{"id":3070,"date":"2016-08-26T02:35:46","date_gmt":"2016-08-26T02:35:46","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/umes-cheminter\/?post_type=chapter&#038;p=3070"},"modified":"2016-08-26T02:35:46","modified_gmt":"2016-08-26T02:35:46","slug":"van-der-waals-forces","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/umes-cheminter\/chapter\/van-der-waals-forces\/","title":{"raw":"Van der Waals Forces","rendered":"Van der Waals Forces"},"content":{"raw":"<div class=\"x-ck12-data-objectives\">\r\n<div class=\"textbox learning-objectives\">\r\n<h3>Learning Objectives<\/h3>\r\n<ul id=\"x-ck12-MjljOTZkNWNmNjJjYzI4YjhhZGQzYTU1ZjE5NjZhNWU.-hns\">\r\n \t<li>Define Van der Waals forces.<\/li>\r\n \t<li>Describe dipole-dipole interactions.<\/li>\r\n \t<li>Describe London dispersion forces<strong>.<\/strong><\/li>\r\n<\/ul>\r\n<\/div>\r\n<div class=\"textbox examples\">\r\n<h3>How to Keep it Cool?<\/h3>\r\n<p id=\"x-ck12-ZTQ4MDRiYTg4NmFlMTJhZTdkNDNiNmZjNGMwMzgyZmI.-rlf\"><span class=\"x-ck12-img-inline\"> <img class=\"aligncenter\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/53\/2014\/08\/19211534\/20140811155258322064.jpeg\" alt=\"NMR machines use liquid nitrogen, which has Van der Waals forces\" width=\"250\" \/><\/span><\/p>\r\n<p id=\"x-ck12-NzEyMTJkM2UxYzM3N2U0M2UwMWI0ZGQ4ZTEwOWZlZjY.-nl5\">Magnetic resonance imaging (NMR) devices use liquid nitrogen to cool the superconducting magnets.\u00a0 Nitrogen is a gas at room temperature and liquefies at -195.8\u00b0C.\u00a0 Its neighbor on the periodic table (oxygen) boils at -182.95\u00b0C.\u00a0 The interactions between nitrogen molecules (N <sub> 2 <\/sub> ) are weaker, so the boiling point is lower.\u00a0 Interactions between non-polar molecules depend on the degree of electron fluctuation within the molecule.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n<h2>Van der Waals Forces<\/h2>\r\n<p id=\"x-ck12-YTdlZmJiNGFmMGVmZTdkZDcwOWZlMzY2ZjNjMGQ1NmY.-ryc\">The first type of intermolecular force we will consider are called van der Waals forces, after Dutch chemist Johannes van der Waals (1837-1923).\u00a0 <strong> Van der Waals forces <\/strong> are the weakest intermolecular force and consist of dipole-dipole forces and dispersion forces.<\/p>\r\n\r\n<h3>Dipole-Dipole Forces<\/h3>\r\n<p id=\"x-ck12-MDU2YTE3MWM1NDNiMTU1MzAwMWM3NmIxM2I3OTRhNTU.-6ba\"><strong> Dipole-dipole forces <\/strong> are the attractive forces that occur between polar molecules.\u00a0 A molecule of hydrogen chloride has a partially positive hydrogen atom and a partially negative chlorine atom.\u00a0 In a collection of many hydrogen chloride molecules, they will align themselves so that the oppositely charged regions of neighboring molecules are near each other.<\/p>\r\n\r\n<div id=\"x-ck12-Y2MwNGQ2ODgyOGZlOGMwZmIxYTgyMWFlYmM1YmFkYTQ.