nuclear nodes (planes that pass through the nucleus where the probability of locating the electron is zero).

In order of decreasing penetration, the subshells are s > p > d > f. A 1s orbital penetrates better than a 2s orbital.

period
Horizontal rows in the periodic table are called periods. For example, all of the elements in the second row are referred to as 'second period elements'. The number of the period is equal to the principal quantum number of the valence shell for all of the elements in the period. All known elements fall in the first seven periods.

periodic law
The periodic law states that physical and chemical properties of the elements recur in a regular way when the elements are arranged in order of increasing atomic number.

periodic table
An arrangement of the elements according to increasing atomic number that shows relationships between element properties.

periodic trend
A regular variation in element properties with increasing atomic number that is ultimately due to regular variations in atomic structure.

photon
A discrete packet of electromagnetic radiation. The photon is a particle with no mass and no charge. Its energy is proportional to the frequency* of the electromagnetic radiation and inversely proportional to wavelength*.

principal quantum number Symbol: n
The quantum number that determines the size and (in hydrogen atoms) the energy of an orbital. n is used to label electron shells. n may take on integer values from 1 to infinity.

pseudocore electrons

s block

shell
A set of electrons with the same principal quantum number. The number of electrons permitted in a shell is equal to 2n2. A shell contains n2 orbitals*, and n subshells*.

shielding

spin pair Symbol:
A pair of electrons with opposite spins, occupying the same orbital. See unpaired spin.

subshell
A set of electrons with the same azimuthal quantum number. The number of electrons permitted in a subshell is equal to 2 + 1.

transition metals

ultraviolet light
Electromagnetic radiation* with wavelength longer than that of x-rays* but shorter than that of visible light*. Ultraviolet light can break some chemical bonds and cause cell damage.

unpaired spin Symbol:
A single electron occupying an orbital. According to Hund's rule, electrons occupy subshells so that the number of unpaired spins is maximized. See spin pair.

valence
the number of bonds an atom typically forms in molecules.

valence electrons

electrons in the shell with the highest value of n. For example, sodium's ground state electron configuration is 1s2 2s2 2p6 3s1; the 3s electron is the only valence electron in the atom. Germanium (Ge) has the ground state electron configuration 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p2; the 4s and 4p electrons are the valence electrons.

valence shell
The shell corresponding to the highest value of principal quantum number in the atom. The valence electrons in this shell are on average farther from the nucleus than other electrons; they are directly involved in chemical reaction.

visible light
Visible light is electromagnetic radiation* with a wavelength* between 400 and 750 nm.
wave
An oscillating motion that moves outward from the source of some disturbance (ripples running away from a pebble tossed in a pond). Waves transmit the energy of the disturbance away from its source.

wavelength
The distance between adjacent peaks (or adjacent troughs) on a wave. Varying the wavelength of light changes its color; varying the wavelength of sound changes its pitch.

x-rays
A very high energy form of electromagnetic radiation* (though not as high energy as gamma rays*). X-rays typically have wavelengths from a few picometers up to 20 nanometers. X-rays easily penetrate soft tissue, which makes them useful in medical imaging and in radiation therapy. Studying x-ray diffraction* patterns gives detailed information on the three-dimensional structure of crystals, surfaces, and atoms.