Week 3 · Lesson 2 The Periodic Table of Elements (A): Mendeleev, Moseley, the modern periodic table, blocks and groups · 3 lessons
2
Moseley and the modern periodic table
Science Prep 1 Term 1, Unit 1: pp. 10–13
GoalSay how Moseley improved the table and describe the modern periodic table.
New words
atomic number · the number of protons in an atom; the order used in the modern tablemodern periodic table · the table in which elements are arranged by atomic numbergroup · a vertical column of the periodic table
Explanation
Mendeleev’s table worked well, but a few elements did not fit when they were ordered by mass. The physicist Moseley showed that the repeating pattern follows the atomic number, not the mass. The elements were reordered by atomic number Z, and the table became the one we use today. The modern periodic table has 7 horizontal rows called periods, 18 vertical columns called groups and 118 elements. The elements go in order of Z from left to right, and a new period starts when a new energy level starts to fill. Elements in the same group have similar chemical properties because they have the same number of electrons in their outer level. Here is a case where mass fails: argon (Z = 18) has atoms that are slightly heavier on average than potassium (Z = 19), yet argon belongs before potassium.
Worked examples
After neon (Z = 10) comes sodium (Z = 11). Z grows by one at each step, so sodium starts the next period. Both Ne and Na are marked in the table.
Check the total of 118 elements. The seven periods hold 2, 8, 8, 18, 18, 32 and 32 elements. Add them: 2 + 8 + 8 + 18 + 18 + 32 + 32 = 118.
Omar says: “Order by mass is simpler.” Mona answers: “With mass, argon would come after potassium and land in the wrong column. With Z, every element lands in the column that matches its properties.”
Class activity
Build the first rows. Write the symbols and Z of the first 18 elements on 18 cards: H 1, He 2, Li 3, Be 4, B 5, C 6, N 7, O 8, F 9, Ne 10, Na 11, Mg 12, Al 13, Si 14, P 15, S 16, Cl 17, Ar 18. Lay them in period rows: 2 in the first, 8 in the second, 8 in the third. Which cards fall in the same column as Li? (Answer: Na.)
Practice
1
What did Moseley show about the repeating pattern?
It follows the atomic number, not the mass.
2
How many periods and groups does the modern periodic table have?
7 periods and 18 groups.
3
How many elements does the modern table hold?
118.
4
Elements are in order of what, from left to right and from one period to the next?
