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Development of the atomic theory. What is an atom ? An atom is the smallest particle into which an element can be divided into and still be the same substance. Atoms make up elements; elements combine to form compounds. What is a theory ? The atomic theory has been around for over 2000 year. A theory is an explanation for a wide range of hypotheses and observations that have been supported by testing. Let us travel back in time. Ancient Greece. Greek philosopher named Democritus. Imagined cutting a silver coin. Proposed that you would eventually end up with an uncuttable particle. Democritus proposes the atom. He called that particle an atom From the Greek word atomos :meaning indivisible. He claimed that atoms are always moving, they form different materials by joining together. Atomic theory and Experiments Scientists had learned that elements combine in specific proportions to form compounds. John Dalton was a British chemist and a school teacher. He developed his atomic theory from observations gathered from many experiments. His theory stated : All elements are made of atoms which are small particles that cannot be divided or destroyed. Atoms of the same elements are exactly alike. Atoms of different elements are different. Atoms join with other atoms to make new compounds (substances). Where do we go from here? The atomic theory has changed as scientists have discovered new information about the atom. It was found that atoms can be divided into even smaller parts, called subparticles. Thomson ,a British scientist, improved Dalton’s theory by using a cathode-ray tube. Rutherford, another British scientist, investigated the structure of the atom by using the Gold foil experiment. Thomson’s cathode-ray Experiment There are two types of electric charges: Positive and Negative. Objects with opposite charges attract each other. Objects with the same charge push each other away. From his experiment he concluded that particles exist in every type of atom and that they are identical. The negatively charged particles found in all atoms are now called electrons . Rutherford’s Gold foil Experiment. The shooting Gallery: Rutherford’s Experiment. Rutherford proposed that in the center of the atom is a tiny, extremely dense, positively charged region called the nucleus . Most of the atom mass is concentrated here. The nucleus is positively charged. The nucleus consists of protons (Positively charged) and neutrons (No charge). Few models have been proposed to describe possible structures of an atom. What is a Model ? A model is a representation of an object or a system. A model presents a picture of what a theory explains. Model showing parts of an atom. The modern theory. According to the current theory, there are regions inside the atom where electrons are likely to be found. These regions are called electron clouds.