Describe the significance of Mendeleev\'s periodic table. Compare the physical a

Question

Describe the significance of Mendeleev's periodic table. Compare the physical and chemical properties of metals and nonmetals. In the periodic table, the element hydrogen is sometimes grouped with the alkali metals and sometimes with the halogens. Explain why hydrogen can resemble an element from each group. Define ionic radius. How does the size of an atom change when it is converted to (a) an anion and (b) a cation? Arrange the following in order of increasing first ionization energy: Na, Cl, Al, S, and Cs. Define electron affinity. Explain why electron affinity measurements are made with gaseous atoms. Which atom should have a smaller first Ionization energy: oxygen or sulfur? Which of the following atoms should have a larger first ionization energy: N or P ? Would the compound BeO be classified as acidic, basic, or amphoteric?

Explanation / Answer

1. Mendeleev's periodic table allows you to elements with similar characteristics in the same groups (columns). You can predict some chemical behavior by knowing the group where belongs an element. On the other hand, the chemical characteristics will slightly change when you move in the same row from left to right.

2. Metals, are placed i the left side of the periodic table and Non-metals are located in the right side. Metals tend to form cationic ions (positive) and non-metals forms anionic ions (negative).

Metals

Most elements are metals. This includes the alkali metals, alkaline earth metals, transition metals, lanthanides, and actinides. On the periodic table, metals are separated from nonmetals by a zig-zag line stepping through carbon, phosphorus, selenium, iodine and radon. These elements and those to the right of them are nonmetals. Elements just to the left of the line may be termed metalloids or semimetals and have properties intermediate between those of the metals and nonmetals. The physical and chemical properties of the metals and nonmetals may be used to tell them apart.

Metal Physical Properties

Metal Chemical Properties

Nonmetals

Nonmetals, with the exception of hydrogen, are located on the right side of the periodic table. Elements that are nonmetals are hydrogen, carbon, nitrogen, phosphorus, oxygen, sulfur, selenium, all of the halogens, and the noble gases.

Nonmetal Physical Properties

Nonmetal Chemical Properties

Both metals and nonmetals take different forms (allotropes), which have different appearances and properties from each other. For example, graphite and diamond are two allotropes of the nonmetal carbon, while ferrite and austenite are two allotropes of iron. While nonmetals may have an allotrope that appears metallic, all of the allotropes of metals look like what we think of as a metal (lustrous, shiny).

3.

Hydrogen's Placement in the Periodic Table

Because hydrogen forms compounds with oxidation numbers of both +1 and -1, many periodic tables include this element in both Group IA (with Li, Na, K, Rb, Cs, and Fr) and Group VIIA (with F, Cl, Br, I, and At).

There are many reasons for including hydrogen among the elements in Group IA. It forms compounds (such as HCl and HNO3) that are analogs of alkali metal compounds (such as NaCl and KNO3). Under conditions of very high pressure, it has the properties of a metal. (It has been argued, for example, that any hydrogen present at the center of the planet Jupiter is likely to be a metallic solid.) Finally, hydrogen combines with a handful of metals, such as scandium, titanium, chromium, nickel, or palladium, to form materials that behave as if they were alloys of two metals.

There are equally valid arguments for placing hydrogen in Group VIIA. It forms compounds (such as NaH and CaH2) that are analogs of halogen compounds (such as NaF and CaCl2). It also combines with other nonmetals to form covalent compounds (such as H2O, CH4, and NH3), the way a nonmetal should. Finally, the element is a gas at room temperature and atmospheric pressure, like other nonmetals (such as O2 and N2).

It is difficult to decide where hydrogen belongs in the periodic table because of the physical properties of the element. The first ionization energy of hydrogen (1312 kJ/mol), for example, is roughly halfway between the elements with the largest (2372 kJ/mol) and smallest (376 kJ/mol) ionization energies. Hydrogen also has an electronegativity (EN = 2.20) halfway between the extremes of the most electronegative (EN = 3.98) and least electronegative (EN = 0.7) elements. On the basis of electronegativity, it is tempting to classify hydrogen as a semimetal.

4. The ionic radius is the radius of an atom's ion. Ions may be larger or smaller than the neutral atom, depending on the ion's electric charge. When an atom loses an electron to form a cation, the other electrons are more strongly attracted to the nucleus, and the radius of the atom gets smaller. Similarly, when an electron is added to an atom, forming an anion, the added electron increases the size of the electron cloud.

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