Elements and the Periodic Table

Elements

Materials on earth are either compounds or pure substances.   Compounds are mixtures of elements chemically combined.    Elements are substances that cannot be broken down any further.   The number of protons in an element is called the atomic number.   The mass number is the total number of electrons and neutrons in an atom, generally twice the atomic number.

History of Elements

Ancient Greeks Defined elements as basic substances from which other materials are made Earth air water and fire   Boyle Defined elements as substances that cannot be broken down further Defined compounds as combinations of such elements   Davy Powerful electrochemical techniques for breaking down substances Isolated potassium, sodium, barium, calcium, and magnesium, through electrochemical techniques   Mosely Found using Xrays that the nucleus of an atom had a positive charge Called this the atomic number

The Periodic Table

Vertical columns are called groups   Horizontal rows are called periods   Above the symbol is the atomic number, below is the atomic mass

History of the Periodic Table

Döberiener Invented the group of triads, or three elements with the same chemical properties Examples are Lithium, and the two elements below it, and calcium, and the three elements below that.   Newlands Invented the law of octaves Properties repeated themselves every eighth element  worked for the first 17 but didn't after that   Mendeleev Made a thorough study of the relationship between atomic weight and the properties of elements There was a periodic reoccurrence of properties when arranged by atomic weight First two rows contained 7 elements, thereafter contained 17 Did not hesitate to rearrange elements to fit chemical properties Left gaps in the periodic table gallium, scandium, and germanium Mosely When he rearranged elements by atomic number, solved the anomaly between iodine and tellurium

Periodic Groups

Alkali metals Physical Properties soft  low densities Chemical Properties metallic shine until reacted with water react with water Alkaline Earth Metals Physical Properties Harder than the prior group Chemical Properties  Less reactive than the prior group Halogens Physical Properties Low melting and boiling points Chemical Properties Become acidic in water react with sodium Noble Gases Physical All gases at SLC Chemical Least reactive of the elements