WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:06.725 Unit C C 6: Covalent bonding. 00:00:09.400 --> 00:00:14.838 A covalent bond is a shared pair of electrons between atoms, usually non-metal atoms. 00:00:14.988 --> 00:00:20.417 Attraction between the shared electrons and both positive nuclei holds the atoms together. 00:00:23.100 --> 00:00:28.624 Sharing can give atoms complete outer shells: hydrogen needs two electrons in its first shell, 00:00:28.774 --> 00:00:30.353 while the common carbon, 00:00:30.503 --> 00:00:31.719 nitrogen, 00:00:31.869 --> 00:00:35.481 oxygen and chlorine examples reach eight outer electrons. 00:00:38.133 --> 00:00:43.253 A single bond shares one pair, a double bond two pairs, and a triple bond three pairs. 00:00:43.403 --> 00:00:47.825 The electrons remain shared rather than transferring to form opposite ions. 00:00:50.500 --> 00:00:54.293 Dot-and-cross diagrams show which atom supplied each electron. 00:00:54.443 --> 00:00:59.819 Dots and crosses are a drawing convention: the electrons have the same physical properties. 00:01:02.500 --> 00:01:09.464 Show outer electrons, including unshared or lone pairs, and count each shared pair towards both atoms' outer shells. 00:01:09.614 --> 00:01:12.727 Inner shells can be omitted in outer-shell diagrams. 00:01:15.400 --> 00:01:19.168 A molecular formula gives numbers of atoms in one molecule. 00:01:19.318 --> 00:01:25.823 A displayed formula shows bonds as lines; each ordinary single line represents one shared electron pair. 00:01:28.500 --> 00:01:33.022 Hydrogen, H 2, has one shared pair between two hydrogen atoms. 00:01:33.172 --> 00:01:37.043 Each atom counts the pair as a complete first shell of two electrons. 00:01:37.193 --> 00:01:41.695 H to H: one shared pair; two electrons count for each hydrogen. 00:01:46.367 --> 00:01:51.314 Chlorine, C L 2, has one shared pair and three lone pairs on each chlorine atom. 00:01:51.464 --> 00:01:55.805 Each chlorine contributes one electron to the bond and has eight around it. 00:01:55.955 --> 00:02:03.517 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. 00:02:08.167 --> 00:02:13.535 Water, H 2 O, has two O to H single bonds and two lone pairs on oxygen. 00:02:13.685 --> 00:02:18.166 Each hydrogen has two electrons; oxygen has eight counted around it. 00:02:18.316 --> 00:02:22.743 Two shared pairs and two oxygen lone pairs; the sketch is not to scale. 00:02:27.400 --> 00:02:32.865 Ammonia, N H 3, has three N to H single bonds and one lone pair on nitrogen. 00:02:33.015 --> 00:02:36.193 Nitrogen contributes one electron to each bond. 00:02:36.343 --> 00:02:39.201 Three shared pairs and one nitrogen lone pair. 00:02:43.867 --> 00:02:49.147 Methane, C H 4, has four C to H single bonds and no lone pairs on carbon. 00:02:49.297 --> 00:02:53.005 Carbon's four outer electrons contribute to four shared pairs. 00:02:53.155 --> 00:02:58.348 Four shared pairs; actual methane has a three-dimensional tetrahedral arrangement. 00:03:03.000 --> 00:03:08.663 Flat diagrams communicate bonding and electron counts but not all three-dimensional shapes. 00:03:08.813 --> 00:03:12.645 Methane is not a flat cross; water is not a straight line. 00:03:15.300 --> 00:03:22.293 Hydrogen chloride, H C L, has one shared pair between hydrogen and chlorine and three lone pairs on chlorine. 00:03:22.443 --> 00:03:25.811 Hydrogen counts two outer electrons and chlorine eight. 00:03:25.961 --> 00:03:30.185 H to C L: one shared pair; three lone pairs belong to chlorine. 00:03:30.335 --> 00:03:33.789 Dots indicate hydrogen and crosses chlorine electrons. 00:03:38.467 --> 00:03:43.661 Typical atoms and small molecules are roughly ten to the power minus 10 metres across. 00:03:43.811 --> 00:03:50.508 Diagrams enlarge them greatly; do not measure a diagram to find an atom’s size unless a scale is provided. 00:03:53.167 --> 00:03:56.827 Oxygen, O 2, has two shared pairs forming a double bond. 00:03:56.977 --> 00:04:02.331 Each oxygen also has two lone pairs, giving eight outer electrons around each atom. 00:04:02.481 --> 00:04:07.421 O double bonded to O: two shared pairs and two lone pairs around each oxygen. 