-dx1\" class=\"x-ck12-img-postcard x-ck12-nofloat\">\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"500\"]<img id=\"x-ck12-OTgwNDUtMTM2MTg2MTcxNS02NC01NS1JbWFnZS0tLTMx\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/53\/2014\/08\/19211535\/20140811155258432114.png\" alt=\"Example of a dipole-dipole force\" width=\"500\" height=\"216\" longdesc=\"Dipole-dipole%20forces%20are%20a%20result%20of%20the%20attraction%20of%20the%20positive%20end%20of%20one%20dipole%20to%20the%20negative%20end%20of%20a%20neighboring%20dipole.\" \/> Figure 1. Dipole-dipole forces are a result of the attraction of the positive end of one dipole to the negative end of a neighboring dipole.[\/caption]\r\n\r\n&nbsp;\r\n\r\n<\/div>\r\n<p id=\"x-ck12-OGVhOTI0NzcyNzQxNjRkMGUyYjUyZmJhODBjNDI4MWM.-td7\">Dipole-dipole forces are similar in nature, but much weaker than ionic bonds.<\/p>\r\n\r\n<h3>London Dispersion Forces<\/h3>\r\n<p id=\"x-ck12-YzJjZjUyMGFkMTRiNjQ5MjNkZjQ4YzA1OGU2N2RjNWI.-e5z\">Dispersion forces are also considered a type of van der Waals force and are the weakest of all intermolecular forces. \u00a0They are often called London forces after Fritz London (1900-1954), who first proposed their existence in 1930.\u00a0 <strong> London dispersion forces <\/strong> are the intermolecular forces that occur between atoms and between nonpolar molecules as a result of the motion of electrons.<\/p>\r\n<p id=\"x-ck12-OGExN2NlODM5YzYzNmRkZGM1ZTBkODgwMjNhNDM4Zjc.-esh\">The electron cloud of a helium atom contains two electrons, which can normally be expected to be equally distributed spatially around the nucleus.\u00a0 However, at any given moment the electron distribution may be uneven, resulting in an instantaneous dipole.\u00a0 This weak and temporary dipole subsequently influences neighboring helium atoms through electrostatic attraction and repulsion.\u00a0 It induces a dipole on nearby helium atoms.<\/p>\r\n\r\n<div id=\"x-ck12-M2RiMTdmNzdlMzA3NDlhNTAyNzI4ZGQyNDVlNGFhZjk.-wqm\" class=\"x-ck12-img-fullpage x-ck12-nofloat\">\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"512\"]<img id=\"x-ck12-OTgwNDUtMTM2MTg2MTc0NS0wMy0xMi1JbWFnZS0tLTMy\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/53\/2014\/08\/19211536\/20140811155258550313.png\" alt=\"An instantaneous dipole in a helium atom\" width=\"512\" height=\"134\" longdesc=\"A%20short-lived%20or%20instantaneous%20dipole%20in%20a%20helium%20atom.\" \/> Figure 2. A short-lived or instantaneous dipole in a helium atom.[\/caption]\r\n\r\n&nbsp;\r\n\r\n<\/div>\r\n<p id=\"x-ck12-OTk3NGQ0Y2IyYmFlYjJmYTA5Y2ZlNmQ0ZWMzZWRkZTI.-zko\">The instantaneous and induced dipoles are weakly attracted to one another.\u00a0 The strength of dispersion forces increases as the number of electrons in the atoms or nonpolar molecules increases.<\/p>\r\n<p id=\"x-ck12-NWFhMGVjOTViOWUxZDBlNDhlYTMyNDYxNjAxNWExNmI.-qo7\">The halogen group consists of four elements that all take the form of nonpolar diatomic molecules.\u00a0 The table below shows a comparison of the melting and boiling points for each.<\/p>\r\n\r\n<div id=\"x-ck12-MzhlZTUzOTE1ZjUwZWMwNWZkMjkwYmRlMDU0YjU3YWM.