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ATOMIC STRUCTURE A History of Atomic Theory Democritus • Around 400 BC, the Greek philosopher Democritus proposed that matter was made of tiny particles that he called atoms • His idea was ignored for 2000 years Dalton • In 1808, John Dalton once again proposed the idea of atoms • This time, the idea was taken seriously by scientists Dalton’s Model of the Atom • Dalton pictured an atom as a solid sphere • He proposed that atoms of an element were alike, and different elements had atoms with different masses Thomson • In 1898, J.J. Thomson discovered the existence of negatively charged particles, which he called electrons Thomson’s Model of the Atom • In this model, negatively charged electrons are embedded in a positively charged sphere • Electrons are able to leave the atom • Thomson’s atomic model became known as the “Plum Pudding” model Rutherford • In his famous1911 experiment, Ernest Rutherford shot a stream of positively charged alpha particles at a very thin sheet of gold foil Here’s what he observed • Most of the particles went right through the foil • A few were deflected or bounced back • What does this tell you about the structure of the gold foil? Rutherford’s Atomic Model • Rutherford’s new model of the atom consisted of a positively charged nucleus surrounded by orbiting electrons and a lot of empty space Bohr • In 1913 Neils Bohr studied the spectra of elements and decided that the electrons must be located in different levels or orbitals Bohr model • Bohr’s new “solar system” model of the atom showed electrons in orbitals or energy levels (also called electron shells) • His model is still used today Atomic Theory Today • An atom is the smallest particle of an element • Atoms are made of smaller subatomic particles • Atoms of different elements contain different numbers of subatomic particles Subatomic particles Atoms consist of three main types of subatomic particles: • Protons (+ charge) • Neutrons (no charge) • Electrons (- charge) • Protons and neutrons are found in the nucleus • Electrons are found outside the nucleus The Nucleus • The nucleus of an atom contains protons and neutrons • These are held together by powerful forces and cannot easily escape Atomic Number • The atomic number of an element is the number of protons in an atom of the element • Every atom of the same element has the same number of protons Mass number • Most of the mass of an atom is found in the nucleus • The mass number of an element = the number of protons + neutrons in an atom Isotopes • Atoms of the same element can have different numbers of neutrons • These different forms are called isotopes Atomic mass • The atomic mass (shown on your periodic table) is the average mass number of atoms of the element • Round this number to the nearest whole number, to find the most common mass number for that element Calculating the number of subatomic particles in an atom: • Number of protons = atomic number • Number of neutrons = mass number atomic number • Number of electrons = number of protons Electrons • Electrons are found in the area surrounding the nucleus • Electrons have very little mass • Electrons are found in “shells” or energy levels • Electrons can move out of their positions Electron shells • Each electron shell has room for a specific number of electrons • 1st electron shell: 2 • 2nd electron shell: 8 • 3rd electron shell: 8 Bohr models • Show number of protons and neutrons in the nucleus • Draw in electron orbitals • Fill from the inside out (28-8-…) Magnesium New Findings in Atomic Theory • The study of quantum physics has led to many new discoveries and theories about the structure of atoms • Protons and neutrons are made of even smaller particles called quarks • Electrons shells are not really circular orbits, but in a series of orbitals with different shapes • The exact location of an electron can’t actually be determined - just the probability of finding in a certain place and time • Particle accelerators are used to do research on atomic structure