Year 7 Science · Unit 2 · Lesson 5
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Learning Goals
Real-world context
In 1871, Russian chemist Dmitri Mendeleev published a version of the periodic table with several deliberate gaps. He predicted that unknown elements would eventually be discovered to fill those gaps, and he even described the properties those elements would have — their approximate atomic mass, density and the types of compounds they would form. In 1875, French chemist Paul-Émile Lecoq de Boisbaudran discovered gallium, an element whose properties matched Mendeleev's predictions almost exactly. Mendeleev had predicted the element four years before it was found.
(a) How was Mendeleev able to predict the existence and properties of gallium before it was discovered? Use the idea of "groups" and "patterns" in your answer.
(b) Why is it impressive that Mendeleev left gaps rather than simply listing elements in order? What does this tell us about how science works?
Read the graph — Noble Gas Atomic Numbers
Study the bar chart showing the atomic numbers of the first five noble gases, then answer the questions below.
Data: IUPAC Periodic Table of the Elements, 2021.
(a) Describe the pattern shown in the atomic numbers of the noble gases as you go down Group 18.
(b) The gaps between consecutive noble gases are: Ne−He = 8, Ar−Ne = 8, Kr−Ar = 18, Xe−Kr = 18. The next noble gas after Xenon is Radon (Rn). Using the pattern, predict Radon's atomic number.
(c) Noble gases are all in Group 18. Using the lesson idea about groups, explain why all noble gases behave in a similar way (extremely unreactive).
Wrap Up
In one sentence, what was the main idea of this lesson?