periodic trends for ionization energy (ei
) vs. atomic number
: note that within
each of the seven periods the ei
(colored circles) of an element begins at a minimum
for the first column of the periodic table (the alkali metals), and progresses to a maximum
for the last column (the noble gases) which are indicated by vertical lines and labelled with a noble gas element symbol, and which also serve as lines dividing the 7 periods. the maximum
ionization energy for each row diminishes
as one progresses from row 1 to row 7 in a given column, due to the increasing distance of the outer electron shell from the nucleus as inner shells are added.
in physics and chemistry, ionization energy (american english spelling) or ionisation energy (british english spelling), denoted ei, is the minimum amount of energy required to remove the most loosely bound electron, the valence electron, of an isolated neutral gaseous atom or molecule. it is quantitatively expressed as
- x(g) + energy → x+(g) + e−
where x is any atom or molecule capable of ionization, x+ is that atom or molecule with an electron removed, and e− is the removed electron. this is generally an endothermic process. generally, the closer the outermost electrons to the nucleus of the atom , the higher the atom's or element's ionization energy.
the sciences of physics and chemistry use different measures of ionization energy. in physics, the unit is the amount of energy required to remove a single electron from a single atom or molecule, expressed as electronvolts. in chemistry, the unit is the amount of energy required for all of the atoms in a mole of substance to lose one electron each: molar ionization energy or enthalpy, expressed as kilojoules per mole (kj/mol) or kilocalories per mole (kcal/mol).
comparison of ei of elements in the periodic table reveals two periodic trends:
- ei generally increases as one moves from left to right within a given period (that is, row).
- ei generally decreases as one moves from top to bottom in a given group (that is, column).
the latter trend results from the outer electron shell being progressively farther from the nucleus, with the addition of one inner shell per row as one moves down the column.
the nth ionization energy refers to the amount of energy required to remove an electron from the species having a charge of (n-1). for example, the first three ionization energies are defined as follows:
- 1st ionization energy
- x → x+ + e−
- 2nd ionization energy
- x+ → x2+ + e−
- 3rd ionization energy
- x2+ → x3+ + e−
the term ionization potential is an older name for ionization energy, because the oldest method of measuring ionization energy was based on ionizing a sample and accelerating the electron removed using an electrostatic potential. however this term is now considered obsolete.
some factors affecting the ionization energy include:
- nuclear charge: the greater the magnitude of nuclear charge the more tightly the electrons are held by the nucleus and hence more will be ionization energy.
- number of electron shells: the greater the size of the atom, the less tightly the electrons are held by the nucleus and ionization energy will be less.
- effective nuclear charge (zeff): the greater the magnitude of electron shielding and penetration the less tightly the electrons are held by the nucleus, the lower the zeff of the electron, and hence less will be the ionization energy.
- type of orbital ionized: the atom having a more stable electronic configuration has less tendency to lose electrons and consequently has high ionization energy.
- occupancy of the orbital matters: if the orbital is half or completely filled then it is harder to remove electrons