 Some elements exhibit similar chemical and physical properties. For example, lithium (Li), sodium (Na), and Potassium (K) can all combine in a 1:1 ratio easily with Chlorine.

 The elements chlorine, bromine, and iodine look very different from each other. But each forms a similar-looking while solid when it reacts with sodium.

 In 1865, an English chemist, John Newlands, arranged the known elements according to their properties and in order of increasing atomic mass.  He noticed that all of the elements in a given row had similar properties, and they repeated every 8 elements (he called this the law of octaves).

 In Newland’s time, determining atomic weights was based on comparing other elements to the lightest element (hydrogen). Some of the elements were given inaccurate values..  Newland was ridiculed by other chemists who felt the table he created was not reliable. He could not get his papers published and returned as chief chemist in a sugar factory and later opened a chemical business with his brother.

 In 1869, a Russian chemist, Dmitri Mendeleev produced the first orderly arrangement (periodic table) of all 63 elements known at the time.

 Mendeleev ( ) rose from very poor beginnings to a position of a renowned Russian chemist in the 19th century. He wrote down information on each element on cards. He ranked the elements from lightest to heaviest.  Mendeleev also put the elements into a table according to their properties. He started a new row each time he noticed that the chemical properties of the elements repeated.

 Mendeleev’s Table contains gaps that elements with particular properties should fill.  He correctly predicted the properties of the missing elements. He even gave them provisional names. These elements were eventually discovered.

 A young English chemist, Henry Moseley, discovered that the elements should be organized according to their atomic numbers, not their atomic weights as was done before.  When Moseley studied the lines in the X-ray spectra of 38 different elements, he found that the wavelength of the lines decreased in a regular manner as atomic number increased.

 Henry Moseley lost his life in 1915 during World War I at the Gallipoli battle in Turkey at the age of 27 and is buried there. His death has been called one of the greatest tragedies of WWI because he was such a brilliant chemist.

 The Periodic Law states that when the elements are arranged according to their atomic numbers, elements with similar properties appear at regular intervals.

 Elements in each column of the table have the same number of electrons in their outer energy level.  These electrons are called valence electrons.

 The outer energy level electrons in an atom are the ones that participate in chemical reactions with other atoms.  Elements with the same number of valence electrons react in similar ways.

 A vertical column on the periodic table is called a group. (These are also called families.) These exhibit similar properties.

 The group an element belongs to is determined by the last notation in its’ electron configuration.  For example, all elements that end in s 1 have the same properties and are in the same group.

 A horizontal row on the periodic table is called a period. Elements in the same period have the same number of occupied energy levels.

 The Periodic Table is organized into blocks— each corresponding to the sublevels. (S, P, D, and F)