-0j8\">\r\n<table id=\"x-ck12-NjU0Y2NiNjllZTNiMWQ3NDI4MjBiZmNkZTA5NmQwOTY.-9jz\" class=\"x-ck12-nofloat\" border=\"1\"><caption>Melting and Boiling Points of Halogens<\/caption>\r\n<tbody>\r\n<tr>\r\n<td><strong> Molecule <\/strong><\/td>\r\n<td><strong> Total Number of Electrons <\/strong><\/td>\r\n<td><strong> Melting Point (\u00b0C) <\/strong><\/td>\r\n<td><strong> Boiling Point ( <\/strong> \u00b0C)<\/td>\r\n<td><strong> Physical State at Room Temperature <\/strong><\/td>\r\n<\/tr>\r\n<tr>\r\n<td>F <sub> 2 <\/sub><\/td>\r\n<td>18<\/td>\r\n<td>-220<\/td>\r\n<td>-188<\/td>\r\n<td>gas<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Cl <sub> 2 <\/sub><\/td>\r\n<td>34<\/td>\r\n<td>-102<\/td>\r\n<td>-34<\/td>\r\n<td>gas<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Br <sub> 2 <\/sub><\/td>\r\n<td>70<\/td>\r\n<td>-7<\/td>\r\n<td>59<\/td>\r\n<td>liquid<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>I <sub> 2 <\/sub><\/td>\r\n<td>106<\/td>\r\n<td>114<\/td>\r\n<td>184<\/td>\r\n<td>solid<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<p id=\"x-ck12-NGI3MGRlNWE1ZTRjYjg4MzQ0YWE0YmViODViZDc0ZjM.-sl4\">The dispersion forces are strongest for iodine molecules because they have the greatest number of electrons.\u00a0 The relatively stronger forces result in melting and boiling points which are the highest of the halogen group.\u00a0 These forces are strong enough to hold iodine molecules close together in the solid state at room temperature.\u00a0 The dispersion forces are progressively weaker for bromine, chlorine, and fluorine and this is illustrated in their steadily lower melting and boiling points.\u00a0 Bromine is a liquid at room temperature, while chlorine and fluorine are gases, whose molecules are much further apart from one another.\u00a0 Intermolecular forces are nearly nonexistent in the gas state, and so the dispersion forces in chlorine and fluorine only become measurable as the temperature decreases and they condense into the liquid state.<\/p>\r\n\r\n<\/div>\r\n<div id=\"x-ck12-MjkwNjEyMTk5ODYxYzMxZDEwMzZiMTg1YjRlNjliNzU.-svt\">\r\n<div class=\"textbox key-takeaways\">\r\n<h3>Summary<\/h3>\r\n<div id=\"x-ck12-MjkwNjEyMTk5ODYxYzMxZDEwMzZiMTg1YjRlNjliNzU.-svt\"><\/div>\r\n<div id=\"x-ck12-NTBiZTEwOWViZjVjYjg5NmIwZWM2OGRjZjNiM2I5YjA.-5ui\">\r\n<ul id=\"x-ck12-NTBiZTEwOWViZjVjYjg5NmIwZWM2OGRjZjNiM2I5YjA.-jqv\">\r\n \t<li>Van der Waals forces are weak interactions between molecules that involve dipoles.<\/li>\r\n \t<li>Polar molecules have permanent dipole-dipole interactions.<\/li>\r\n \t<li>Non-polar molecules can interact by way of London dispersion forces.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<div id=\"x-ck12-OGMwMDRkN2UzYjhhNWE4NTBhYjI0NTljM2FmYzJmNTc.-lkn\">\r\n<div class=\"textbox exercises\">\r\n<h3>Practice<\/h3>\r\n<div id=\"x-ck12-OGMwMDRkN2UzYjhhNWE4NTBhYjI0NTljM2FmYzJmNTc.-lkn\">\r\n<p id=\"x-ck12-NmNlM2JkYzM5ZTU0NDcyNTAzOGUwZTg3Yjc3MWRiMWU.