Questions and Answers on Hydrogen in Metals I-Basic Properties

Response to Q.1.

Noble gases appear in the group (viii), which is the last group of the periodic table in red color. Halogens appear in-group (vii) of the periodic table with a blue color. Alkali metals are group (i) elements on the periodic table with a green color. Alkali earth metals are group (ii) elements with a deep blue color. All the elements that are in bold are metals while the other are non-metals.

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Atomic radius increases down the group due to increase in the energy levels but decreases across the period due to increase in the nuclear charge (Greenwood & Earnshaw, 2012). Electron affinity decreases down the group while increasing across the period. Ionization energy increases across the period while decreasing down the group.

Response to Q.2.

From group one, I have chosen sodium element. It has a boiling point of 882C and a melting point of 98C. It has an electronegativity of 0.93 because of the low nuclear charge. Sodium is a good conductor and a very reactive element (Worlock, 1966). It is a silver-white when in solid form since it can exist in all forms of the state, i.e., as a solid, liquid, gas and plasma. The electron configuration of solid is [Ne]3S1. The outermost electron in the S shell is responsible the reactivity of sodium because it can easily be lost in a chemical reaction. Sodium forms ionic bonds because it can be able to lose its outermost electron to another element, e.g., chlorine. The reactivity of group one elements increases down the group hence sodium is more reactive than lithium but less reactive than potassium. The reactivity decreases as you move across the period because there is an increase in the nuclear charge, which makes it difficult to lose electrons. An increase in the quantum shells increases the nuclear charge of the element. Hence it becomes more shielded making it quite difficult to react.

From group ii elements, I have chosen magnesium. It is a silver-white solid with a melting point of 650C and a boiling point of 1107C. Magnesium has an electronegativity of 1.31 and is a good electrical and heat conductor. The melting point of magnesium is higher than that of sodium due to the crease in the nuclear charge. Magnesium can form either covalent or ionic bond because it has two electrons in the outermost quantum shell. The two delocalized electrons are responsible for the conductivity. It is, however, less reactive than sodium but more reactive than aluminum. The melting point of magnesium is lower than the melting point of calcium because, as you move down the group, there is an increase in the quantum shell hence increasing the size of calcium. The conductivity of magnesium is lower than the conductivity of calcium despite them being in the same group.

From group iii elements, I have chosen aluminum. Aluminum is a silvery-grey metal that has a boiling point of 2470C and a melting point of 660C. It is a good electrical conductor due to the three delocalized electrons in the outermost shell having an electronegativity of 1.61. Aluminum forms covalent bonds because it can share the three electrons. It is less reactive than Mg but more reactive than silicon. Aluminum has a higher melting point than boron. As you go down the group, there is an increase in the quantum shell. This means that the size of the atom also increases making it have a higher melting point.

Response to Q.3.

Sodium is a very reactive element and forms many compounds. Examples of the compounds formed are halides, oxides, and hydrides. Halides are formed when sodium reacts with halogens, e.g., chlorine and the compounds are usually linear. Oxides compounds are formed when sodium reacts with oxygen while hydrides are formed when sodium reacts with hydrogen. The shape of sodium oxide is tetrahedral. The melting point of halides increases down the group starting from sodium fluoride to sodium iodide. On the other hand, the boiling point decreases down the group. The oxide of sodium has high melting and boiling point. The melting point of metal hydride is lower than the oxides and halides. Sodium has one electron in the outermost energy level/quantum shell. This electron is responsible for forming the bond with halides. In solid form, the compound does not conduct electricity, but they conduct electricity in molten form, (Kaufmann, 1960)

[Na]+[Cl]- diagram for NaCl

O: Na:O Lewis dot diagram for sodium oxide

Magnesium reacts with halogens to form halides. Since magnesium has two electrons in the outermost energy level, it will react with one halide and another compound so that it can become stable. For example, magnesium reacts to form RMgX where X is the halide while R is an alkyl group. The melting point of magnesium halide decreases down the group. This is because the bond formed between magnesium and fluorine is so strong due to its small size. A large element will form a weak since the interaction is not strong. Magnesium can also form other compounds like oxides. The bond formed has a melting point of 2852C. The bond formed between magnesium and oxygen is a covalent bond because magnesium loses its two electrons while oxygen gains two electrons. The shape of the compound is linear since there only two elements involved in the reaction.

: Lewis structure of magnesium oxide

H3C:Mg: Br Lewis structure for Grignard reagent

() Linear

Aluminum reacts with oxygen, halides, and hydrogen. Aluminum reacts with hydrogen to form a trigonal planar compound. This is because it has three electrons in the outermost energy level while hydrogen has one electron. Hence, to obtain an octet state, three moles of hydrogen will be required. The melting point is low (150C) because the bond between the two elements is weak, hence it can easily be broken. An example of a halide is aluminum chloride. The compound has a melting point of 180C meaning that the bond formed is stronger than the halide. The shape of the compound is still a trigonal planar (Read, 2008). The molar mass of the compound is higher than that of hydride hence having a higher melting point.

The trigonal planar shape of AlH3

Response to Q.4.

Hydrogen is the first element in the periodic table. It is the smallest element in the periodic table since it has only one mass number. It exists as a gas nature and is usually in a pair. The element does not satisfactorily fit on any group in the periodic table due to its properties and reactivity. For instance, it is usually placed above the alkali metals. However, it has very different properties from those of the alkali metals. The ionization energy for hydrogen is very high (1312KJ/mol) while that of lithium is very low (520KJ/mol). Secondly, hydrogen is a non-metal and is sometimes placed in the group belonging to halogens. It is capable of forming H- like the halogens. Just like halogens, hydrogen can also form dihydrogen (H2). However, hydrogen is not supposed to be in the halogen group because it has a smaller electron affinity than the halogens.

The reactivity of hydrogen is also different from the elements in-group one. For instance, group one elements like sodium react with halogens to form an ionic compound, and the bond type is ionic. However, hydrogen does not form an ionic compound with halogens. For instance, the bond formed between hydrogen and chlorine is polar covalent due to the difference in electronegativity. Hydrogen can react with both metal and non-metal while alkali metals cannot react with another metal (Alefeld and Volkl, 1978). This properties and relativities make it difficult to know the kind of group that hydrogen belongs.

References

Alefeld, G. and Volkl, J., 1978. Hydrogen in metals I-Basic properties. In Berlin and New York, Springer-Verlag (Topics in Applied Physics. Volume 28), 1978. 442 p.(For individual items see A79-16057 to A79-16061) (Vol. 28).

Greenwood, N.N., and Earnshaw, A., 2012. Chemistry of the Elements. Elsevier.

Kaufmann, D.W., 1960. Sodium chloride: the production and properties of salt and brine.

Read, D., 2008. Shapes of Molecules.

Worlock, J.M., 1966. Thermal conductivity in sodium chloride crystals containing silver colloids. Physical Review, 147(2), p.636.