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Introduction to the Atom Understanding the basis for all matter in the known world. Development of the atomic theory 430 BC Democritus proposed the idea that matter is formed of pieces that are so small they could not be seen. Democritus was the first to describe these small pieces as “atomos” which is Greek for “uncuttable”. We today use a variation of this word called the “Atom”. Daltons Atomic theory John Dalton used evidence from a variety of experiments and inferred that atoms in nature had different characteristics. Dalton theory stated that atoms of the same element are the same, atoms of different elements have different masses, and that one atom can not be changed into another type of atom. The Thomson model of the atom Thomson figured out that atoms themselves had no electrical charge, so he figured that each atom must have both a positive and a negatively charged particle within them which allows the atom to remain neutral. The negative portion of an atom became known as the electron. Rutherford’s atomic model Rutherford figured out that not all portions of an atom weighed the same. The heaviest part of the atom was the nucleus. Rutherford’s work allowed us to understand how the atomic weight of an atom is always determined by the contents of the nucleus, and that electrons carry almost no mass or weight. The nucleus consists of protons and neutrons. Cloud model Established in the 1920’s Based on the understanding of how electrons do not orbit around the nucleus like planets around the sun. Electrons can be found in a cloud which surrounds the nucleus in a unpredictable manner. The electron cloud is a visual model. The electrons movement is related to the energy level of the electron itself. The modern atomic theory 1932 the neutron is discovered in the nucleus. The neutron has no electrical charge. With the discovery of the neutron and the understanding of the protons and electrons the atomic theory was established. electron neutron proton The Atomic Theory The atomic theory has five important points. 1) Elements are made of atoms. 2) Atoms of a single element are identical and have the same properties. 3) Atoms of different elements combine to form compounds. 4) Compounds contain atoms in given ratios 5) Atoms can combine in different ratios to form multiple atoms. Particles of an atom Atoms consist of three types of particles. Electrons, protons and neutrons. Protons and Neutrons are found in the nucleus. Electrons are found in the “shell” which surrounds the outside of the nucleus. Particle Symbol Charge Mass (amu) Proton P+ 1+ 1 Neutron N 1 1 Electron e- 1- 1/1836 What is the amu and the Atomic number? The amu is the measurement of weight of an atom. The amu is the atom mass units. The amu system of measurement assigns one (1) amu to both protons and neutrons. The atomic number represents the number of protons an atom contains in the nucleus. An example is oxygen. The atomic number of oxygen is eight. What are isotopes Isotope Protons Neutrons Mass Number Carbon 12 6 6 12 Carbon 13 6 7 13 Carbon 14 6 8 14 Isotopes are atoms of an element which have different number of neutrons which has an effect on the weight of the atom. Isotopes are identified by the mass number which is the number of protons and neutrons in the nucleus. An example is on the left which are all variations of the element carbon.