-kbf\">Use the link below to answer the following questions:<\/p>\r\n\r\n<\/div>\r\n<div id=\"x-ck12-ZGIxN2Q3OTkyNWFkYjUxMTNmNjhkMjQzYjI2ZDMxODQ.-d63\">\r\n<p id=\"x-ck12-ZGIxN2Q3OTkyNWFkYjUxMTNmNjhkMjQzYjI2ZDMxODQ.-qg3\"><a href=\"http:\/\/www.chemguide.co.uk\/atoms\/bonding\/vdw.html\"> http:\/\/www.chemguide.co.uk\/atoms\/bonding\/vdw.html <\/a><\/p>\r\n\r\n<\/div>\r\n<div id=\"x-ck12-MjFkMGE4NWNmZjhlMDJlZTFlZTAxODkwYTdmNzZkM2U.-fkv\">\r\n<ol id=\"x-ck12-MjFkMGE4NWNmZjhlMDJlZTFlZTAxODkwYTdmNzZkM2U.-fcp\">\r\n \t<li>What are intermolecular attractions?<\/li>\r\n \t<li>How cold must helium get before it forms a liquid?<\/li>\r\n \t<li>Can large numbers of molecules be held together by dispersion forces?<\/li>\r\n \t<li>Do long thin molecules develop stronger or weaker dipoles than short fat molecules?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<div id=\"x-ck12-NDU3ZGQ1NTE4NGZhZWRiNzg4NWFmZDQwMDlkNzAxNjM.-1qy\">\r\n<div class=\"textbox exercises\">\r\n<h3>Review<\/h3>\r\n<div id=\"x-ck12-NDU3ZGQ1NTE4NGZhZWRiNzg4NWFmZDQwMDlkNzAxNjM.-1qy\"><\/div>\r\n<div id=\"x-ck12-MjNjNzFlZDNhMzM5NTk2ZjliMmY5NzMwYjAzYmZhZWQ.-o6r\">\r\n<ol id=\"x-ck12-MjNjNzFlZDNhMzM5NTk2ZjliMmY5NzMwYjAzYmZhZWQ.-ypr\">\r\n \t<li>What attractive forces develop between polar molecules?<\/li>\r\n \t<li>What creates London dispersion forces?<\/li>\r\n \t<li>Are London dispersion forces permanent or temporary?<\/li>\r\n \t<li>Are the dispersion forces for Cl <sub> 2 <\/sub> stronger or weaker than the ones for Br<sub>2<\/sub>?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<h2 class=\"x-ck12-data-problem-set\">\u00a0Glossary<\/h2>\r\n<div id=\"x-ck12-OGJlYjYyOGYwZDgyMDFmYzA5N2M5M2NkNGZmZDQ0NWI.-50e\" class=\"x-ck12-data-vocabulary\">\r\n<ul id=\"x-ck12-Mzg3NjUxMzU3MzA3N2MwZTNlNTQxM2U0YTYwYzc2ZWQ.-o8t\">\r\n \t<li><strong> dipole-dipole forces: <\/strong> The attractive forces that occur between polar molecule <em> s <\/em><\/li>\r\n \t<li><strong> London dispersion forces: <\/strong> The intermolecular forces that occur between atoms and between nonpolar molecules as a result of the motion of electrons.<\/li>\r\n \t<li><strong> Van der Waals forces: <\/strong> The weakest intermolecular force and consist of dipole-dipole forces and dispersion forces <em> .<\/em><\/li>\r\n<\/ul>\r\n[reveal-answer q=\"836080\"]Show References[\/reveal-answer]\r\n[hidden-answer a=\"836080\"]\r\n<h2>References<\/h2>\r\n<ol>\r\n \t<li>User:MartinSaunders\/Wikimedia Commons.<a href=\"http:\/\/commons.wikimedia.org\/wiki\/File:HWB-NMR_-_900MHz_-_21.2_Tesla.jpg\">http:\/\/commons.wikimedia.org\/wiki\/File:HWB-NMR_-_900MHz_-_21.2_Tesla.jpg<\/a>.<\/li>\r\n \t<li>CK-12 Foundation - Jodi So.<\/li>\r\n \t<li>CK-12 Foundation - Zachary Wilson.<\/li>\r\n<\/ol>\r\n[\/hidden-answer]\r\n\r\n<\/div>","rendered":"<div class=\"x-ck12-data-objectives\">\n<div class=\"textbox learning-objectives\">\n<h3>Learning Objectives<\/h3>\n<ul id=\"x-ck12-MjljOTZkNWNmNjJjYzI4YjhhZGQzYTU1ZjE5NjZhNWU.-hns\">\n<li>Define Van der Waals forces.