Week 3 · Lesson 2 The Periodic Table of Elements (A): Mendeleev, Moseley, the modern periodic table, blocks and groups · 3 lessons
1
Mendeleev sorts the elements
Science Prep 1 Term 1, Unit 1: pp. 10–13
GoalExplain how Mendeleev arranged the elements and what “periodic” means.
New words
periodic table · a chart that arranges all the elements in rows and columnsperiod · a horizontal row of the periodic tableperiodic · repeating again and again after regular steps
Explanation
By the 1800s chemists knew dozens of elements and wanted an order for them. In 1869 the Russian chemist Dmitri Mendeleev arranged the elements in rows by increasing atomic mass. He noticed that the properties of the elements repeat again and again at regular steps. When a row filled up, he started a new row so that elements with similar properties stood in the same column. We say the properties repeat periodically, like the days of the week, which repeat every seven days. This idea gave us the periodic table. The element mendelevium, symbol Md, is named in his honour.
elements in a row by increasing mass→similar properties come back again and again→new row, so similar ones line up in columns
Worked examples
1: soft metal
2: hard metal
3: yellow solid
4: gas
5: soft metal
6: hard metal
7: yellow solid
8: gas
9: soft metal
10: hard metal
11: yellow solid
12: gas
Try Mendeleev’s idea with twelve invented elements, numbered by mass. Their properties repeat every four elements, so we start a new row after every fourth one. Now each column holds similar elements.
Suppose a new invented element 13 is found. Where would it go, and what would it probably be like? It starts a fourth row, in the first column, so it is probably a soft, shiny metal like 1, 5 and 9.
The word “period” is used for the rows of the table because the pattern repeats row after row. Elements in one column have similar properties; the word for a column is a group.
Class activity
Invent a table. Write twelve pretend elements on cards, each with a mass from 1 to 12 and a property that repeats in a pattern of your choice (for example every three elements). Mix the cards, then sort them by mass and start a new row where the pattern restarts. Give them to a friend and ask: where would element 13 go?
Practice
1
In which year, and by which scientist, was the first well-known periodic table published?
In 1869, by the Russian chemist Mendeleev.
2
How did Mendeleev arrange the elements in rows?
By increasing atomic mass.
3
What does it mean that the properties are “periodic”?
