3.2 Organizing the Elements Vocabulary: Atomic mass – Periodic table – Chemical symbol – Period – Group – Dmitri Mendeleev, a Russian chemist created the first version of the periodic table in 1869. By arranging the 63 known elements according to their atomic masses, he predicted that new elements would be discovered. Within 17 years, chemists had discovered these missing elements. Mendeleev knew that some elements had similar chemical and physical properties. He wrote each element’s melting point, density, atomic mass, and color on cards. The atomic mass of an element is the average mass of all the isotopes of that element. He tried arranging them in different ways. Mendeleev noticed that a pattern of properties appeared when he arranged the elements in order of increasing atomic mass. He found that some properties repeated regularly. For example, lithium, sodium, and potassium all react violently in water. Mendeleev lined up the cards for these elements to form their own group. He did the same for other elements that shared similar properties. A periodic table is an arrangement of element showing the repeating pattern of their properties. As scientists discovered new elements and learned more about atomic structure, the period table changed. It is now known that the number of protons in the nucleus, given by the atomic number, determines the chemical properties of an element. Modern periodic tables are arranged in order of increasing atomic number. Information contained in the Periodic Table 1. Atomic number - The first piece of information is 2. Chemical symbol – the the number 19, the atomic letter K is the symbol for number of potassium. Potassium. Chemical Every potassium atom has symbols contain one or 19 protons in its nucleus. two letters. Often it is an abbreviation of the element’s name in English. Other elements have 3. Atomic mass – For abbreviations of their Potassium, that value is 39.098 Latin names. amu. The atomic mass is an average because most elements consist of a mixture of isotopes. An element’s properties can be predicted from its location in the periodic table. This predictability is the reason that the periodic table is so useful to chemists. Periods – the periodic table is arranged in ROWS called periods. Metals are shown on the left of the table, and nonmetals are located on the right. Metalloids are found between the metals and nonmetals. The pattern is repeated in each period. The lanthanides and actinides are placed off the table to save space. Groups – the modern periodic table has 7 periods, which form 18 columns. The elements in a column form a group. Groups are also known as families. Elements in each group have similar characteristics. For example, all elements in Group 1 (except hydrogen) are all metals that react violently with water. Group 17 elements are also very reactive, but Group 18 is generally nonreactive. https://www.youtube.com/watch?v=0RRVV4Diomg .