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Quiz 7 The Periodic Table

Skills to develop

Elemental Properties

The following are some typical elemental properties. Since the properties are required often, abbreviations are often used. Some of the common abbreviations are given in parentheses. Common units, if any, are also given but make sure to note the proper units for your data. Properties of an element are related to its electronic configuration.

Magnetochemistry

The study of magnetic properties is called magnetochemistry. In this regard, we classify material according to three categories: diamagnetic, paramagnetic, and ferromagnetic.

Diamagnetic materials such as water, nitrogen, copper, calcium carbonates are slightly repelled by a magnetic field if they are placed between poles of a magnet. They are not attracted by such a magnetic field. Diamagnetism is a very weak interaction of all material with a magnetic field.

All materials exhibit diamagnetism, but some materials have other intrinsic but stronger magnetic properties than diamagnetism. As a result, they are classified according to the stronger magnetic properties.

If a material is attracted into a magnetic field when placed between magnetic poles, the material is said to be paramagnetic. A paramagnetism material has unpaired electrons in the molecular or atomic orbitals.

When atoms of an element have unpaired electrons, the element will be paramagnetic. For example, the electronic configuration of Co is [Ar]4s2 3d7, and 3 of the 7 d electrons are unpaired. You would expect Co to exhibit paramagnetism.

Iron, Fe, with electronic configuration [Ar]4s2 3d7, is a typical ferromagnetic material. Iron is greatly attracted by magnets. The "tinny magnets" of the atoms align themselves to enhance each other in ferromagnetic material. The aligned tinny magnets are called magnetic domains.

Diamagnetis is exhibited by all materials, including paramagnetic and ferromagnetic materials. But paramagnetism and ferromagnetism overpower the diamagnetism.

Atomic and Ionic radii

Generally speaking, atomic radii vary rather regularly among elements in the same group and amoung elements in the same period. Within the same period, atomic radii decreases as the atomic number increase. The atomic radii of the 3rd period follow this trend: Na > Mg > Al > Si > P > S > Cl For elements of the same group, atomic radii increase as atomic number increase. Thus, atomic radii of the inert gases has the trend: He < Ne < Ar < Kr < Rn Radius of positive ions is smaller than that of a neutral atom, which in turn is smaller than that of a negative ion. Thus the sizes of the atoms have the following order: S2- > Cl- > Ar > K+ > Ca2+. These species have the same number of electrons, but their nuclei have increasing number of protons from S, Cl to Ca. Thus, the electrons are held tighter as we go from S2- to Cl- to Ca2+, making them smaller.

Apply the same reasoning to figure out trends for elements in the periodic table.

Ionization energy, Electron affinity and Electronegativity

Ionization energy (IE) or ionization potential (IP) is the energy required to remove an electron from a species. For example, the energy required for the following reaction is called the ionization energy. Mg(g) ® Mg+(g) + e-;       (IE) Note that all species are in gas phase. In the above equation, the first electron is removed from Mg atom, and such an IE is called the first IE.

Note the trend of the first IE, or 2nd IE in periods and in groups. Generally speaking, the IE is the highest for the lightest element. In terms of IE, the order for the first group is:

Li > Na > K > Rb > Cs Removing an electron from Li is harder than from Na, etc.

Electron affinity (EA) is the energy or enthalpy given off when a species acquires an electron:

Cl(g) + e- ® Cl-(g)       (EA) The variation of EA is irregular, no particular trend can be found. In general, the electron affinity for an element in the fluorine group is usually large, indicating a very high tendency for it to acquire an electron.

The most electronegative element, F, releases -322 kJ/mol whereas chlorine, Cl, releases the most energy when it acquires an electron, -348.7 kJ/mol. For some unknown reasons, the electron affinity for chlorine is larger than that of fluorine.

If you reverse the above reaction, it looks like the ionization of the negative ion. Thus, the negative of EA is the zeroth ionization energy (0th IE) of the neutral atom, Cl in this case.

Cl-(g) + e- ® Cl-(g);     (0th IE) Thus, electron affinity can also be defined as the energy required to remove a electron from a negative ion (the reverse reaction of acquiring an electron). This is equivalent to the ionization energy of a negative ion. This is often called the zeroth ionization potential.

IE and EA are quantities of energy when an electron is removed from or added to an atom. They convey no information regarding the bonding electrons between atoms. Pauling invented the concept called electronegativity to indicate the degree of attraction of bonding electrons. Electronegativity is a scale based on information of IE, EA, bondlength and bond energy. Many schalors have used different methods to evaluate electronegativity, and Pauling's scale is the most common one in text books.

Electronegativity of some elements are given below for your reference:

            B     C    N    O    F
           2.0   2.6  3.0  3.4  4.0

  Cs              Si   P    S    Cl    Kr   Xe
  0.8            1.9  2.2  2.6  3.2    2.9  2.6
The higher the electronegativity, the more attraction the element has towards bonding electrons. Bonding between two elements with large electronegativity difference tends to be ionic. It is interesting to note that inert gases Kr and Xe have elenegativities similar to those of nitrogen and carbon.

On the periodic table, there is a general trend. An element close to F has a large electronegativity, whereas an element close to Cs on the opposit corner of the periodic table from F has the least electronegativity.

The Periodic Table as a Summary of Chemical Properties

The periodic table is a convenient way to correlate chemical properties. For example, from their position on the periodic table, we easily recognize them as metals, semimetals (metalloids), or nonmetals.

Most elements are metals (M), some are metalloids (o), nonmetals (-), and inert gases (i), on the top right hand of the long periodic table.

   -                              i
   Mo                        o----i
   MM                        oo---i
   MM              MMMMMMMMMMMoo--i
   MMMMMMMMMMMMMMMMMMMMMMMMMMMMoo-i
   MMMMMMMMMMMMMMMMMMMMMMMMMMMMMooi
Look at some metals, and see if you can describe their characteristics. If not, please check some resource books to see if they give a description you like?

Metals are characterized by having high melting point, high electric and heat conductivity, metallic bonding, malleable, ductile, form positive ions Cu2+, Fe3+, and form alloys with one another. Check out a metal, and you'll be amazed how close it has come to your life.

Nonmetals have low melting points, form molecules, atoms in their solids are covalently bonded. They are poor conductors of heat and electricity, and they form negative ions or form molecular compounds. Here are some non-metals. Note however, that diamond is an exceptionally good heat conductor, but it does not conduct electricity.

Nobel gases: He, Ne, Ar, Kr, Xe, Rn
Oxygen group: O, S, Se
Nitrogen group: N, P, As
Carbon group: C, Si
Boron

Between metals and nonmetals lie the metalloids as indicated above.

The chemistry of these elements is best discussed in GROUPS such as alkali metals, carbon group, halogens, inert gases, etc.

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