Periodic Table of the Elements Aim PT1 – How did the Periodic Table develop?

Why is the Periodic Table important to me? The periodic table is the most useful tool to a chemist. It organizes information about all the known elements It shows trends and patterns of elements in the table as well You get to use it on every test.

Pre-Periodic Table Chemistry … Imagine going to a grocery store with no organization!! There was initially no organization of the elements either It was difficult to relate elements to each other Chemistry and the elements didn’t make sense

Elements in the Periodic Table needed to be classified Classification –grouping elements based on similarities and differences Cross – classification –the classifying of elements into more than one category at a time –Examples Deck of cards – Suits and Numbers Periodic Table – Groups and Periods

Mendeleev's Periodic Table Dmitri Mendeleev (1869) prepared a card for each of the known elements which listed each element’s: –symbol –atomic mass –chemical properties –Reactivity of each High reactivity vs low reactivity vs no reactivity He arranged the cards in order of increasing atomic mass A pattern, emerged which developed into the Periodic Law

MENDELEEV'S PERIODIC LAW: When the elements are arranged in increasing order of atomic mass, the chemical properties repeat themselves periodically Each row or PERIOD was arranged in order of increasing atomic mass At the end of a row or PERIOD, the properties of the elements repeated themselves Example – whenever a total of eight electrons showed up in the valence shell, a Noble Gas (an unreactive element) was reached

MENDELEEV'S PERIODIC LAW: In each row or PERIOD –Each element in a period had the same number of energy levels –And the characteristics would repeat themselves in the next row or PERIOD Therefore, each vertical column was called a GROUP or FAMILY –As you go down a group –you find the same physical and chemical characteristics in each element of the GROUP –Example – He, Ne, Ar, Xe, all are Noble gases and are unreactive elements due to the Duet and Octet Rules

Example: is sodium more similar to potassium or magnesium? Sodium and magnesium are in the same period of the table –So they are both filling the same valence shell –Sodium’s electron configuration: 2 – 8 – 1 –magnesium’s configuration:2 – 8 – 2 –Both are filling the 3 rd shell of their atoms –But sodium is much more reactive in water than magnesium

But sodium and potassium are in the same group –Both have 1 electron in the valence shell –Both react violently in water –Both are soft metals The number of electrons in the valence shell determine the chemical properties and reactivity of each element Elements with the same number of valence electrons have similar properties

Moseley's Periodic Table (or the Modern Periodic Table) Henry Moseley noticed that when all the elements were arranged in order of mass a few were not in the right family with respect to properties Moseley used a procedure called X- ray diffraction to determine the atomic number of the elements When the elements were arranged in increasing order of atomic number, the discrepancies in Mendeleev's table disappeared

Moseley's Periodic Table (or the Modern Periodic Table) THE PERIODIC LAW: When the elements are arranged in increasing order of atomic number, the chemical properties repeat themselves periodically The modern Periodic Table is arranged in order of increasing atomic number Side note – Moseley died at the Battle of Gallipoli in WWI after volunteering for the Army. He would have won the Noble Prize in 1916 if he had not been killed. The British government never allowed prominent scientists or scholars to volunteer again for active service