Hi, and welcome back to Free Science Lessons. By the end of this video, you should be able to describe how Group 7 elements consist of molecules with two atoms. You should then be able to describe how the melting and boiling points of the Group 7 elements change down the group. OK, I'm showing you Group 7 here. Group 7 is also called the halogens, and you could be asked that in your exam.
All the elements in Group 7 are non-metals. Now a key fact is that Group 7 elements have 7 electrons in their outer energy level. And I'm showing you that here with fluorine and with chlorine. I'm using crosses to represent electrons. Now remember that atoms react in order to achieve a full outer energy level.
If we take two atoms of a Group 7 element, then they react with each other. And I'm showing you that here with fluorine. Each fluorine atom has seven outer electrons. I'm showing the electrons as two different colours to make it easier to see. These two atoms now overlap their outer energy levels and they share a pair of electrons.
We call this shared pair of electrons a covalent bond. And we've produced a fluorine molecule F2. Every group 7 element forms a molecule consisting of two atoms joined by a covalent bond. I'm showing you here the chlorine molecule Cl2.
And again, we can see the shared pair of electrons in the covalent bond. Now we can represent the covalent bond as a stick between the two atoms. So here are the halogens forming molecules consisting of two atoms joined by a covalent bond.
Now I should point out that we'll be seeing covalent bonding again in much more detail in a later topic. OK, now the melting and boiling points of Group 7 elements change as we move down the group. And I'm showing you the melting and boiling points of the Group 7 elements here. We can use the melting and boiling points to work out the physical state of each halogen. In other words, whether it's a solid, liquid or gas at room temperature.
And remember that room temperature is around 20 degrees Celsius. As you can see, the melting and boiling points increase as we move down Group 7. Fluorine and chlorine are both gases at room temperature, as their boiling points are lower than 20 degrees Celsius. Bromine is a liquid at room temperature. That's because the melting point of bromine is lower than 20 degrees Celsius, but the boiling point of bromine is higher than 20 degrees Celsius. Finally, iodine is a solid at room temperature.
That's because both the melting point and boiling point of iodine are higher than 20 degrees Celsius. Now, there's one final thing that we need to look at. The second column shows you the relative molecular mass of the halogens. Relative molecular mass gives us an idea of the size of a molecule, and we'll be looking at relative molecular mass in a later video. You can see that as we move down Group 7, the relative molecular mass increases.
In other words, the molecules get bigger moving down Group 7, and you need to learn that. You'll find plenty of questions on this topic in my revision workbook, which you can get by clicking on the link above.