<\/li>\n<li>Describe dipole-dipole interactions.<\/li>\n<li>Describe London dispersion forces<strong>.<\/strong><\/li>\n<\/ul>\n<\/div>\n<div class=\"textbox examples\">\n<h3>How to Keep it Cool?<\/h3>\n<p id=\"x-ck12-ZTQ4MDRiYTg4NmFlMTJhZTdkNDNiNmZjNGMwMzgyZmI.-rlf\"><span class=\"x-ck12-img-inline\"> <img decoding=\"async\" class=\"aligncenter\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/53\/2014\/08\/19211534\/20140811155258322064.jpeg\" alt=\"NMR machines use liquid nitrogen, which has Van der Waals forces\" width=\"250\" \/><\/span><\/p>\n<p id=\"x-ck12-NzEyMTJkM2UxYzM3N2U0M2UwMWI0ZGQ4ZTEwOWZlZjY.-nl5\">Magnetic resonance imaging (NMR) devices use liquid nitrogen to cool the superconducting magnets.\u00a0 Nitrogen is a gas at room temperature and liquefies at -195.8\u00b0C.\u00a0 Its neighbor on the periodic table (oxygen) boils at -182.95\u00b0C.\u00a0 The interactions between nitrogen molecules (N <sub> 2 <\/sub> ) are weaker, so the boiling point is lower.\u00a0 Interactions between non-polar molecules depend on the degree of electron fluctuation within the molecule.<\/p>\n<\/div>\n<\/div>\n<h2>Van der Waals Forces<\/h2>\n<p id=\"x-ck12-YTdlZmJiNGFmMGVmZTdkZDcwOWZlMzY2ZjNjMGQ1NmY.-ryc\">The first type of intermolecular force we will consider are called van der Waals forces, after Dutch chemist Johannes van der Waals (1837-1923).\u00a0 <strong> Van der Waals forces <\/strong> are the weakest intermolecular force and consist of dipole-dipole forces and dispersion forces.<\/p>\n<h3>Dipole-Dipole Forces<\/h3>\n<p id=\"x-ck12-MDU2YTE3MWM1NDNiMTU1MzAwMWM3NmIxM2I3OTRhNTU.-6ba\"><strong> Dipole-dipole forces <\/strong> are the attractive forces that occur between polar molecules.\u00a0 A molecule of hydrogen chloride has a partially positive hydrogen atom and a partially negative chlorine atom.\u00a0 In a collection of many hydrogen chloride molecules, they will align themselves so that the oppositely charged regions of neighboring molecules are near each other.<\/p>\n<div id=\"x-ck12-Y2MwNGQ2ODgyOGZlOGMwZmIxYTgyMWFlYmM1YmFkYTQ.-dx1\" class=\"x-ck12-img-postcard x-ck12-nofloat\">\n<div style=\"width: 510px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" id=\"x-ck12-OTgwNDUtMTM2MTg2MTcxNS02NC01NS1JbWFnZS0tLTMx\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/53\/2014\/08\/19211535\/20140811155258432114.png\" alt=\"Example of a dipole-dipole force\" width=\"500\" height=\"216\" longdesc=\"Dipole-dipole%20forces%20are%20a%20result%20of%20the%20attraction%20of%20the%20positive%20end%20of%20one%20dipole%20to%20the%20negative%20end%20of%20a%20neighboring%20dipole.\" \/><\/p>\n<p class=\"wp-caption-text\">Figure 1. Dipole-dipole forces are a result of the attraction of the positive end of one dipole to the negative end of a neighboring dipole.<\/p>\n<\/div>\n<p>&nbsp;<\/p>\n<\/div>\n<p id=\"x-ck12-OGVhOTI0NzcyNzQxNjRkMGUyYjUyZmJhODBjNDI4MWM.-td7\">Dipole-dipole forces are similar in nature, but much weaker than ionic bonds.