Learning Outcomes

Describe and also explain the observed fads in atomic size, ionization energy, and also electron affinity of the elements

The elements in groups (vertical columns) of the periodic table exhibit similar chemical behavior. This similarity occurs since the members that a team have the exact same number and distribution of electron in their valence shells. However, there are likewise other fads in chemistry properties ~ above the routine table. For example, together we relocate down a group, the metallic character of the atom increases. Oxygen, at the peak of team 16 (6A), is a colorless gas; in the center of the group, selenium is a semiconducting solid; and, toward the bottom, polonium is a silver-grey solid the conducts electricity.

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As we go throughout a duration from left to right, we include a proton to the nucleus and an electron come the valence shell with each successive element. Together we go down the aspects in a group, the variety of electrons in the valence shell stays constant, yet the principal quantum number increases by one each time. An expertise of the digital structure the the elements permits us come examine several of the properties the govern their chemical behavior. These properties differ periodically together the electronic structure the the elements changes. They are (1) dimension (radius) that atoms and ions, (2) ionization energies, and (3) electron affinities.


Explore visualizations the the routine trends disputed in this section (and many more trends) ~ above the Atomic variety of the elements website. With simply a couple of clicks, friend can develop three-dimensional versions of the routine table reflecting atomic size or graphs of ionization energies from every measured elements.

Variation in Covalent Radius

The quantum mechanical snapshot makes it an overwhelming to create a definite size of one atom. However, there space several handy ways to define the radius of atoms and, thus, to determine their relative sizes that offer roughly similar values. We will usage the covalent radius (Figure 1), i m sorry is characterized as one-half the distance in between the nuclei the two identical atoms once they are joined by a covalent bond (this measurement is possible because atom within molecules still retain lot of your atomic identity). We understand that as we scan under a group, the primary quantum number, n, increases by one because that each element. Thus, the electrons space being included to a an ar of an are that is increasingly far-off from the nucleus. Consequently, the size of the atom (and the covalent radius) must rise as we increase the distance of the outermost electron from the nucleus. This tendency is shown for the covalent radii of the halogens in Table 1 and Figure 1. The patterns for the whole periodic table deserve to be watched in figure 1.

Table 1. Covalent Radii that the Halogen team ElementsAtomCovalent radius (pm)Nuclear charge
F64+9
Cl99+17
Br114+35
I133+53
At148+85

Figure 1. (a) The radius of an atom is identified as one-half the distance in between the nuclei in a molecule consisting of two similar atoms join by a covalent bond. The atomic radius for the halogens boosts down the team as n increases. (b) Covalent radii of the aspects are presented to scale. The basic trend is that radii increase down a group and decrease across a period.


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Figure 2. Within every period, the trend in atom radius decreases together Z increases; for example, indigenous K come Kr. Within each group (e.g., the alkali metals shown in purple), the trend is that atomic radius rises as Z increases.


As presented in number 2, as we move across a period from left to right, us generally discover that each aspect has a smaller sized covalent radius than the element preceding it. This might seem counterintuitive because it suggests that atom with more electrons have actually a smaller atomic radius. This have the right to be described with the principle of effective atom charge, Zeff. This is the traction exerted top top a certain electron by the nucleus, taking right into account any type of electron–electron repulsions. For hydrogen, over there is just one electron and also so the nuclear fee (Z) and also the efficient nuclear fee (Zeff) room equal. For all other atoms, the inner electrons partly shield the external electrons native the traction of the nucleus, and thus:

Z_ exteff=Z- extshielding

Shielding is established by the probability of another electron being in between the electron the interest and also the nucleus, and by the electron–electron repulsions the electron of interest encounters. Core electrons are adept at shielding, while electron in the exact same valence shell execute not block the atom attraction experienced by each various other as efficiently. Thus, every time we relocate from one element to the next across a period, Z increases by one, but the shielding rises only slightly. Thus, Zeff increases as we move from left to right throughout a period. The stronger pull (higher efficient nuclear charge) experienced by electrons on the right side the the periodic table draws lock closer come the nucleus, making the covalent radii smaller.

Thus, together we would certainly expect, the outermost or valence electron are simplest to remove since they have actually the greatest energies, are shielded more, and also are farthest from the nucleus. Together a basic rule, as soon as the representative elements type cations, they do so by the lose of the ns or np electrons the were included last in the Aufbau process. The shift elements, ~ above the various other hand, shed the ns electrons before they begin to lose the (n – 1)d electrons, even though the ns electrons are included first, follow to the Aufbau principle.


Figure 3. The radius because that a cation is smaller sized than the parental atom (Al), due to the lost electrons; the radius because that an anion is bigger than the parent (S), because of the gained electrons.


Cations with bigger charges are smaller than cations with smaller sized charges (e.g., V2+ has actually an ionic radius that 79 pm, while that of V3+ is 64 pm). Proceeding under the teams of the regular table, we discover that cations of successive facets with the same charge typically have bigger radii, corresponding to an increase in the principal quantum number, n.

