![]() ![]() The bonding between the two molecules is co-ordinate, using lone pairs on the chlorine atoms. It exists as a dimer (two molecules joined together). Measurements of the relative formula mass of aluminium chloride show that its formula in the vapour at the sublimation temperature is not AlCl 3, but Al 2Cl 6. There is likely to be a similarity, because aluminium and boron are in the same group of the Periodic Table, as are fluorine and chlorine. The dots-and-crosses diagram shows only the outer electrons.ĪlCl 3, like BF 3, is electron deficient. The implication is that it when it sublimes at this relatively low temperature, it must be covalent. If it simply contained ions it would have a very high melting and boiling point because of the strong attractions between the positive and negative ions. We shan't use this method again - it's more confusing than just using an arrow.Īluminium chloride sublimes (turns straight from a solid to a gas) at about 180☌. The nitrogen end of the bond has become positive because the electron pair has moved away from the nitrogen towards the boron - which has therefore become negative. The second diagram shows another way that you might find co-ordinate bonds drawn. Using lines to represent the bonds, this could be drawn more simply as: The lone pair on the nitrogen of an ammonia molecule can be used to overcome that deficiency, and a compound is formed involving a co-ordinate bond. BF 3 is described as being electron deficient. The boron only has 3 pairs of electrons in its bonding level, whereas there would be room for 4 pairs. If you have recently read the page on covalent bonding, you may remember boron trifluoride as a compound which doesn't have a noble gas structure around the boron atom. The reaction between ammonia and boron trifluoride, BF 3 When a hydrogen ion breaks away again, it could be any of the three. Note that once the co-ordinate bond has been set up, all the hydrogens attached to the oxygen are exactly equivalent. When it reacts with something (an alkali, for example), the hydrogen ion simply becomes detached from the water molecule again. If you write the hydrogen ion as H + (aq), the " (aq)" represents the water molecule that the hydrogen ion is attached to. A raw hydrogen ion is simply a proton, and is far too reactive to exist on its own in a test tube. In an introductory chemistry course (such as GCSE), whenever you have talked about hydrogen ions (for example in acids), you have actually been talking about the hydroxonium ion. The H 3O + ion is variously called the hydroxonium ion, the hydronium ion or the oxonium ion. A hydrogen ion (H +) is transferred from the chlorine to one of the lone pairs on the oxygen atom. The arrow points from the atom donating the lone pair to the atom accepting it.ĭissolving hydrogen chloride in water to make hydrochloric acid In simple diagrams, a co-ordinate bond is shown by an arrow. Although the electrons are shown differently in the diagram, there is no difference between them in reality. Once the ammonium ion has been formed it is impossible to tell any difference between the dative covalent and the ordinary covalent bonds. The hydrogen's electron is left behind on the chlorine to form a negative chloride ion. When the ammonium ion, NH 4 +, is formed, the fourth hydrogen is attached by a dative covalent bond, because only the hydrogen's nucleus is transferred from the chlorine to the nitrogen. If these colourless gases are allowed to mix, a thick white smoke of solid ammonium chloride is formed.Īmmonium ions, NH 4 +, are formed by the transfer of a hydrogen ion from the hydrogen chloride to the lone pair of electrons on the ammonia molecule. The reaction between ammonia and hydrogen chloride A co-ordinate bond (also called a dative covalent bond) is a covalent bond (a shared pair of electrons) in which both electrons come from the same atom.įor the rest of this page, we shall use the term co-ordinate bond - but if you prefer to call it a dative covalent bond, that's not a problem! In the formation of a simple covalent bond, each atom supplies one electron to the bond - but that doesn't have to be the case. The atoms are held together because the electron pair is attracted by both of the nuclei. Important! If you are uncertain about covalent bonding follow this link before you go on with this page.Ī covalent bond is formed by two atoms sharing a pair of electrons. You need to have a reasonable understanding of simple covalent bonding before you start. This page explains what co-ordinate (also called dative covalent) bonding is.
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