<\/p>\n<h3>London Dispersion Forces<\/h3>\n<p id=\"x-ck12-YzJjZjUyMGFkMTRiNjQ5MjNkZjQ4YzA1OGU2N2RjNWI.-e5z\">Dispersion forces are also considered a type of van der Waals force and are the weakest of all intermolecular forces. \u00a0They are often called London forces after Fritz London (1900-1954), who first proposed their existence in 1930.\u00a0 <strong> London dispersion forces <\/strong> are the intermolecular forces that occur between atoms and between nonpolar molecules as a result of the motion of electrons.<\/p>\n<p id=\"x-ck12-OGExN2NlODM5YzYzNmRkZGM1ZTBkODgwMjNhNDM4Zjc.-esh\">The electron cloud of a helium atom contains two electrons, which can normally be expected to be equally distributed spatially around the nucleus.\u00a0 However, at any given moment the electron distribution may be uneven, resulting in an instantaneous dipole.\u00a0 This weak and temporary dipole subsequently influences neighboring helium atoms through electrostatic attraction and repulsion.\u00a0 It induces a dipole on nearby helium atoms.<\/p>\n<div id=\"x-ck12-M2RiMTdmNzdlMzA3NDlhNTAyNzI4ZGQyNDVlNGFhZjk.-wqm\" class=\"x-ck12-img-fullpage x-ck12-nofloat\">\n<div style=\"width: 522px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" id=\"x-ck12-OTgwNDUtMTM2MTg2MTc0NS0wMy0xMi1JbWFnZS0tLTMy\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/53\/2014\/08\/19211536\/20140811155258550313.png\" alt=\"An instantaneous dipole in a helium atom\" width=\"512\" height=\"134\" longdesc=\"A%20short-lived%20or%20instantaneous%20dipole%20in%20a%20helium%20atom.\" \/><\/p>\n<p class=\"wp-caption-text\">Figure 2. A short-lived or instantaneous dipole in a helium atom.<\/p>\n<\/div>\n<p>&nbsp;<\/p>\n<\/div>\n<p id=\"x-ck12-OTk3NGQ0Y2IyYmFlYjJmYTA5Y2ZlNmQ0ZWMzZWRkZTI.-zko\">The instantaneous and induced dipoles are weakly attracted to one another.\u00a0 The strength of dispersion forces increases as the number of electrons in the atoms or nonpolar molecules increases.<\/p>\n<p id=\"x-ck12-NWFhMGVjOTViOWUxZDBlNDhlYTMyNDYxNjAxNWExNmI.-qo7\">The halogen group consists of four elements that all take the form of nonpolar diatomic molecules.\u00a0 The table below shows a comparison of the melting and boiling points for each.<\/p>\n<div id=\"x-ck12-MzhlZTUzOTE1ZjUwZWMwNWZkMjkwYmRlMDU0YjU3YWM.-0j8\">\n<table id=\"x-ck12-NjU0Y2NiNjllZTNiMWQ3NDI4MjBiZmNkZTA5NmQwOTY.-9jz\" class=\"x-ck12-nofloat\">\n<caption>Melting and Boiling Points of Halogens<\/caption>\n<tbody>\n<tr>\n<td><strong> Molecule <\/strong><\/td>\n<td><strong> Total Number of Electrons <\/strong><\/td>\n<td><strong> Melting Point (\u00b0C) <\/strong><\/td>\n<td><strong> Boiling Point ( <\/strong> \u00b0C)<\/td>\n<td><strong> Physical State at Room Temperature <\/strong><\/td>\n<\/tr>\n<tr>\n<td>F <sub> 2 <\/sub><\/td>\n<td>18<\/td>\n<td>-220<\/td>\n<td>-188<\/td>\n<td>gas<\/td>\n<\/tr>\n<tr>\n<td>Cl <sub> 2 <\/sub><\/td>\n<td>34<\/td>\n<td>-102<\/td>\n<td>-34<\/td>\n<td>gas<\/td>\n<\/tr>\n<tr>\n<td>Br <sub> 2 <\/sub><\/td>\n<td>70<\/td>\n<td>-7<\/td>\n<td>59<\/td>\n<td>liquid<\/td>\n<\/tr>\n<tr>\n<td>I <sub> 2 <\/sub><\/td>\n<td>106<\/td>\n<td>114<\/td>\n<td>184<\/td>\n<td>solid<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p id=\"x-ck12-NGI3MGRlNWE1ZTRjYjg4MzQ0YWE0YmViODViZDc0ZjM.