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.
Click here to buy a book, photographic periodic table poster, card deck, or 3D print based on the images you see here! 18 8 valence electrons (except helium which has only 2 valence electrons) The atomic number of cesium is 55 which belongs to s -block elements. Cesium is represented by Cs.
- Caesium is element number 55. Its electronic configuration is mathXe 6s^1/math. So, the principal quantum number, which describes the shell, is mathn=6/math, as it is in the 6th 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.
Cs Valence Electrons
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. Chrome icon for mac. 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.
As mentioned above, the characteristic chemical property of a metalatom is to lose one or more of its electrons to form a positive ion. However, certain metals lose electrons much more readily than others. In particular, cesium (Cs) can give up its valence electron more easily than can lithium (Li). In fact, for the alkali metals (the elements in Group 1), the ease of giving up an electron varies as follows: Cs > Rb > K > Na > Li with Cs the most likely, and Li the least likely, to lose an electron. In going down the group, the metals become more likely to lose an electron because the electron being removed lies increasingly farther from the positive nucleus. That is, the electron lost from Cs to form Cs+ lies at a much greater distance from the attractive positive nucleus—and is thus easier to remove—than the electron that must be removed from a lithium atom to form Li+. The same trend also is seen among the Group 2 elements (the alkaline-earth metals); the farther down in the group the metal resides, the more likely it is to lose an electron.
Just as metals vary somewhat in their properties, so do nonmetals. As a general rule, the most chemically active metals appear in the lower left-hand region of the periodic table, whereas the most chemically active nonmetals appear in the upper right-hand region. The properties of the semimetals, or metalloids, lie between those of the metals and the nonmetals.
The ionization energy of an element is the energy required to remove an electron from an individual atom. Here M(g) represents a metal in the vapour state.
Metal atoms lose electrons to nonmetal atoms because metals typically have relatively low ionization energies. Metals at the bottom of a group lose electrons more easily than those at the top. That is, ionization energies tend to decrease in going from the top to the bottom of a group. Nonmetals, which are found in the right-hand region of the periodic table, have relatively large ionization energies and therefore tend to gain electrons. Ionization energies generally increase in going from left to right across a given period. Thus, the elements that appear in the lower left-hand region of the periodic table have the lowest ionization energies (and are therefore the most chemically active metals), while the elements that occur in the upper right-hand region of the periodic table have the highest ionization energies (and are thus the most chemically active nonmetals).
As mentioned above, when a nonmetallic element reacts with a metallic element, electrons are transferred from the atoms of the metal to the atoms of the nonmetal, forming positive ions (cations) and negative ions (anions), respectively. This produces an ionic compound. For example, lithium and fluorine (F) react to form lithium fluoride (LiF), which contains Li+ and F− ions.
In contrast, when two nonmetallic elements react, the atoms combine to form molecules by sharing electrons. Bonds formed by electron sharing between atoms are called covalent bonds. The electrons are shared rather than transferred, because the two nonmetal atoms have comparable attractive powers for the electrons in the bond. For example, fluorinegas consists of F2 molecules in which the fluorine atoms are bound together by sharing a pair of electrons, one contributed by each atom. In addition, hydrogen and fluorine react to form hydrogen fluoride, which contains HF molecules. The hydrogen and fluorine atoms are bound together by a pair of electrons, one electron contributed by the hydrogen atom and one by the fluorine atom. Although the electrons are shared between the hydrogen and the fluorine atoms, in this case they are not shared equally. This is clear from the fact that the HF molecule is polar; the hydrogen atom has a partial positive charge (δ+), while the fluorine atom has a partial negative charge (δ−): H―F
δ+ δ− (In this example the symbol δ stands for a number less than one.) This electrical polarity occurs because the shared electrons spend more time close to the fluorine atom than to the hydrogen atom. That is, fluorine has greater affinity for the shared electrons than does hydrogen. This leads to a polar covalent bond.
The ability of an atom to attract the electrons shared with another atom is termed its electronegativity. The relative electronegativities of the various atoms can be determined by measuring the polarities of the bonds involving the atoms in question. Fluorine has the greatest electronegativity value (4.0, according to the Pauling scale), and cesium and francium have the smallest values (0.79 and 0.7, respectively). In general, nonmetal atoms have higher electronegativities than metal atoms. In the periodic table, electronegativity typically increases in moving across a period and decreases in going down a group. When elements with very different electronegativities (such as fluorine and cesium) react, one or more electrons are transferred to form an ionic compound. For example, cesium and fluorine react to form CsF, which contains Cs+ and F− ions. When nonmetal atoms with differing electronegativities react, they form molecules with polar covalent bonds.
Cesium Valence Electrons Number
Another important atomic property is atomic size. The sizes of atoms vary; atoms generally tend to become larger in going down a group on the periodic table and smaller in going from left to right across a period.