The periodic table
The periodic table — the chapter map
SB p.30Open with a claim worth pausing on: the periodic-table shows all the elements in the universe and is one of the most important tools a chemist has. Not one of the most important tools in chemistry — one of the most important tools, full stop. The reason is one word, worth emphasising twice: arrangement. Because the elements are laid out in a fixed, meaningful order, the table lets you understand trends in properties and make predictions — including predictions about elements nobody had ever seen.
The prediction power is not hypothetical. The modern Periodic Table was first presented in 1869 by the Russian chemist Dmitri Mendeleev, and his table is famous precisely because it left gaps for elements that had not been discovered yet and correctly predicted their properties. This chapter explores some of the features of that table: how the elements are ordered, what the groups and periods mean, how the table lets you count protons, neutrons and electrons, and how it answers the question "metal or non-metal?".
By the end of this lesson you will be able to:
- say how the elements are arranged in the Periodic Table — in order of atomic number, in groups (columns) and periods (rows);
- deduce the electronic configurations of the first 20 elements from their positions in the table;
- classify an element as a metal or a non-metal, both from its position and from electrical conductivity and the acid-base character of its oxides;
- explain why elements in the same group have similar chemical properties, and why the noble gases barely react at all.
From Lesson u1:c03 you know the counting recipe — protons from the atomic number, electrons = protons, neutrons = mass number − atomic number — and you know electrons sit in shells (energy levels) around the nucleus. From Lesson u1:c02 you know that the 118 elements are the building blocks of everything. This chapter finally explains how that list of elements is organised — and why the organisation is not arbitrary.