Energy, ionization

Ionization energy (IE) is the minimum energy required to remove an electron from an atom in the gas phase. Typically, we express ionization energy in kJ/mol, the number of kilojoules required to remove a mole of electrons from a mole of gaseous atoms. Sodium, for example, has an ionization energy of 495.8 kJ/mol, meaning that the energy input required to drive the process 

Student Annotation As with atomic radius, ionization energy changes in a similar but somewhat less regular way among the transition elements. 

It is possible to remove additional electrons in subsequent ionizations, giving/E2, /E3, and so on. The second and third ionizations of sodium, for example, can be represented, respectively, as 

However, the removal of successive electrons requires ever-increasing amounts of energy because it is harder to remove an electron from a cation than from an atom (and it gets even harder as the charge on the cation increases). Table 7.2 lists the ionization energies of the elements in period 

2 and of sodium. These data show that it takes much more energy to remove core electrons than to remove valence electrons. There are two reasons for this. First, core electrons are closer to the nucleus, and second, core electrons experience a greater effective nuclear charge because there are fewer filled shells shielding them from the nucleus. Both of these factors contribute to a greater attractive force between the electrons and the nucleus, which must be overcome to remove the electrons. 

The ionization energy, also known as the ionization potential, is the energy required to remove an electron from a gaseous atom or ion  

At the fourth 2p electron, at oxygen, a similar decrease in ionization energy occurs. Here, the fourth electron shares an orbital with one of the three previous 2p electrons 

A plot of the first ionization energies for selected elements as a function of increasing atomic number. The peaks occur for the noble gases. 

For any given element, the second and third ionization energies are always larger than the first ionization energy. Table 5.7 lists the first several ionization energies for the more common elements. 

Representation of the periodic table showing the division line between the metals and the nonmetals. 

The increase in first ionization energy from left to right across a period and from bottom to top in a group for representative elements. 

The magnitude of ionization energy is a measure of how tightly the electron is held in the atom. The higher the ionization energy, the more difficult it is to remove the electron. For a matty-electron atom, the amoimt of energy required to remove the first electron from the atom in its groimd state. 

When an electron is removed from an atom, the repulsion among the remaining electrons decreases. Because the nuclear charge remains constant, more energy is needed to remove another electron from the positively charged iom Thus, ionization energies always increase in the following order  

Variation of the first ionization energy with atomic number. Note that the noble gases have high ionization energies, whereas the alkali metals and alkaline earth metals have low ionization energies. 

The Group 2A elements (the alkaline earth metals) have higher first ionization energies than the alkali metals do. The alkaline earth metals have two valence electrons (the outermost electron configuration is n ). Because these two 5 electrons do not shield each other well, the effective nuclear charge for an alkaline earth metal atom is larger than that for the preceding alkali metal. Most alkaline earth compounds contain dipositive ions (Mg, Ca, Sr, Ba ). The Be ion is isoelectronic with Li and with He, Mg, is isoelectronic with Na and with Ne, and so on. 

Sample Problem 7.4 shows how to use these trends to compare first ionization energies, and subsequent ionization energies, of specific atoms. 

Would you expect Na or Mg to have the greater first ionization energy (/ i) Which should have the greater second ionization energy (IE,)  

Strategy Consider effective nuclear chaige and electron configuration to compare the ionization energies. Effective nuclear charge increases from left to right in a period (thus increasing IE), and it is more difficult to remove a paired core electron than an unpaired valence electron. 

Penning ionization occurs with the (trace) gas M having an ionization energy lower than the energy of the metastable state of the excited (noble gas) atoms A. The above ionization processes have also been employed to construct mass spectrometer ion sources. [21,24] However, Penning ionization sources never escaped the realm of academic research to find widespread analytical application. 

It is obvious that ionization of the neutral can only occur when the energy deposited by the electron-neutral collision is equal to or greater than the ionization energy (IE) of the corresponding neutral. Formerly, the ionization energy has been termed ionization potential (IP). 

Definition The ionization energy (IE) is defined as the minimum amount of energy which has to be absorbed by an atom or molecule in its electronic and vibrational ground states form an ion that is also in its ground states by ejection of an electron. 

Removal of an electron from a molecule can formally be considered to occur at a a-bond, a 7i-bond or at a lone electron pair with the a-bond being the least favorable and the lone electron pair being the most favorable position for charge-localization within the molecule. This is directly reflected in the lEs of molecules (Table 2.1). Nobel gases do exist as atoms having closed electron shells and there- 

Molecules with m-bonds have lower lEs than those without, causing the IE of ethene to be lower than that of ethane. Again the IE is reduced further with increasing size of the alkene. Aromatic hydrocarbons can stabilize a single charge even better and expanding m-systems also help making ionization easier. 

