Hi all, I am a Chemistry Physics Tuition teacher giving small-group tuition in Yishun. Today, I am doing a recap on a Chemistry topic – Periodic Table this week with her Sec 3s and 4 students. This post covers 5 common mistakes that students uaually make. So, have a read and dont repeat them, ok? 🙂
(1) Group 1 metals are called alkali metals not because they are alkali but because they produce OH- ions in water when they react with steam or cold water. Note, which metals produce hydroxides and which metals produce produces another base (oxides). The more reactive metals (Top 4 in the metal reactivity series produce hydroxides). Also note, some of the unreactive metals do not react with water at all too. Yes, there are lots of memorising to do in this chapter. Stop complaining! Memorising is a muscle. Flex it, use it, train it. It will serve you well, I promise. 👍😎
(2) Students, please ensure you know the reasons behind the trends down Group 1 and Group 7 especially in terms of reactivity. As we move down the group, the atoms get larger. The number of electron shells increased and hence the valence electron gets further away from the nucleus. The forces of attraction between the outermost electron and the nucleus gets weaker get it goes down the group. For Group 1 metals, the atoms lose electron more easily as we move from the group. Hence reactivity increased down the group for Group 1 metals. For Group 7, the atoms gain electrons less easily as we move down the group to form a stable electronic configuration. Hence reactivity increased down the group for Group 7 Halogens. 🤞
(3) Also, students often make observation error for halogens displacement reactions. Chlorine gas is yellowish-green gas but sodium chloride solution is colourless not yellowish. When chlorine gas is bubbled into NaI solution, the colourless solution turns reddish brown because iodine is produced as it is being displaced from NaI. The solution did not turn purplish black although students studied the colour of Iodine to be purplish black. This is because iodine which exist in solid state in room temperature and pressure, dissolves in the aqueous solution and the colour tone goes one shade lighter to reddish brown. 👌?
(4) Please also note that displacement reaction is a type of redox reaction! When Iodine is displaced from sodium iodide, the oxidation state increased from -1 in iodide to 0 in iodine. 🙂 So, it is oxidised. Hence, one can also say chlorine is the oxidising agent in this reaction. And it is reduced. You can check that the oxidation state of chlorine decreased from 0 in chlorine to -1 in NaCl.
(5) Noble gas exists as atoms not molecules like the rest of the gases. (O2, N2, Cl2). Why? They already have a full outermost electron shell. Hence, they are relatively inert and do not need to lose/gain/share electrons with other elements to form a stable electronic configuration. Please note the other properties for noble gases because I realize they are pretty common in examinations. Other characteristics of the noble gases are that they all conduct electricity, fluoresce, are odorless and colorless, and are used in many conditions when we need to create a safe and inert environment for reactive elements or substances.
I may be doing a youtube video on these 5 common mistakes soon. And I will upload it here. Stay tuned! Meanwhile, you can watch more Chemistry Physics tuition videos that I have made according to the feedback given by my Yishun students. This one below is on salt preparation which is also another killer Chemistry topic. 🙂