Covalent bonds valences, common elements

Covalent bonds are commonly formed between atoms of different elements to form compounds. For example, methane (natural gas) is the simplest hydrocarbon (a compound made up only of carbon and hydrogen). In the following diagram only the outer or valence electrons are shown. The carbon atom has 4 outer electrons, and shares with 4 hydrogen atoms, each sharing its one electron ... 

Section 1 3 The most common kind of bonding involving carbon is covalent bond ing A covalent bond is the sharing of a pair of electrons between two atoms Lewis structures are written on the basis of the octet rule, which limits second row elements to no more than eight electrons m their valence shells In most of its compounds carbon has four bonds... 

Elements that can expand their valence shells commonly show variable covalence, the ability to form different numbers of covalent bonds. Elements that have variable covalence can form one number of bonds in some compounds and a different number in others. Phosphorus is an example. It reacts directly with a limited supply of chlorine to form the toxic, colorless liquid phosphorus trichloride ... 

Atoms in a molecule are joined by bonds. Bonds are formed when the valence or outermost electrons of two or more atoms interact. The nature of the bond between atoms goes a long way toward determining the properties of the molecule. Chapter 5 introduced the two common types of chemical bonds covalent and ionic. Elements with similar electronegativities share electrons and form covalent bonds. But elements with greatly different electronegativities exchange one or more electrons. This is called an ionic bond. 

All elements of the group form Zintl compounds with electropositive metals (see Topic D5 ). Continuous networks of covalently bonded atoms are generally found, rather than the clusters common with group 14. For example, NaAl and NaTl have tetrahedral diamond-like networks of A1 or Tl, which can be understood on the basis that AT and TF have the same valence electron count as carbon. 

Compounds with the zincblende or wurtzite structure are commonly formed when one element belongs to the nth. and the other to the (8—rc)th B sub-groups. In these structures each atom is bound to four neighbours by purely covalent bonds and it is necessary that the number of valency electrons available for the formation of these bonds should average four per atom. It is not, however, necessary that these should be contributed equally by the atoms of the two types, and the condition is therefore satisfied if these atoms are related in the manner indicated. Some B-B compounds with the zincblende and wurtzite structures are shown in table 13.09. These structures are not, of course, confined to systems of the type B-B we have encountered numerous examples of their occurrence in compounds of quite other kinds. 

The Group 6A elements can form covalent bonds with other nonmetals. For example, they combine with hydrogen to form a series of covalent hydrides of the general formula H2X. Those members of the group that have valence d orbitals available (all except oxygen) commonly form molecules in which they are surrounded by more than eight electrons. Examples are SF4, SFg, TeG, and SeBr4. 

A natural question chemistry students ask is Why are there so many compounds of carbon The answer Carbon contains four valence electrons and so can form four covalent bonds to other carbons or elements. (A common mistake organic chemistry students make when drawing structures is not ensuring that every carbon has four bonds attached to it.) The bonds that carbon forms are strong covalent bonds (Chapter 7 covers covalent bonds), and carbon has the ability to bond to itself in long chains and rings. It can form double and triple bonds to another carbon or to another element. No other element, with the possible exception of silicon, has this ability. (And the bonds silicon makes aren t nearly as strong as carbon s.) These properties allow carbon to form the vast multitude of compounds needed to make an amoeba or a butterfly or a baby. 

Double bond n. A type of covalent bond (pi or n bond in C=C), common in organic chemistry, in which two pairs of electrons are shared between two elements. The double bond may be symbolized either by or = , as in ethylene, CH2 CH2, or dimethyl ketone (CH3)2C=0. Elements possess a definite number of valency linkages or bonds, and these are employed in uniting with other elements. When all these bonds are completely employed in such unions, saturated compounds result. If, however, a residual bond or linkage remains, this may be involved with another in forming a reactive or unsaturated... 

The cohesion above is for ionic solids covalent and metallic bonds are different. In covalent bonds, electrons between atoms are shared, whereas in metallic solids, atoms of the same (or different) elements donate their valence electrons to form an electron gas throughout the space occupied by the atoms. Giving up then-electrons to a common pool, known as an electron cloud or electron gas , these atoms actually become positive (similar to positive ions). They are held together by forces similar to those of ionic bonds, but acting between ions and electrons. The electrostatic interaction between the positive ions and the electron gas holds metals together. Unlike other crystals, metals may be deformed without fracture, because the electron gas permits atoms to slide past one another, acting as a lubricant. In non-ductile materials, such as most ceramics, this is not possible and renders them brittle. 

The hydrogen atom and the halogen atoms form only one covalent bond to other atoms in stable neutral compounds. However, the carbon, oxygen, and nitrogen atoms can bond to more than one atom. The number of covalent bonds an atom can form is called the valence of the atom. The valence of a given atom is the same in most stable neutral organic compounds. Table 1.2 lists the valences of some common elements contained in organic compounds. 

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