Note Ionization energies of most molecules are in the range of 7-15 eV. 

An important property of atoms that is related to their chemical behavior is the ionization potential or ionization energy. In general, ionization energy can be defined as the energy needed to remove an electron from a gaseous atom. For hydrogen, there is only one ionization potential because the atom has only one electron. Atoms having more than one electron have an ionization potential for each electron, and these often differ markedly. 

After the first electron is removed, succeeding electrons are removed from an ion that is 

The highest first ionization energy, about 2400 kJ mol-1, is for He. As a group, the noble gases have the highest ionization energies and the alkali metals have the lowest. 

For some elements, the first ionization energy alone is not always relevant because the elements may not exhibit a stable oxidation state of +1. For example, in Group IIA, the 

The effect of closed shells is apparent. For example, sodium has a first ionization energy of only 495.8 kJ mol-1, whereas the second is 4562.4 kJ mol-1. The second electron removed comes from Na+, and it is removed from the filled 2p shell. For Mg, the first two ionization potentials are 737.7 and 1450.7 kJ moF1, and the difference represents the additional energy necessary to remove an electron from a +1 ion. Thus, the enormously high second ionization energy for Na is largely due to the closed shell effect. 

The ease with which electrons can be removed from an atom or ion has a major impact on chemical behavior. The ionization energy of an atom or ion is the minimum energy required to remove an electron from the ground state of the isolated gaseous atom or ion. We first encountered ionization in our discussion of the Bohr model of the hydrogen atom. CEO (Section 6.3) If the electron in an H atom is excited from n = 1 (the ground state) to = the electron is completely removed from the atom the atom is ionized. 

In general, the first ionization energy, f, is the energy needed to remove the first electron from a neutral atom. For example, the first ionization energy for the sodium atom is the energy required for the process 

Atoms or groups of atoms may lose electrons to form positive ions, known as cations. 

Positives attract negatives. In order to pull the negative electron away from the positive nucleus, energy will be required to break the attractive force. 

like atoms, have size. For ions, the term is ionic radii. For cations, the loss of electrons results in a decrease in size, since (for the representative metals) an entire energy level is usually lost. A sodium ion, Na+, is smaller than a sodium atom. The greater the number of electrons removed, the greater the decrease in radius. This applies to any element and its cations as illustrated by the trend in radii of Fe Fe2+ Fe3+. 

This is an isoelectronic series, with all ions having 18 electrons. In such a series, size decreases as nuclear charge (atomic number) increases. The atomic numbers of the ions are S 16, Cl 17, K 19, Ca 20. Thus, the ions decrease in size in the order CF K Ca.  

The greater the ionization energy, the more difficult it is to remove an electron. 

For the transition elements, the variations are not so regular because electrons are being added to an inner shell, but the general trend of decreasing radii certainly continues as one moves across the d- or/-block metals. All transition elements have smaller radii than the preceding Group lA and 2A elements in the same period. 

Arrange the following elements in order of increasing atomic radii. Justify your order. 

Both K and Cs are Group 1A metals, whereas F and Cl are halogens (7A nonmetals). 

Remember the trends in atomic size—across within a row and verticaiiy within a coiumn. 

In the previous sections, we have seen how the number of electrons and the number of protons affects the size of an atom or ion. However, we have not considered how the number of neutrons affects the size of an atom. Why not Would you expect isotopes— for example, C-12 and C-13—to have different atomic radii  

The energy required to remove the second electron is the second ionization energy lEf), the energy required to remove the third electron is the third ionization energy (JEf), and so on. We represent the second ionization energy of sodium as  

Notice that the second ionization energy is not the energy required to remove two electrons from sodium (that quantity is the sum of lEi and IE2), but rather the energy required to remove one electron from Na. We look at trends in lEi and IE2 separately. 

PRACTICE EXAMPLE A Refer only to the periodic table on the inside front cover, and arrange the following species in order of increasing size Ti, Ca , Br, and Sr.  

On the blank periodic table in the margin, locate the following  

Ionization energies are usually measured through experiments in which gaseous atoms at low pressures are bombarded with photons of sufficient energy to eject an electron from the atom. Here are two typical values. 

M A distinction between valence electrons and core electrons can be made based on the ionization energies for removing electrons one by one. The ionization energies of valence electrons are much smaller and show a big jump when the first core electron is removed. 