00:04:12.100 --> 00:04:20.199 Carbon dioxide, C O 2, has structure O double bonded to C double bonded to O: two C double bonded to O double bonds. 00:04:20.349 --> 00:04:25.042 Four pairs are shared in the whole molecule, and each oxygen has two lone pairs. 00:04:25.192 --> 00:04:33.234 O double bonded to C double bonded to O: four shared pairs in total; carbon electrons are crosses and oxygen electrons dots. 00:04:33.384 --> 00:04:35.688 Each oxygen has two lone pairs. 00:04:40.367 --> 00:04:45.529 In C O 2 carbon contributes four outer electrons, two to each double bond. 00:04:45.679 --> 00:04:50.627 Each oxygen contributes two to its double bond and retains four in lone pairs. 00:04:53.300 --> 00:04:59.603 The number of shared pairs differs from the number of atoms: C O 2 contains three atoms but four shared pairs. 00:04:59.753 --> 00:05:02.429 Count lines or electron pairs carefully. 00:05:05.100 --> 00:05:13.919 Molecular oxygen is an element because both atoms are oxygen; carbon dioxide is a compound because carbon and oxygen are different elements. 00:05:16.600 --> 00:05:20.139 Strong covalent bonds need considerable energy to break. 00:05:20.289 --> 00:05:27.860 A change of state of a simple molecular substance mainly overcomes attractions between molecules, not its covalent bonds. 00:05:30.533 --> 00:05:34.620 Simple molecular substances are made of separate small molecules. 00:05:34.770 --> 00:05:44.507 They often have low melting and boiling points because the attractions between molecules (intermolecular forces) are relatively weak and need little energy to overcome. 00:05:47.167 --> 00:05:51.363 Low boiling point does not mean the bonds inside each molecule are weak. 00:05:51.513 --> 00:05:57.410 During ordinary boiling, intact molecules separate rather than decomposing into individual atoms. 00:05:57.560 --> 00:06:02.796 Boiling separates molecules; it does not normally break H to H covalent bonds. 00:06:07.467 --> 00:06:17.641 Simple molecular substances usually do not conduct electricity because they have no charged particles that can move freely: no mobile ions or delocalised electrons. 00:06:17.791 --> 00:06:21.348 Moving neutral molecules cannot carry an electric current. 00:06:24.000 --> 00:06:29.321 Solubility varies: some molecular substances dissolve in water and others do not. 00:06:29.471 --> 00:06:33.084 Avoid claiming that every covalent substance is insoluble. 00:06:35.767 --> 00:06:45.429 Some molecules react or ionise in water, giving ions in solution: aqueous hydrogen chloride conducts even though molecular H C L is covalent. 00:06:45.579 --> 00:06:47.819 Structure and conditions matter. 00:06:50.500 --> 00:06:58.626 Giant covalent structures such as diamond are exceptions to simple molecular melting-point patterns and are covered in C C7. 00:07:01.300 --> 00:07:07.301 Some small molecules, called monomers, can chemically join into long-chain molecules called polymers. 00:07:07.451 --> 00:07:09.947 Not every small molecule is a monomer. 00:07:12.600 --> 00:07:16.849 Ethene can join by addition polymerisation to form poly(ethene). 00:07:16.999 --> 00:07:24.944 The C double bonded to C bond is converted into single bonds as repeating units link; the polymer still contains covalent bonds. 00:07:27.600 --> 00:07:34.351 Polymers have strong covalent bonds within long molecules; attractions between chains influence their properties. 00:07:34.501 --> 00:07:37.594 They are not giant covalent lattices like diamond. 00:07:40.267 --> 00:07:47.081 Ball-and-stick models show how atoms join and their three-dimensional arrangement, but exaggerate the gaps between them. 00:07:47.231 --> 00:07:49.130 The sticks represent bonds. 00:07:49.280 --> 00:07:53.792 Space-filling models show the space atoms occupy but can hide the bonds. 00:07:56.467 --> 00:08:04.635 Dot-and-cross diagrams reveal shared and lone pairs but do not show actual atom sizes, bond lengths or full three-dimensional shape. 00:08:04.785 --> 00:08:07.291 Choose a model for the information required. 00:08:09.967 --> 00:08:12.057 That completes Covalent bonding. 00:08:12.207 --> 00:08:15.553 Revisit the notes and test yourself on the revision website.