-sl4\">The dispersion forces are strongest for iodine molecules because they have the greatest number of electrons.\u00a0 The relatively stronger forces result in melting and boiling points which are the highest of the halogen group.\u00a0 These forces are strong enough to hold iodine molecules close together in the solid state at room temperature.\u00a0 The dispersion forces are progressively weaker for bromine, chlorine, and fluorine and this is illustrated in their steadily lower melting and boiling points.\u00a0 Bromine is a liquid at room temperature, while chlorine and fluorine are gases, whose molecules are much further apart from one another.\u00a0 Intermolecular forces are nearly nonexistent in the gas state, and so the dispersion forces in chlorine and fluorine only become measurable as the temperature decreases and they condense into the liquid state.<\/p>\n<\/div>\n<div id=\"x-ck12-MjkwNjEyMTk5ODYxYzMxZDEwMzZiMTg1YjRlNjliNzU.-svt\">\n<div class=\"textbox key-takeaways\">\n<h3>Summary<\/h3>\n<div id=\"x-ck12-MjkwNjEyMTk5ODYxYzMxZDEwMzZiMTg1YjRlNjliNzU.-svt\"><\/div>\n<div id=\"x-ck12-NTBiZTEwOWViZjVjYjg5NmIwZWM2OGRjZjNiM2I5YjA.-5ui\">\n<ul id=\"x-ck12-NTBiZTEwOWViZjVjYjg5NmIwZWM2OGRjZjNiM2I5YjA.-jqv\">\n<li>Van der Waals forces are weak interactions between molecules that involve dipoles.<\/li>\n<li>Polar molecules have permanent dipole-dipole interactions.<\/li>\n<li>Non-polar molecules can interact by way of London dispersion forces.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<\/div>\n<div id=\"x-ck12-OGMwMDRkN2UzYjhhNWE4NTBhYjI0NTljM2FmYzJmNTc.-lkn\">\n<div class=\"textbox exercises\">\n<h3>Practice<\/h3>\n<div id=\"x-ck12-OGMwMDRkN2UzYjhhNWE4NTBhYjI0NTljM2FmYzJmNTc.-lkn\">\n<p id=\"x-ck12-NmNlM2JkYzM5ZTU0NDcyNTAzOGUwZTg3Yjc3MWRiMWU.-kbf\">Use the link below to answer the following questions:<\/p>\n<\/div>\n<div id=\"x-ck12-ZGIxN2Q3OTkyNWFkYjUxMTNmNjhkMjQzYjI2ZDMxODQ.-d63\">\n<p id=\"x-ck12-ZGIxN2Q3OTkyNWFkYjUxMTNmNjhkMjQzYjI2ZDMxODQ.-qg3\"><a href=\"http:\/\/www.chemguide.co.uk\/atoms\/bonding\/vdw.html\"> http:\/\/www.chemguide.co.uk\/atoms\/bonding\/vdw.html <\/a><\/p>\n<\/div>\n<div id=\"x-ck12-MjFkMGE4NWNmZjhlMDJlZTFlZTAxODkwYTdmNzZkM2U.-fkv\">\n<ol id=\"x-ck12-MjFkMGE4NWNmZjhlMDJlZTFlZTAxODkwYTdmNzZkM2U.-fcp\">\n<li>What are intermolecular attractions?<\/li>\n<li>How cold must helium get before it forms a liquid?<\/li>\n<li>Can large numbers of molecules be held together by dispersion forces?<\/li>\n<li>Do long thin molecules develop stronger or weaker dipoles than short fat molecules?<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<\/div>\n<div id=\"x-ck12-NDU3ZGQ1NTE4NGZhZWRiNzg4NWFmZDQwMDlkNzAxNjM.