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Home , Ionization energy, Periodic trends

Periodic Trends:

Learning Objectives

• Define . • Describe how the ionization energy changes within a . • Describe how the ionization energy changes within a . • Analyze the importance of ionization energy in determining element reactivity

Which animals in the herd are the easiest to hunt?

In any nature show, from the classic “Wild Kingdom” shows hosted by Marlin Perkins to the modern BBC World-produced series hosted by David Attenborough (“Planet Earth” and “Blue Planet” are two of the multi-volume examples), there is a sequence (or several) showing a hunt between predators (plural) and prey (also plural). Lions, tigers, wolves, hyenas, and other well- equipped carnivores or secondary consumers will systematically attack a group or herd to obtain their food. As they hunt, whether as an organized pack or in small groups, they must weigh the need to eat with the need to conserve useful energy, and not be injured or possibly killed in the process, with no meal to show for it.

As a result, these hunters and predators tend to stay to the outside, looking for the weakest prey that is closest to the edge, and therefore the easiest to remove.

The low-hanging fruit

It is very likely that you have gone apple-picking, or at least seen someone else participate in the activity. On fall weekends all over central New England (famously John “Johnny Appleseed” Chapman hailed from Leominster), orchards open to family festivals where the central activity is picking apples directly off the trees. Without fail, the first fruits picked are the ones closest to the picker and easiest to retrieve, or the “low-hanging fruit,” and the trees closer to the entrance (at the edge of the orchard) are the first attacked by eager toddlers and their families trying to keep everyone together and not have to worry about wanderers. As the day wears on, trees situated more toward the center are the next to be exploited in the mad ritual, and eventually adult shoulders and small platforms or carefully monitored ladders are relied upon to grab fruit that is higher up in the tree (it depends on what kind of facilities are available, as well as insurance…) by the end of the weekend, each tree looks like a picture painted by someone who started at the top and ran out of red paint halfway to the bottom.

In either case, whether one considers the unfortunate (but biologically necessary) fate of a hunted animal, or the classic experience of a piece of fruit to be enjoyed, the first ones to go are the easiest ones to obtain and remove, the ones that require the least amount of energy to do so. Animals or fruits that are more difficult to remove require more energy, and usually that energy is not available in the system, either because of fatigue, an instinctive calculation, or the rules of the farm.

Ionization Energy

Ionization is the process by which become , specifically positive ions (cations, pronounced cat-ions, like a charged feline). If there is enough energy available, an will be removed from an . The energy required to remove an electron is called ionization energy, and it varies from one element to another, but is very predictable with the trends in the .

The energy required to remove an electron from a neutral atom is known as the first ionization energy, and is given by the general formula model

Z + energy → Z+ + e-

The energy required to remove two follows (a second electron is removed),

Z+ + energy → Z2+ + 2e-

…and so on…

This energy is usually expressed in kJ/mol (kilojoules per ), which means the amount of energy it takes for all the atoms in a mole of the substance to lose one electron each.

Trends in ionization energy

Ionization energy is closely related to Coulomb’s Law. If a negatively-charged electron is closer to the positively-charged nucleus, it will have a greater attractive force, and therefore be held more tightly by the nucleus. As a result, elements with smaller atoms tend to have higher ionization . Contributing to this feature, atoms in a certain period (horizontal row on the periodic table) with more protons tend to have a greater force holding on to electrons in the same . Therefore, Ionization energy increases from left to right across the periodic table.

If an electron is further away from the nucleus, the attractive force decreases, as described and predicted by Coulomb’s Law. The outer electrons (known as the electrons) are the first to go, because they are the furthest away and require the least amount of energy to overcome the attractive force. Also, inner electrons (known as shielding electrons) provide an additional repulsive force that reduces the effect of the attraction of a nucleus. As a result, atoms with more energy levels tend to lower ionization energies, as they require less energy to remove electrons. Therefore, Ionization energy decreases from top to bottom down a group on the periodic table.

The trend in First Ionization Energy (IE1) is well-known in and charts and diagrams are very common and found quite easily.

Period 1 Element Symbol First Ionization Energy (IE1), kJ/mol 1 (H) 1312 2 (He) 2372

Period 2 Atomic number Element Symbol First Ionization Energy (IE1), kJ/mol 3 (Li) 520 4 (Be) 900 5 (B) 801 6 (C) 1087 7 (N) 1402 8 (O) 1314 9 (F) 1681 10 (Ne) 2081

Period 3 Atomic number Element Symbol First Ionization Energy (IE1), kJ/mol 11 (Na) 496 12 (Mg) 738 13 Aluminum (Al) 580 14 (Si) 787 15 (P) 1012 16 (S) 1000 17 (Cl) 1251 18 (Ar) 1521

Again, we are considering overall trends, and while one element in a series may not specifically follow that trend, these exceptions may be explained by other factors and ignored in favor of the larger trend.