An anion (negative ion) is formed by the enhancement of one or an ext electrons to the valence shell of one atom. This outcomes in a better repulsion among the electrons and also a decrease in Zeff every electron. Both impacts (the increased number of electrons and also the decreased Zeff) cause the radius of an anion come be larger than that of the parental atom (Figure 3). Because that example, a sulfur atom (3s23p4) has actually a covalent radius that 104 pm, whereas the ionic radius that the sulfide anion (3s23p6) is 170 pm. Because that consecutive elements proceeding down any group, anions have larger principal quantum number and, thus, bigger radii.

Atoms and also ions that have actually the very same electron construction are stated to be isoelectronic. Examples of isoelectronic species are N3–, O2–, F–, Ne, Na+, Mg2+, and also Al3+ (1s22s22p6). Another isoelectronic collection is P3–, S2–, Cl–, Ar, K+, Ca2+, and also Sc3+ (3s23p6). For atoms or ions that room isoelectronic, the number of protons identify the size. The greater the nuclear charge, the smaller the radius in a series of isoelectronic ions and also atoms.

Variation in Ionization Energies

The lot of power required to remove the many loosely bound electron indigenous a gas atom in its soil state is dubbed its an initial ionization energy (IE1). The first ionization energy for an element, X, is the power required to kind a cation v +1 charge:

extXleft(g ight)longrightarrow extX^ ext+left(g ight)+ exte^- extIE_1

The power required to remove the 2nd most loose bound electron is called the second ionization energy (IE2).

extX^ ext+left(g ight)longrightarrow extX^2+left(g ight)+ exte^- extIE_2

The energy required to eliminate the 3rd electron is the 3rd ionization energy, and also so on. Power is constantly required to remove electrons from atoms or ions, so ionization processes are endothermic and IE values are always positive. For larger atoms, the most loosely bound electron is located farther from the nucleus and also so is less complicated to remove. Thus, as size (atomic radius) increases, the ionization energy should decrease. Relating this reasonable to what we have just learned around radii, we would certainly expect first ionization energies to decrease under a group and also to increase throughout a period.

Figure 4 graphs the relationship in between the very first ionization energy and the atomic variety of several elements. The worths of very first ionization power for the facets are offered in figure 5. Within a period, the IE1 normally increases with enhancing Z. Under a group, the IE1 value normally decreases with boosting Z. There are some systematic deviations indigenous this trend, however. Note that the ionization power of boron (atomic number 5) is much less than the of beryllium (atomic number 4) also though the nuclear charge of boron is higher by one proton. This can be explained since the power of the subshells increases as l increases, as result of penetration and also shielding (as disputed previously in this chapter). Within any kind of one shell, the s electrons are reduced in energy than the p electrons. This means that an s electron is more difficult to remove from one atom 보다 a p electron in the same shell. The electron removed during the ionization that beryllium (2s2) is an s electron, conversely, the electron removed during the ionization that boron (2s22p1) is a p electron; this results in a lower first ionization power for boron, even though that is nuclear fee is higher by one proton. Thus, we watch a little deviation from the suspect trend emerging each time a brand-new subshell begins.


Figure 4. The an initial ionization energy of the elements in the an initial five durations are plotted against their atomic number.



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Figure 5. This version of the periodic table shows the first ionization power (IE1), in kJ/mol, the selected elements.


Another deviation occurs together orbitals become much more than one-half filled. The very first ionization power for oxygen is slightly less than that for nitrogen, despite the trend in boosting IE1 values throughout a period. Looking in ~ the orbital diagram of oxygen, we deserve to see the removing one electron will get rid of the electron–electron repulsion led to by pairing the electrons in the 2p orbital and will result in a half-filled orbital (which is energetically favorable). Analogous transforms occur in succeeding periods (note the dip for sulfur after phosphorus in number 5).

Removing an electron indigenous a cation is more challenging than removing an electron indigenous a neutral atom because of the better electrostatic attraction come the cation. Likewise, removed an electron indigenous a cation v a greater positive charge is more complicated than remove an electron indigenous an ion v a reduced charge. Thus, succeeding ionization energies because that one element constantly increase. As seen in Table 2, there is a huge increase in the ionization energies (color change) for each element. This jump coincides to removed of the main point electrons, which space harder to remove than the valence electrons. Because that example, Sc and Ga both have actually three valence electrons, therefore the rapid boost in ionization energy occurs ~ the third ionization.

Table 2. Successive Ionization Energies because that Selected facets (kJ/mol)ElementIE1IE2IE3IE4IE5IE6IE7
K418.83051.84419.65876.97975.59590.611343
Ca589.81145.44912.46490.68153.010495.712272.9
Sc633.11235.02388.77090.68842.910679.013315.0
Ga578.81979.42964.661808298.710873.913594.8
Ge762.21537.53302.14410.69021.4N/AN/A
As944.51793.62735.54836.86042.912311.5N/A

Example 2: Ranking Ionization Energies

Predict the stimulate of increasing energy for the adhering to processes: IE1 because that Al, IE1 for Tl, IE2 because that Na, IE3 for Al.


