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Introduction to Chemistry - Review Chemistry is the study of matter. Matter is anything that takes up space and has mass. All matter is made up of small particles called atoms. All known types of atoms are found on the periodic table of the elements. Atom and element are often used synonymously. Atoms - Review The atom is a basic unit of matter. Atoms are made of protons, electrons and neutrons. Atoms of different elements have different numbers of protons, neutrons and electrons. How Can Elements Be Organized? Scientists started trying to organize the known elements in the early 1800’s. Could the elements be organized based on properties like colour, smell or taste? Not really, because the characteristics or properties were not unique. Early scientists found a property unique to each element, atomic mass. Dimitri Mendeleev Dmitri Mendeleev was the first scientist to organize the elements by unique, repeatable patterns. He arranged the 64 elements known in his time in order of increasing atomic mass. When an element or group of elements seemed to repeat other properties he had seen before, he started a new row. Periodic Table Columns Eventually, Mendeleev found that elements with similar properties fit into the same vertical columns. A vertical column is a group or family. There are 18 groups in the modern periodic table. Periodic Table Rows Each horizontal row is called a period. All of the elements in a row belong to the same period. There are seven periods in the modern periodic table. Mendeleev’s Periodic Table Modern Periodic Law Today the elements are organized by their atomic number, symbol Z. The atomic number is the unique number of protons inside the nucleus. Modern periodic law states: –If the elements are arranged according to their atomic number, a pattern can be seen in which similar properties occur regularly. Can you find one place in the periodic table where the atomic mass does not increase from left to right? Hint: Look near the end of the fifth period.
