WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:06.703 Unit S C 6: Covalent bonding. 00:00:09.367 --> 00:00:14.805 A covalent bond is a shared pair of electrons between atoms, usually non-metal atoms. 00:00:14.955 --> 00:00:20.384 Attraction between the shared electrons and both positive nuclei holds the atoms together. 00:00:23.067 --> 00:00:28.591 Sharing can give atoms complete outer shells: hydrogen needs two electrons in its first shell, 00:00:28.741 --> 00:00:30.319 while the common carbon, 00:00:30.469 --> 00:00:31.685 nitrogen, 00:00:31.835 --> 00:00:35.448 oxygen and chlorine examples reach eight outer electrons. 00:00:38.100 --> 00:00:43.219 A single bond shares one pair, a double bond two pairs, and a triple bond three pairs. 00:00:43.369 --> 00:00:47.791 The electrons remain shared rather than transferring to form opposite ions. 00:00:50.467 --> 00:00:54.260 Dot-and-cross diagrams show which atom supplied each electron. 00:00:54.410 --> 00:00:59.785 Dots and crosses are a drawing convention: the electrons have the same physical properties. 00:01:02.467 --> 00:01:09.431 Show outer electrons, including unshared or lone pairs, and count each shared pair towards both atoms' outer shells. 00:01:09.581 --> 00:01:12.694 Inner shells can be omitted in outer-shell diagrams. 00:01:15.367 --> 00:01:19.135 A molecular formula gives numbers of atoms in one molecule. 00:01:19.285 --> 00:01:25.789 A displayed formula shows bonds as lines; each ordinary single line represents one shared electron pair. 00:01:28.467 --> 00:01:32.989 Hydrogen, H 2, has one shared pair between two hydrogen atoms. 00:01:33.139 --> 00:01:37.010 Each atom counts the pair as a complete first shell of two electrons. 00:01:37.160 --> 00:01:41.662 H to H: one shared pair; two electrons count for each hydrogen. 00:01:46.333 --> 00:01:51.281 Chlorine, C L 2, has one shared pair and three lone pairs on each chlorine atom. 00:01:51.431 --> 00:01:55.772 Each chlorine contributes one electron to the bond and has eight around it. 00:01:55.922 --> 00:02:03.483 C L to C L: one shared pair and three lone pairs around each C L; dots belong to one chlorine and crosses to the other. 00:02:08.133 --> 00:02:13.502 Water, H 2 O, has two O to H single bonds and two lone pairs on oxygen. 00:02:13.652 --> 00:02:18.133 Each hydrogen has two electrons; oxygen has eight counted around it. 00:02:18.283 --> 00:02:22.709 Two shared pairs and two oxygen lone pairs; the sketch is not to scale. 00:02:27.367 --> 00:02:32.832 Ammonia, N H 3, has three N to H single bonds and one lone pair on nitrogen. 00:02:32.982 --> 00:02:36.159 Nitrogen contributes one electron to each bond. 00:02:36.309 --> 00:02:39.168 Three shared pairs and one nitrogen lone pair. 00:02:43.833 --> 00:02:49.113 Methane, C H 4, has four C to H single bonds and no lone pairs on carbon. 00:02:49.263 --> 00:02:52.971 Carbon's four outer electrons contribute to four shared pairs. 00:02:53.121 --> 00:02:58.315 Four shared pairs; actual methane has a three-dimensional tetrahedral arrangement. 00:03:02.967 --> 00:03:08.629 Flat diagrams communicate bonding and electron counts but not all three-dimensional shapes. 00:03:08.779 --> 00:03:12.611 Methane is not a flat cross; water is not a straight line. 00:03:15.267 --> 00:03:22.259 Hydrogen chloride, H C L, has one shared pair between hydrogen and chlorine and three lone pairs on chlorine. 00:03:22.409 --> 00:03:25.777 Hydrogen counts two outer electrons and chlorine eight. 00:03:25.927 --> 00:03:30.152 H to C L: one shared pair; three lone pairs belong to chlorine. 00:03:30.302 --> 00:03:33.755 Dots indicate hydrogen and crosses chlorine electrons. 00:03:38.433 --> 00:03:43.628 Typical atoms and small molecules are roughly ten to the power minus 10 metres across. 00:03:43.778 --> 00:03:50.475 Diagrams enlarge them greatly; do not measure a diagram to find an atom’s size unless a scale is provided. 00:03:53.133 --> 00:03:56.793 Oxygen, O 2, has two shared pairs forming a double bond. 00:03:56.943 --> 00:04:02.297 Each oxygen also has two lone pairs, giving eight outer electrons around each atom. 00:04:02.447 --> 00:04:07.387 O double bonded to O: two shared pairs and two lone pairs around each oxygen. 