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S = Heat of sublimation of sodium D = Dissociation energy of chlorine / = Ionization energy of sodium = Electron affinity of chlorine Uq = Lattice energy of sodium chloride AHf = Heat of formation of sodium chloride. 

Lias S G 1998 ionization energy evaiuation NIST Chemistry WebBook, NIST Standard Reference Database Number 69 ed W G Maiiard and P J Linstrom (Gaithersburg, MD Nationai institute of Standards and Technoiogy)... 

Lias S G 1997 ionization energies of gas phase moiecuies Handbook of Chemistry and Physics ed D R Lide (Boca Raton, FL CRC Press)... 

Such a series of lines is called a Rydberg series [26]. These lines also converge to the ionization energy of the atom or molecule, and fitting the lines to this fonuula can give a very accurate value for the ionization energy. In the case of molecules there may be resolvable vibrational and rotational stmcture on the lines as well. 

Figure Bl.6.12 Ionization-energy spectrum of carbonyl sulphide obtained by dipole (e, 2e) spectroscopy [18], The incident-electron energy was 3.5 keV, the scattered incident electron was detected in the forward direction and the ejected (ionized) electron detected in coincidence at 54.7° (angular anisotropies cancel at this magic angle ). The energy of the two outgoing electrons was scaimed keeping the net energy loss fixed at 40 eV so that the spectrum is essentially identical to the 40 eV photoabsorption spectrum. Peaks are identified with ionization of valence electrons from the indicated molecular orbitals.

Time-of-flight mass spectrometers have been used as detectors in a wider variety of experiments tlian any other mass spectrometer. This is especially true of spectroscopic applications, many of which are discussed in this encyclopedia. Unlike the other instruments described in this chapter, the TOP mass spectrometer is usually used for one purpose, to acquire the mass spectrum of a compound. They caimot generally be used for the kinds of ion-molecule chemistry discussed in this chapter, or structural characterization experiments such as collision-induced dissociation. Plowever, they are easily used as detectors for spectroscopic applications such as multi-photoionization (for the spectroscopy of molecular excited states) [38], zero kinetic energy electron spectroscopy [39] (ZEKE, for the precise measurement of ionization energies) and comcidence measurements (such as photoelectron-photoion coincidence spectroscopy [40] for the measurement of ion fragmentation breakdown diagrams). 

Used effects Phonon excitation (20 meV-1 eV) Plasmon and interband excitations (1-50 eV) Inner-shell ionization (A = ionization energy loss) Emission of x-ray (continuous/characteristic, analytical EM)... 

The photon statistical weight is g = 2, corresponding to the two directions of polarization of the photon. The photon energy E is related to its momentum p and wavenumber k and to the ionization energy of the... 

In recent years, these methods have been greatly expanded and have reached a degree of reliability where they now offer some of the most accurate tools for studying excited and ionized states. In particular, the use of time-dependent variational principles have allowed the much more rigorous development of equations for energy differences and nonlinear response properties [81]. In addition, the extension of the EOM theory to include coupled-cluster reference fiuictioiis [ ] now allows one to compute excitation and ionization energies using some of the most accurate ab initio tools. 

The ionization energies and impurity levels are shown in the flat-band figure next to the configuration diagram. 

The donor level (-1- / 0) corresponds to the ionization energy essentially identical configurations. The ionization energy measured by pSR is very close to the donor level obtained for hydrogen by DLTS [30], 0.175 0.005 eV. 

The ionization energy for hydrogen is the minimum amount of energy that is required to bring about the reaction... 

The ionization energy for hydrogen (or other hydrogen-like systems) can be found from the Rydberg equation... 

Procedure. Use Mathcad, QLLSQ, or TableCurve (or, preferably, all three) to determine a value of the ionization energy of hydrogen from the wave numbers in Table 3-4 taken from spectroscopic studies of the Lyman series of the hydrogen spectrum where ni = 1. 

Note that we are interested in nj, the atomic quantum number of the level to which the electron jumps in a spectroscopic excitation. Use the results of this data treatment to obtain a value of the Rydberg constant R. Compare the value you obtain with an accepted value. Quote the source of the accepted value you use for comparison in your report. What are the units of R A conversion factor may be necessary to obtain unit consistency. Express your value for the ionization energy of H in units of hartrees (h), electron volts (eV), and kJ mol . We will need it later. 

The MNDO output from this four-line input file eontains the ionization energy along with other information (Fig. 9-3). The results for the three methods are MNDO 11.91, AMI 11.40, and PM3 13.07 eV. The experimental value is 13.61 eV. 

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