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STUDYING ATOMS Ancient Greeks Ancient Greeks believed that all matter consisted of extremely small particles that could not be divided. Atoms comes from the Greek word atomos, which means uncut or indivisible. Dalton’s Atomic Theory Dalton proposed the theory that all matter is made up of individual particles called atoms, which cannot be divided. Dalton’s Atomic Theory Main points: All elements are composed of atoms All atoms of the same element have the same mass Compounds contain atoms of more than one element Atoms of different elements always combine in the same way Dalton’s Atomic Theory Thomson’s Model Thomson used a cathode ray tube to discover electrons. It is called the plum pudding model. Protons had already been discovered by this time. Thomson’s Atomic Theory Rutherford’s Atomic Theory • Rutherford used the Gold Foil Experiment to discover the nucleus. • The nucleus is a dense, positively charged mass located in the center of the atom. • According to Rutherford’s model, all of an atom’s positive charge is concentrated in its nucleus. Rutherford’s Model STRUCTURE OF AN ATOM Structure of an Atom • Protons, electrons, and neutrons are subatomic particles. • Protons are positive and weigh 1 amu. • Neutrons are neutral and weigh 1 amu. • Electrons are negative and weigh 1/1836 amu. Structure of an Atom Protons and neutrons are large and in the nucleus of an atom. Electrons are small and are outside the nucleus. Atomic Number The atomic number of an element equals the number of protons and electrons in an atom of that element. Atoms of different elements have different numbers of protons. Mass Number The mass number of an atom is the sum of the protons and neutrons. To find neutrons: Mass number – Atomic number = # of neutrons Isotopes Isotopes of an element have the same atomic number but different mass numbers because they have different numbers of neutrons. EX: O-16 and O-17 MODERN ATOMIC THEORY Bohr’s Model Bohr focused on energy levels. Energy levels are the possible energies that electrons can have in an atom. An electron in an atom can move from one energy level to another when the atom gains or loses energy. Bohr’s Model Energy Levels, Orbitals, and Electrons Energy Level Max # of electrons 1 2 2 8 3 18 4 32 Electron Configurations The most stable electron configuration is the one in which the electrons are in orbitals with the lowest possible energies. The ground state is the lowest possible energy where the atom is most stable. Electron Cloud Model • An electron cloud is a visual model of the most likely locations for electrons in an atom. • The cloud is denser where the probability of finding an electron is high. • Scientists use the electron cloud model to describe the possible locations of electrons around the nucleus. Electron Cloud The electron cloud represents all the orbitals in an atom. An orbital is a region of space around the nucleus where an electron is likely to be found. Electron Cloud Model