Representative element compounds

An internationally accepted chemical notation makes use of symbols to represent elements and compounds, and advises on naming chemical compounds. In this notation, the elements are represented by one or two letters, many of which are drawn from the elements Latin or Greek names. The number of atoms of an element in a molecule is represented by a subscript written after the symbol thus Au (the first two letters of aurum, the Latin name for gold) represents an atom of gold Cu (the first two letters of cuprum, the Latin name for copper), an atom of copper and C (the first letter of carbon), an atom of carbon O represents an atom of oxygen and 02, a molecule of oxygen. The symbols listed below provide examples of the presently accepted form of chemical notation ... 

Considering the influence of electronic configurations on crystal structures it may be asked, whether certain structure t5rpes are restricted to fluorine compounds of the transition elements. Apart from the structure types distorted by the Jahn-Teller effect such a limitation is not obvious at all. On the contrary quite a number of structure prototypes are represented by compounds of the main group elements. Bonding thus must be similar in both, main group and transition element fluorides, at least as for the factors that influence crystal structmes. 

Dalton too would not divorce himself from the use of pictures to represent elements and compounds. Dalton resorted to a series of circles containing various patterns and letters to represent substances. A sample of Dalton s nomenclature is shown in Table 5.2. 

Berzelius method of assigning letters to represent elements was also applied to compounds. Berzelius used superscripts to denote the number of atoms in a compound. Thus, water would be O and carbon dioxide CO. Later these superscripts were changed to the current practice of designating the number of atoms using subscripts. The absence of a subscript implies the subscript one. Using symbols we can write the chemical formula for ammonia ... 

Berzelius s method of assigning letters to represent elements was also applied to compounds. Berzelius used superscripts to denote the number of atoms in a compound. Thus water would... 

T. Carnelley has further shown that if in a series of compounds A Xm, B Xm, C Xm,. .. in which A, B, C,. . . represent elements belonging to the same subgroup of Mendeleeff s Table, the colour passes wholly or partially through the series ... 

The 112 known elements—there may be more by the time you read this—combine to form millions of compounds. That is far more than we could study individually. Moreover just learning a string of isolated facts would not build the insight we need to devise new compounds. It is far more useful to study a select group of representative elements and their compounds. In this and the next two chapters, we use the periodic table as our guide in this highly selective journey. The topics of these chapters are commonly called descriptive chemistry—the description of the preparation, properties, and applications of elements and their compounds. 

Experimental work in transition metal chemistry is particularly enjoyable because most transition metal compounds have brilliant colors. In this section, we ll look at the chemistry of some representative elements commonly encountered in the laboratory. 

Ionic Compounds To name an ionic compound, you just name the cation and then the anion. There is a crucial difference between naming ionic compounds and molecular compounds. In molecular compounds you must include prefix multipliers (di, tri, etc.) to indicate the number of each kind of atom in the molecule. In ionic compounds you must not include prefix multipliers, because the number of each ion in the formula unit is controlled by the charges on the ions. If the cation is a representative element, it is not necessary to indicate the charge, because (with few exceptions) these metals form cations with an ionic charge equal to the group number. 

NaCI represents the compound containing one atom of sodium and one atom of chlorine. The masses on these elements are too small to determine individually or as individual ionic units. However, using the mole we can determine the mass of the compound represented by the formula NaCI. One mole of sodium atoms has a mass of 23.0 grams and one mole of chlorine atoms has a mass of 35.5 grams. Therefore, one mole of sodium chloride molecules has a mass of 58.5 grams. 

Take as an illustration the case of antimony, which is by far the least-represented element in this review. From the initial indications of reactivity and structural patterns, it often behaves differently from both arsenic and bismuth. Why, for example, does Sb[Co(CO)4]3 seem to have only a fleeting existence while Bi[Co(CO)4]3 is stable Furthermore, it has not been possible to isolate SbCo3(CO)9 when both the arsenic (as the cyclic trimer) and bismuth compounds are known. And even though Sb[Co(CO)4]3 may have a fleeting existence, it spontaneously disproportionates to give [Sb2Co4(CO)n]1, whereas Bi[Co(CO)4]3 is a stable, isolable molecule. There are no molecules with the Sb3 ion present, whereas examples exist for P, As, and Bi. 

Hudson et al. [26] describes a method called the Most Descriptive Compound (MDC) method for selecting representative subsets and a sphere exclusion method for selecting sets of compounds that cover the available property space. The MDC method aims to select subsets that most effectively represent the compounds in the original collection. It operates by calculating a vector / of N elements where there are N compounds. For each compound, the other compounds are ranked in order of distance to it. The reciprocal of the rank of each compound n is then stored in vector position / . The process... 

Many representative elements attain at least a share of eight electrons in their valence shells when they form molecular or ionic compounds there are some limitations. 

Chemists write symbols together in formulas to identify compounds. For example, the letters CO represent a compound of carbon and oxygen. Be careful to distinguish the formula CO from the symbol Co, which represents the element cobalt. The capitalization of letters is very important Formulas are sometimes written with subscripts to tell the relative proportions of the elements present. For example, H2O represents water, which has two atoms of hydrogen for every atom of oxygen present. More about formulas will be presented in Section 5.1. 

Formulas are used to identify molecules of free elements. A molecule contains two or more nonmetallic atoms bonded together. Many free (uncombined) non-metallic elements exist as molecules, such as H2, N2, O2, F2, CI2, Br2, and I2, as well as P4 and Sg (Figure 5.1). The formula P4 indicates four phosphorus atoms bonded together. This formula does not represent a compound, because only one kind of atom is present. Elemental phosphorus in its lowest energy form occurs in such molecules. 

The formula mass of a compound is the sum of the atomic masses (Chap. 3) of all the atoms (not merely each kind of atom) in the formula. Thus, in the same way that a symbol is used to represent an element, a formula is used to represent a compound or a molecule of an element, such as H2, and also one unit of either. The formula mass of the substance or the mass of 1 mol of the substance is easily determined on the basis of the formula (Sec. 7.4). Note that just as formula unit may refer to uncombined atoms, molecules, or atoms combined in an ionic compound, the term formula mass may refer to the atomic mass of an atom, the molecular mass of a molecule, or the formula mass of a formula unit of an ionic compound. 

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