The question wants you to recognize the fact that cesium, 'Cs', is a metal, or an alkali metal, to be precise, and so it cannot react with other metals. With the exception of hydrogen, elements located in group 1 of the periodic table are called alkali metals and are all characterized by the fact that they have one valence electron. That is, these elements have one electron located in. Caesium or cesium is a chemical element with symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of 28 °C (82 °F), which makes it one of only five elemental metals that are liquid at or near room temperature. Its 58 electrons are arranged in the configuration Xe4f 1 5d 1 6s 2, of which the four outer electrons are valence electrons. The 4f, 5d, and 6s energy levels are very close to each other, and the transfer of one electron to the 5d shell is due to strong interelectronic repulsion in the compact 4f shell. A cesium atom has 1 valence electron. It is an alkali metal, and all alkali metals have 1 valence electron. The electron configuration for cesium is (Rn)7s1. The single electron in the 7s sublevel. Lithium, sodium, potassium, rubidium, cesium. Place the elements above in order from smallest energy needed to remove the outermost electron to the largest energy needed to remove the outermost electron. How many valence electrons are present in the dot notation for potassium? Which alkaline earth metals should have the smallest radius?
To understand how remarkable the cesium atom is as a basis for the cesium atomic clock, it is necessary to examine the details of the structure of this atom. Having 55 protons in its nucleus, it must have 55 electrons in orbit around that nucleus to be a neutral atom. The electron states are described by four quantum numbers, and those of cesium fill all the electron states that are part of the noble gas xenon (54 electrons) and then there is just one additional electron outside that symmetric electron distribution. Following the order of filling of the electron shells, the xenon structure fills all the levels up through the 5p electrons. The next available energy level is the 6s electron, so the chemistry of cesium is determined by that lone 6s electron.
It is typical for the quantum energy state of such a valence electron to be split by fine structure arising from the magnetic interaction of the electron spin with the orbital angular momentum of the inner electrons. But the inner core of the electrons of cesium is perfectly symmetrical and has zero angular momentum, so there is no fine structure.
However, the cesium nucleus exhibits a magnetic influence associated with the nuclear spin, which has the large value 7/2. Though quantum mechanical in nature, the interaction can be visualized as the interaction between two magnets - that of the nucleus and that of the electron, and there are two very closely spaced energy levels for the 6s electron based on whether the nuclear and electron spins are parallel or antiparallel. This splitting is called 'hyperfine structure'. It is this pair of precise and closely spaced energy levels that makes possible the cesium clock mechanism.
One remarkable aspect of the interaction which makes possible the cesium clock is the vast separation, relatively speaking, between the nuclear spin and the electron spin with which it interacts. The nuclear radius for a nucleus of mass number A=133 is 6.1 x 10-15 meters or 6.1 Fermis. The radius of the cesium atom where the 6s electron resides can be found from the periodic table to be 3.34 x 10-10 meters or 0.334 nm, almost 55,000 times as large!
Another remarkable comparison is that of the energy levels involved. The 6s valence electron can be ejected from the atom by an ultraviolet photon of quantum energy 3.9 electron volts. By comparison, the pair of hyperfine levels involved in the cesium clock are separated by only 0.000038 eV, about 100,000 times smaller. This energy is in the microwave region, and is about a thousand times smaller than the random thermal energy of about 0.04 eV associated with the 100 °C temperature at which the cesium clock is typically operated.
Most of us learned in high school chemistry class that chemical bonds can only form when electrons are shared or given away from one atom’s outer shell to another’s. But this may not be strictly true. A chemist has calculated that under very high pressure not just the outer electrons but the inner ones, too, could form bonds.
Inside atoms, electrons are organized into energy levels, called shells, which can be thought of as buckets of increasing size that can each hold only a fixed number of electrons. Atoms prefer to have filled buckets, so if their outer shell is missing just one or two electrons, they are eager borrow form another atom that might have one or two to spare. But sometimes, a new study suggests, atoms can be incited to share not just their outer valence electrons, but those from their full inner shells. “It breaks our doctrine that the inner-shell electrons never react, never enter the chemistry domain,” says Mao-sheng Miao, a chemist at the University of California, Santa Barbara, and the Beijing Computational Science Research Center in China. Miao predicted such bonds using so-called first-principles calculations, which rely purely on the known laws of physics, and reported his findings in a paper published September 23 in Nature Chemistry. Such bonding has yet to be demonstrated in a lab. Nevertheless, “I’m very confident that this is real,” he says. (Scientific American is part of Nature Publishing Group.)
His calculations show that two possible molecules could form between cesium and fluorine atoms under extremely high pressure—about 30 gigapascals (higher than the pressure at the bottom of the ocean, but less than at Earth’s center). Cesium, all the way on the left side of the periodic table, has one superfluous electron in its outer, or sixth shell. Fluorine, on the other hand, is toward the far right of the table, just next to the column of noble gases with completely full shells (which is why noble gases are notoriously unreactive—they have little incentive to gain or lose electrons) and is one electron short of a full outer shell. “Under normal pressure, cesium gives an electron completely to fluorine and they bind together,” Miao says. “But under high pressure, the electrons from cesium’s inner shells start to form molecules with fluorine.”
Cesium Valence Electron Count
Miao identified two compounds that could form and remain stable up to very high pressures: cesium trifluoride (CsF3), where cesium has shared its one valence electron and two from an inner shell with three fluorine atoms, and cesium pentafluoride (CsF5), where cesium shares its valence electron and four inner-shell electrons to five fluorine atoms. “That forms a very beautiful molecule, like a starfish,” Miao says. Both the shape of the resulting molecules and the possibility of their formation are “very surprising,” says chemist Roald Hoffmann, a professor emeritus at Cornell University, who was not involved in the calculations. “This is the first clear case of an alkali metal not only losing its single easily ionized valence electron in bonding, but also ‘breaking into the core’ in its bonding with several fluorines.”
Does Cesium Have Valence Electrons
The reason these reactions may occur has to do with enthalpy—a measure of the total energy of a system at constant pressure. Chemical reactions tend to move toward products with lower enthalpy. Miao calculated the enthalpy of cesium fluoride (the basic one-to-one bond of two atoms that forms naturally) and the enthalpy of the possible compounds cesium trifluoride and cesium pentafluoride. He found that above certain pressure thresholds, these larger molecules had lower enthalpy—and therefore were likely to form. “Pretty much everything we see in terms of structure and bonding is merely a manifestation of the system finding ways to minimize the potential energy, balancing the energy gain from the energy cost,” says Purdue University chemist Paul Wenthold, who was not involved in the study. “Although inner-shell electron oxidation is not normally expected for something like cesium, if you put it in close enough proximity of something with a high enough propensity to accept electrons, sure enough, it will do it.”
Cesium Element Valence Electrons
So far, no one has attempted experiments to make these molecules in a laboratory, but Miao says it should be possible, although fluorine is difficult to work with. The pressures necessary are well within reach of modern equipment. “Experimental studies on these systems will provide an excellent opportunity to calibrate theoretical chemistry of systems at high pressure,” says chemist Lee Sunderlin of Northern Illinois University, who also was not involved in the study. “This is an excellent example of a theoretical approach that can guide experimentalists in choosing systems that should exhibit unprecedented properties.” Now that this basic rule of high school chemistry has been overwritten, who knows what other molecular surprises could be in store?