The Periodic Table

The modern periodic table is the result of many years of work by scientists from all corners of the scientific world. Before 1790 only 27 elements were known to exist. By 1811, the total number of elements had risen to 60.

The Periodic Table Along with these discoveries, more and more were being learned about the chemical reactivity and physical properties of the elements. A real breakthrough came in 1869 when the Russian chemist Dmitri Mendeleev proposed his version of the periodic table in which the elements were arranged in order of increasing atomic mass and grouped according to their chemical reactivity.

Mendeleev’s Periodic Table An insightful feature of Mendeleev’s periodic table was that he left gaps for elements that he believed should exist because the elements on either side of the gap matched the expected chemical properties of their groups.

Mendeleev’s Periodic Table

The Modern Periodic Table There are many different ways of presenting the periodic table and there are several different ways of labeling various sections of the table.

The Modern Periodic Table Some of the common conventions include: ▫The elements are arranged in order of increasing atomic number. ▫There is a division between metals and non- metals. ▫The vertical columns are called groups. ▫Some of the groups have accepted names. ▫The horizontal rows are called periods.

The Modern Periodic Table

▫The long metal periods are labeled transition metals, lanthanides and actinides. ▫There is difficulty placing hydrogen in the table. It is placed at the top of group 1 because it has some similarities to other group 1 elements, but it is a non-metal. ▫Unlike other noble gases, helium has only 2 electrons in its valence shell, but as this is shell 1, this constitutes a full valence shell.

The Modern Periodic Table Groups 3 to 6 contain both metal and non-metal elements. Seven elements (B, Si, Ge, As, Sb, Te, & Po) are called metalloids because they have characteristics of both metal and non-metal elements. Most of the elements are solid at room temperature and 1 atm pressure, except for the 11 gaseous elements (H, N, F, Cl & all group 0 elements) and the 2 liquid elements (Br and Hg).

The Modern Periodic Table The arrangement of the elements in the periodic table is linked to their electron configurations. ▫Elements with the same outer-shell electron configuration belong to the same group of the periodic table.

Atomic Electron Configuration

The Modern Periodic Table For example, the alkali metals (group 1) all have one electron in their outer shell. Lithium2, 1 Sodium2, 8, 1 Potassium2, 8, 8, 1 Rubidium2, 8, 18, 8, 1 The similarity in the number of out-shell electrons of each of these alkali metals leads to similarities in their physical properties and their chemical reactions.

Sodium Atom

The Modern Periodic Table The electron arrangement of an element can be predicted from the position of the element on the periodic table. ▫Elements in group 1 have 1 electron in their outer shell. ▫Group 2 elements have 2 electrons in their outer shell. ▫This trend continues across to group 7, and then group 0 with a full outer shell of 8 electrons.

The Modern Periodic Table The number of the outer shell can be read directly from the number of the horizontal period of the periodic table. ▫The elements sodium to chlorine are all in period 3, so their third electron shell is being filled. ▫Argon has 8 electrons in its third electron shell, which fills this shell.

The Modern Periodic Table This means that, given a periodic table, the electron configuration of any element up to Z=20 can be predicted and written. ▫Sulfur is in period 3 and group 6 of the periodic table.  It must therefore have 3 shells of electrons, with 6 electrons in the third shell.  Its electron arrangement is 2, 8, 6.