Show Solution

Removing the 6p1 electron from Tl is less complicated than removed the 3p1 electron from Al because the greater n orbit is farther native the nucleus, so IE1(Tl) 1(Al). Ionizing the 3rd electron from extAlleft( extAl^2+longrightarrow extAl^3++ exte^ ext- ight) requires much more energy due to the fact that the cation Al2+ exerts a stronger pull ~ above the electron than the neutral Al atom, for this reason IE1(Al) 3(Al). The 2nd ionization power for sodium gets rid of a core electron, i m sorry is a much greater energy procedure than remove valence electrons. Putting this all together, we obtain: IE1(Tl) 1(Al) 3(Al) 2(Na).


Check her Learning

Which has actually the lowest value for IE1: O, Po, Pb, or Ba?


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Figure 6. This variation of the routine table displays the electron affinity worths (in kJ/mol) for selected elements.


The properties discussed in this section (size of atoms and also ions, efficient nuclear charge, ionization energies, and also electron affinities) are central to understanding chemical reactivity. For example, since fluorine has an energetically favorable EA and a large energy barrier to ionization (IE), the is much less complicated to form fluorine anions 보다 cations. Metallic properties consisting of conductivity and malleability (the capacity to be created into sheets) count on having electrons that can be eliminated easily. Thus, metallic character increases as we move down a group and also decreases across a duration in the very same trend observed because that atomic size because it is much easier to eliminate an electron that is farther far from the nucleus.

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Key Concepts and Summary

Electron configurations allow us come understand many periodic trends. Covalent radius boosts as we move down a group due to the fact that the n level (orbital size) increases. Covalent radius mainly decreases as we move left come right throughout a duration because the efficient nuclear charge skilled by the electrons increases, and the electrons space pulled in tighter come the nucleus. Anionic radii are larger than the parental atom, while cationic radii room smaller, because the variety of valence electrons has changed while the nuclear charge has actually remained constant. Ionization energy (the energy connected with developing a cation) decreases under a group and mostly increases across a duration because the is less complicated to remove an electron indigenous a larger, higher energy orbital. Electron affinity (the energy linked with developing an anion) is an ext favorable (exothermic) when electrons are inserted into lower energy orbitals, closer to the nucleus. Therefore, electron affinity i do not care increasingly an unfavorable as we relocate left come right throughout the regular table and decreases together we move down a group. Because that both IE and also electron affinity data, there space exceptions to the trends once dealing with totally filled or half-filled subshells.



Try It

Based on your positions in the periodic table, suspect which has the smallest atomic radius: Mg, Sr, Si, Cl, I.Based on their positions in the periodic table, predict which has the biggest atomic radius: Li, Rb, N, F, I.Based on your positions in the periodic table, predict which has actually the largest an initial ionization energy: Mg, Ba, B, O, Te.Based on their positions in the regular table, suspect which has actually the smallest an initial ionization energy: Li, Cs, N, F, I.Based on their positions in the routine table, location the following atoms in order of increasing very first ionization energy: F, Li, N, RbBased on their positions in the routine table, rank the following atoms or link in bespeak of increasing very first ionization energy: Mg, O, S, SiAtoms that which team in the routine table have a valence shell electron configuration of ns2np3?Atoms the which group in the regular table have a valence covering electron configuration of ns2?Based on their positions in the regular table, perform the adhering to atoms in bespeak of increasing radius: Mg, Ca, Rb, Cs.Based on your positions in the regular table, perform the adhering to atoms in stimulate of raising radius: Sr, Ca, Si, Cl.Based on your positions in the periodic table, list the complying with ions in stimulate of enhancing radius: K+, Ca2+, Al3+, Si4+.List the adhering to ions in order of enhancing radius: Li+, Mg2+, Br–, Te2–.Which atom and/or ion is (are) isoelectronic with Br+: Se2+, Se, As–, Kr, Ga3+, Cl–?Which the the following atoms and ions is (are) isoelectronic through S2+: Si4+, Cl3+, Ar, As3+, Si, Al3+?Compare both the numbers of protons and also electrons existing in each to rank the complying with ions in order of raising radius: As3–, Br–, K+, Mg2+.Of the five facets Al, Cl, I, Na, Rb, which has the most exothermic reaction? (E represents an atom.) What surname is offered to the energy for the reaction? Hint: keep in mind the process depicted go not correspond to electron affinity extE^ ext+left(g ight)+ exte^-longrightarrow extEleft(g ight)Of the five elements Sn, Si, Sb, O, Te, which has the most endothermic reaction? (E represents an atom.) What name is offered to the power for the reaction? extEleft(g ight)longrightarrow extE^ ext+left(g ight)+ exte^-The ionic radii the the ions S2–, Cl–, and K+ space 184, 181, 138 afternoon respectively. Define why these ion have various sizes also though castle contain the same number of electrons.Which main team atom would be expected to have actually the lowest second ionization energy?Explain why Al is a member of team 13 fairly than group 3?

1. Cl