-1qy\">\n<div class=\"textbox exercises\">\n<h3>Review<\/h3>\n<div id=\"x-ck12-NDU3ZGQ1NTE4NGZhZWRiNzg4NWFmZDQwMDlkNzAxNjM.-1qy\"><\/div>\n<div id=\"x-ck12-MjNjNzFlZDNhMzM5NTk2ZjliMmY5NzMwYjAzYmZhZWQ.-o6r\">\n<ol id=\"x-ck12-MjNjNzFlZDNhMzM5NTk2ZjliMmY5NzMwYjAzYmZhZWQ.-ypr\">\n<li>What attractive forces develop between polar molecules?<\/li>\n<li>What creates London dispersion forces?<\/li>\n<li>Are London dispersion forces permanent or temporary?<\/li>\n<li>Are the dispersion forces for Cl <sub> 2 <\/sub> stronger or weaker than the ones for Br<sub>2<\/sub>?<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<\/div>\n<h2 class=\"x-ck12-data-problem-set\">\u00a0Glossary<\/h2>\n<div id=\"x-ck12-OGJlYjYyOGYwZDgyMDFmYzA5N2M5M2NkNGZmZDQ0NWI.-50e\" class=\"x-ck12-data-vocabulary\">\n<ul id=\"x-ck12-Mzg3NjUxMzU3MzA3N2MwZTNlNTQxM2U0YTYwYzc2ZWQ.-o8t\">\n<li><strong> dipole-dipole forces: <\/strong> The attractive forces that occur between polar molecule <em> s <\/em><\/li>\n<li><strong> London dispersion forces: <\/strong> The intermolecular forces that occur between atoms and between nonpolar molecules as a result of the motion of electrons.<\/li>\n<li><strong> Van der Waals forces: <\/strong> The weakest intermolecular force and consist of dipole-dipole forces and dispersion forces <em> .<\/em><\/li>\n<\/ul>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q836080\">Show References<\/span><\/p>\n<div id=\"q836080\" class=\"hidden-answer\" style=\"display: none\">\n<h2>References<\/h2>\n<ol>\n<li>User:MartinSaunders\/Wikimedia Commons.<a href=\"http:\/\/commons.wikimedia.org\/wiki\/File:HWB-NMR_-_900MHz_-_21.2_Tesla.jpg\">http:\/\/commons.wikimedia.org\/wiki\/File:HWB-NMR_-_900MHz_-_21.2_Tesla.jpg<\/a>.<\/li>\n<li>CK-12 Foundation &#8211; Jodi So.<\/li>\n<li>CK-12 Foundation &#8211; Zachary Wilson.<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<\/div>\n\n\t\t\t <section class=\"citations-section\" role=\"contentinfo\">\n\t\t\t <h3>Candela Citations<\/h3>\n\t\t\t\t\t <div>\n\t\t\t\t\t\t <div id=\"citation-list-3070\">\n\t\t\t\t\t\t\t <div class=\"licensing\"><div class=\"license-attribution-dropdown-subheading\">CC licensed content, Shared previously<\/div><ul class=\"citation-list\"><li>Chemistry Concepts Intermediate. <strong>Authored by<\/strong>: Calbreath, Baxter, et al.. <strong>Provided by<\/strong>: CK12.org. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"http:\/\/www.ck12.org\/book\/CK-12-Chemistry-Concepts-Intermediate\/\">http:\/\/www.ck12.org\/book\/CK-12-Chemistry-Concepts-Intermediate\/<\/a>. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by-nc\/4.0\/\">CC BY-NC: Attribution-NonCommercial<\/a><\/em><\/li><\/ul><\/div>\n\t\t\t\t\t\t <\/div>\n\t\t\t\t\t <\/div>\n\t\t\t <\/section>","protected":false},"author":17,"menu_order":15,"template":"","meta":{"_candela_citation":"[{\"type\":\"cc\",\"description\":\"Chemistry Concepts Intermediate\",\"author\":\"Calbreath, Baxter, et 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