When graphed as a line, the trends appear quite readily. Note the tendency for the Group 18 Noble gases to appear as peaks, and the Group 1 Alkali to appear as low points, documenting the ends and beginnings of periods respectively. Note also that additional lines may be drawn connecting these to show group (column) trends: noble

Simplified charts show just the noble gases:

Similar trends appear for Second Ionization Energies (IE2), third ionization energies (IE3), and so on: Ionization Energies (kJ/mol)

IE1 IE2 IE3 IE4 IE5 IE6 IE7 IE8 H 1312 He 2372 5250 Li 520 7297 11810 Be 899 1757 14845 21000 B 800 2426 3659 25020 32820 C 1086 2352 4619 6221 37820 47260 N 1402 2855 4576 7473 9442 53250 64340 O 1314 3388 5296 7467 10987 13320 71320 84070 F 1680 3375 6045 8408 11020 15160 17860 92010 Ne 2080 3963 6130 9361 12180 15240 20000 23100 Na 496 4563 6913 9541 13350 16600 20113 25666 Mg 737 1450 7731 10545 13627 17995 21700 25662

As more electrons are removed, it becomes much more difficult to remove remaining electrons because nuclear attraction increases greatly when shielding is not present. Each successive ionization energy shows a similar trend, but shifted. Second Ionization Energy is greatest in the first group (alkali metals) and lowest in the second group (alkaline earth metals), Third Ionization Energy is greatest in the second group, and lowest in group 13 (Boron family) Importance to reactivity In a , understanding ionization energy is important in order to understand the behavior of whether various atoms make ionic or covalent bonds with each other, or whether they will bond with other elements at all. Ionic bonds form between metals and , when atoms will lose (transfer) their outer electrons and atoms will gain them. Covalent bonds occur when nonmetal atoms share their electrons.

For instance, the ionization energy of Sodium () is 496 kJ/mol whereas Chlorine's first ionization energy is 1251.1 kJ/mol. Sodium’s relatively low ionization energy makes it easy to remove its outer electron, while Chlorine will gain an electron rather easily (the result of a few factors). Due to this difference in their ionization energy, when they chemically combine they make an ionic bond, and a salt forms. Elements that reside close to each other in the periodic table or elements that do not have much of a difference in ionization energy make polar covalent or covalent bonds (also called nonpolar). For example, carbon and oxygen reside close to each other on a periodic table, and combine, for example, to make CO2 (Carbon dioxide). Due to their relatively high ionization energies, neither element tends to lose electrons, so they, therefore, form a . Carbon and chlorine, although not next to each other, are near each other (both are nonmetals toward the right of the periodic table), combine to make CCl4 (Carbon tetrachloride) another that is covalently bonded.

The fact that some elements tend to lose electrons more easily makes them generally more reactive. The alkali metals (for example and cesium on the far left of the periodic table) tend to be more reactive than elements toward the middle of the periodic table (such as or ). They will react with nonmetals very easily, sometimes simply in moist air. Nonmetals also react with one another by similar methods, but there are other factors involved which will be covered later.

SUMMARY Trends

• Ionization energy refers to the amount of energy needed to remove an electron from an atom. • The ionization energy of the elements within a period generally increases from left to right. This is due to valence shell stability. • The ionization energy of the elements within a group generally decreases from top to bottom. This is due to electron shielding. • The noble gases possess very high ionization energies because of their full valence shells as indicated in the graph. Note that helium has the highest ionization energy of all the elements. • Elements with low ionization energy tend to be more reactive, and metals to the left tend to be more reactive than metals in the center

QUESTIONS:

1) a. What is an ?

b. What is ionization energy?

2) Infer how energy may be supplied to an atom to ionize it (think about some things we use in chemistry labs, just as one example):

3) a. What is the trend in IE across a period (row) from left to right? b. What causes this trend?

4) a. What is the trend in IE down a group (column) from top to bottom? b. What causes this trend?

5) Ionization energy is the ability to capture an electron: TRUE or FALSE? Justify your response.

6) The second ionization energy of Mg is greater than second ionization energy of Al: TRUE or FALSE? Justify your response.

7) Which group would generally have the lowest first ionization energy? a. Transition Metals (Groups 3-12) b. Alkali Metals (Group 1) c. Noble Gases (Group 18) d. Alkaline Earth Metals (Group 2) e. (Group 17) Justify your response.

8) Sulfur has a first ionization energy of 999.6 kJ/mol. has a first ionization energy of 403 kJ/mol. What bond do they form when chemically combined? a. Covalent b. Polar Covalent c. Ionic

9) Low first ionization energy is considered a property of a. Metals b. Nonmetals Justify your response

The following are “higher order” questions for Honors, but CP are encouraged to try as well. 10) Ionization energy, when supplied to an atom, results in a(n) a. Anion and a proton b. Cation and a proton c. Cation and an Electron d. Anion and an electron

11) has a first ionization energy of 580 kJ/mol, and Lithium 520 kJ/mol. Justify this based on atomic structure and .

12) Evaluate how the comparisons of hunting among predators and prey, and picking apples, to ionization energy are useful in understanding the ability of elements to react chemically.