00:04:12.067 --> 00:04:20.166 Carbon dioxide, C O 2, has structure O double bonded to C double bonded to O: two C double bonded to O double bonds. 00:04:20.316 --> 00:04:25.009 Four pairs are shared in the whole molecule, and each oxygen has two lone pairs. 00:04:25.159 --> 00:04:33.201 O double bonded to C double bonded to O: four shared pairs in total; carbon electrons are crosses and oxygen electrons dots. 00:04:33.351 --> 00:04:35.655 Each oxygen has two lone pairs. 00:04:40.333 --> 00:04:45.495 In C O 2 carbon contributes four outer electrons, two to each double bond. 00:04:45.645 --> 00:04:50.594 Each oxygen contributes two to its double bond and retains four in lone pairs. 00:04:53.267 --> 00:04:59.569 The number of shared pairs differs from the number of atoms: C O 2 contains three atoms but four shared pairs. 00:04:59.719 --> 00:05:02.396 Count lines or electron pairs carefully. 00:05:05.067 --> 00:05:13.885 Molecular oxygen is an element because both atoms are oxygen; carbon dioxide is a compound because carbon and oxygen are different elements. 00:05:16.567 --> 00:05:20.105 Strong covalent bonds need considerable energy to break. 00:05:20.255 --> 00:05:27.827 A change of state of a simple molecular substance mainly overcomes attractions between molecules, not its covalent bonds. 00:05:30.500 --> 00:05:34.587 Simple molecular substances are made of separate small molecules. 00:05:34.737 --> 00:05:44.474 They often have low melting and boiling points because the attractions between molecules (intermolecular forces) are relatively weak and need little energy to overcome. 00:05:47.133 --> 00:05:51.329 Low boiling point does not mean the bonds inside each molecule are weak. 00:05:51.479 --> 00:05:57.377 During ordinary boiling, intact molecules separate rather than decomposing into individual atoms. 00:05:57.527 --> 00:06:02.763 Boiling separates molecules; it does not normally break H to H covalent bonds. 00:06:07.433 --> 00:06:17.607 Simple molecular substances usually do not conduct electricity because they have no charged particles that can move freely: no mobile ions or delocalised electrons. 00:06:17.757 --> 00:06:21.315 Moving neutral molecules cannot carry an electric current. 00:06:23.967 --> 00:06:29.288 Solubility varies: some molecular substances dissolve in water and others do not. 00:06:29.438 --> 00:06:33.051 Avoid claiming that every covalent substance is insoluble. 00:06:35.733 --> 00:06:45.395 Some molecules react or ionise in water, giving ions in solution: aqueous hydrogen chloride conducts even though molecular H C L is covalent. 00:06:45.545 --> 00:06:47.785 Structure and conditions matter. 00:06:50.467 --> 00:06:58.593 Giant covalent structures such as diamond are exceptions to simple molecular melting-point patterns and are covered in C C7. 00:07:01.267 --> 00:07:07.268 Some small molecules, called monomers, can chemically join into long-chain molecules called polymers. 00:07:07.418 --> 00:07:09.914 Not every small molecule is a monomer. 00:07:12.567 --> 00:07:16.815 Ethene can join by addition polymerisation to form poly(ethene). 00:07:16.965 --> 00:07:24.911 The C double bonded to C bond is converted into single bonds as repeating units link; the polymer still contains covalent bonds. 00:07:27.567 --> 00:07:34.317 Polymers have strong covalent bonds within long molecules; attractions between chains influence their properties. 00:07:34.467 --> 00:07:37.561 They are not giant covalent lattices like diamond. 00:07:40.233 --> 00:07:47.048 Ball-and-stick models show how atoms join and their three-dimensional arrangement, but exaggerate the gaps between them. 00:07:47.198 --> 00:07:49.097 The sticks represent bonds. 00:07:49.247 --> 00:07:53.759 Space-filling models show the space atoms occupy but can hide the bonds. 00:07:56.433 --> 00:08:04.602 Dot-and-cross diagrams reveal shared and lone pairs but do not show actual atom sizes, bond lengths or full three-dimensional shape. 00:08:04.752 --> 00:08:07.258 Choose a model for the information required. 00:08:09.933 --> 00:08:12.024 That completes Covalent bonding. 00:08:12.174 --> 00:08:15.519 Revisit the notes and test yourself on the revision website.