<text><span class="style10">hemical Bonds (1 of 6)</span><span class="style7">Although there are only 109 known elements, there are millions of chemical substances found in nature or made artificially. These substances are not simply mixtures of two or more elements: they are specifically determined chemical compounds, formed by combining two or more elements together in a chemical reaction. The chemical 'glue' that holds these compounds together is known as </span><span class="style26">chemical bonding.</span><span class="style7"> The properties of compol cube containing just 27 ions.highly reactive, others inert; some are solids with high melting points, others are gases. Furthermore, the properties of a compound are generally very different from those of its constituent elements. To understand how and why these differences arise, we need to understand the different types of chemical bond.</span><span class="style10">Ionic bonding</span><span class="style7">The atoms of the element neon have a full outer shell of electrons, with the electron configuration 2.8. This arrangement is very stable and neon is not known to form chemical bonds with any other element. An atom of the element sodium (Na) has one more electron than neon (configuration 2.8.1), while an atom of the element fluorine (F) has one electron less (configuration 2.7). If an electron is transferred from a sodium atom to a fluorine atom, two species are produced with the same stable electron configuration as neon. Unlike neon, however, the species are charged and are known as </span><span class="style26">ions</span><span class="style7">. The sodium atom, having lost a (negative) electron, has a net positive charge and is known as a </span><span class="style26">cation</span><span class="style7"> (written Na+), while the fluorine atom, having gained an electron, has a net negative charge and is called a fluoride </span><span class="style26">anion</span><span class="style7"> (written F-). When oppositely charged ions such as Na+ and F- are brought together, there is a strong attraction between them; a large amount of energy is released - the same amount of energy as would have to be supplied in order to separate the ions again. This force of attraction is called an </span><span class="style26">ionic</span><span class="style7"> (or </span><span class="style26">electrovalent</span><span class="style7">) </span><span class="style26">bond</span><span class="style7">. The energy released more than compensates for the energy input required to transfer the electron from the sodium atom to the fluorine atom. Overall there is a net release of energy and a solid crystalline compound - sodium fluoride (NaF) - is formed. The structure of a similar ionic compound - potassium chloride (KCl) - is illustrated in the box.Atoms that have two more electrons than the nearest noble gas (such as magnesium, configuration 2.8.2) or two less (such as oxygen, 2.6) also form ions having the noble-gas configuration by transfer of electrons - in this case Mg2+ and O2-. The ionic compound magnesium oxide (MgO; see photo, p. 47) has the same arrangement of ions as NaF, but since the ions in MgO have a greater charge, there is a stronger force between them. Thus more energy must be supplied to overcome this force of attraction, and the melting point of MgO is higher than that of NaF. Al though the ions are fixed in position in the solid crystal, they become free to move when the solid is melted. As a liquid, therefore, the compound becomes electrolytic and is able to conduct electricity.Many other more complex ionic structures are known. The formula of any ionic compound can be worked out by balancing the charges of its ions. For example, Mg2+ and F- form MgF2, while Na+ and O2- form Na2O.</span></text>
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<text>ΓÇó ATOMS AND SUBATOMIC PARTICLESΓÇó WHAT IS CHEMISTRY?ΓÇó ELEMENTS AND THE PERIODIC TABLEΓÇó CHEMICAL REACTIONSΓÇó SMALL MOLECULESΓÇó THE REACTIVITY SERIES </text>
