GCSE Chemistry - Ionic Bonding - Formation | Dot and Cross Diagrams (2026/27 exams) — Transcript

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1. 0:07In today's video we're going to take a look at how particles can bond together through
2. 0:11ionic bonds.
3. 0:13To explain this we'll take a look at some dot and cross diagrams.
4. 0:19First though, I just want to recap what ions are.
5. 0:23We said in a previous video that ions are formed when atoms lose or gain electrons, and
6. 0:30we can show this happening with equations.
7. 0:33For example, a sodium atom will go to form a sodium one plus ion plus one electron.
8. 0:40We know this because if we look at a diagram of a sodium atom, it has one electron in its
9. 0:45outermost shell that it needs to lose in order to become stable.
10. 0:50Because remember, stability is all about having a full outer shell.
11. 0:54Meanwhile, for chlorine, we'd write that chlorine plus an electron, which we can see
12. 1:00it needs to complete its outer shell, goes to form a one minus chloride ion.
13. 1:07Now this is all well and good in theory, but in real life these reactions don't happen
14. 1:12in isolation.
15. 1:13Instead, we normally talk about a transfer of electrons, from an atom that has too many,
16. 1:20like sodium, to an atom that doesn't have enough, like clorine
17. 1:25Once this electron has been transferred, both atoms become ions, with full outer shells
18. 1:31of electrons.
19. 1:33So we put big square brackets around them, and they're charged in the top right corner.
20. 1:39The important bit here is that the two ions have opposite charges, so they'll be attracted
21. 1:44to each other by electrostatic forces, to form an ionic compound.
22. 1:50We call this force an ionic bond.
23. 1:53And it's really strong, similar in strength to covalent bonds, which we cover in another
24. 1:59video.
25. 2:03The way that we've drawn our compound here is known as a dot and cross diagram, and you'll
26. 2:08often be asked to draw things this way in your exam.
27. 2:12To do it properly, there are a couple of features to notice though.
28. 2:15One is that we've drawn the electrons of one atom as dots, and the other as crosses.
29. 2:20This is so that we can tell which electrons bond with which atoms
30. 2:25And you should show the movement of any electrons with an arrow.
31. 2:29Notice that in this dot and cross diagram, we've shown every electron shell of the atoms.
32. 2:35Sometimes though, you'll be told you only have to draw the outer shell, which is a bit
33. 2:39quicker to draw.
34. 2:41And for our example, it would look like this.
35. 2:47Let's consider a harder example.
36. 2:49Draw the dot and cross diagram
37. 2:52for the formation of magnesium chloride, MgCl2.
38. 2:58Only draw the outermost shells.
39. 3:01Now this time, we can see that we have three atoms in the compound, rather than two.
40. 3:06To start, let's draw out our reactants.
41. 3:09We have magnesium, which has two electrons in its outer shell that it wants to get rid of
42. 3:14And we have two chlorines, both of which have seven outer electrons, so we need one more
43. 3:20each.
44. 3:22The next step is to think about where the electrons could move to make all the electrons
45. 3:26happy with a full outer shell.
46. 3:29And as a general rule, electrons will move from the metal to the non-metal.
47. 3:34So in this case, magnesium can give one electron to each of the two chlorines.
48. 3:39As a result, we'll end up with a Mg2 plus ion, and two Cl1- ions.
49. 3:47This is now pretty much done.
50. 3:49However, in dot and cross diagrams involving more than two ions
51. 3:53will generally arrange the ions like they would be arranged in a real compound
52. 3:57So, because the chlorides will both be attracted to the positive magnesium, we place them on
53. 4:03either side of it.
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