Page 5 - Curriculum Visions Dynamic Book

Shell diagrams
 An argon atom (Ar) has an atomic number of 18 and therefore has 18 protons and 18 electrons.The electrons can be arranged as shown in this shell diagram.
The Periodic Law
The Periodic Law is one of the most important ways of making sense of the chemical elements.The Periodic Law states that the chemical elements show repeating, or periodic, patterns if they are arranged according to their atomic number (see
page 4).The pattern of elements arranged according to the Periodic Law is called the Periodic Table (see pages 6–7).
The Periodic Table was first described by a Russian teacher, Dmitri Ivanovich Mendeleev, between 1869 and 1870. At that time only 57 elements were known, but he arranged them as a chart (the Periodic Table) on the assumption that there was some order to the elements. He left blank spaces where elements seemed to be missing. Using this chart, he was able to predict, in detail, the chemical
and physical proprieties of elements that had not yet been discovered.As soon as scientists looked for the missing elements, using clues provided by Mendeleev’s table, they quickly began to find them.
The only element that Mendeleev could not fit into his scheme was hydrogen, so he put it in a box on its own. Otherwise, the elements were all placed
in order horizontally. Similarities among the elements can be found by reading up and down the table.These columns are the groups. Each of the rows is a period.
There are eight numbered groups or columns of elements in the Periodic Table. The number of the group represents the number of electrons in the outer shell of the atom. For example, neon and argon have eight electrons in their outer shell and so belong to what can be numbered group 8. However, since helium (with a stable outer shell of only two electrons in this exceptional case) is included in this group
The nucleus at the centre of the atom contains protons and neutrons. The number of protons is given by the atomic number, more recently called the proton number.
A circle with electrons represents a shell.
These show the number of electrons in each shell.
The outer shell of an argon atom is full and so this atom does not react – it is inert.
 A chlorine atom (Cl) has an atomic number of 17 and therefore has 17 protons and 17 electrons.The electrons can be arranged as shown in this shell diagram.
The outer shell of a chlorine atom has seven instead of eight electrons, and so
this atom is highly
reactive, being strongly attractive to an electron.
The background colour
shows whether the element is
a metal, non-metal, metalloid or inner transition metal.The Periodic Table on pages 6–7 also has a colour key.
5