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Atoms. Chapter 4. Section 1: “The Development of the Atomic Theory”. The atom is the smallest unit of all matter. Section 1: “The Development of the Atomic Theory”. Who: Democritis When: 4 th century B.C. What: Suggested that the universe was made of indivisible units.
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Atoms Chapter 4
Section 1: “The Development of the Atomic Theory” • The atom is the smallest unit of all matter.
Section 1: “The Development of the Atomic Theory” • Who: Democritis • When: 4th century B.C. • What: Suggested that the universe was made of indivisible units. • He called these units atoms. “Atom” comes from atomos, a Greek word that means “unable to be cut or divided.”
Section 1: “The Development of the Atomic Theory” • Who: John Dalton • When: 1803 • What: Created the Atomic Theory: • Every element is made of tiny, unique particles called atoms that cannot be subdivided. • Atoms of the same element are exactly alike. • Atoms of different elements can join to form molecules.
Section 1: “The Development of the Atomic Theory” • Who: J.J. Thomson • When: 1897 • What: Used a cathode-ray tube experiment to discover negatively charged subatomic particles called electrons.
Section 1: “The Development of the Atomic Theory” • What: Proposed a new model of an atom called the plum-pudding model.
Section 1: “The Development of the Atomic Theory” • Who: Ernest Rutherford • When: 1911 • What: Used the gold foil experiment to propose that most of the mass of the atom was concentrated at the atom’s center (the nucleus).
Section 1: “The Development of the Atomic Theory” • Rutherford’s model disclaimed Thomson’s plum-pudding model.
Section 1: “The Development of the Atomic Theory” • Who: Niels Bohr • When: 1922 • What: Developed Bohr Model of the atom.
Section 1: “The Development of the Atomic Theory” • Who: James Chadwick • When: 1932 • What: Discovered the neutron.
Section 1: “The Development of the Atomic Theory” • Quantum Mechanical Look at the Atom
Section 2: “The Structure of Atoms” • Subatomic Particles of an Atom
Section 2: “The Structure of Atoms” • Electric force from the attraction between the positive proton and negative electron holds the atom together.
Section 2: “The Structure of Atoms” • The overall charge electric charge of an atom is neutral or 0. This is because the number of protons in an atom is equal to the number of electrons. • This hydrogen atom has 1 proton and 1 electron (+1 + -1 = 0)
Section 2: “The Structure of Atoms” • Atomic number is the number of protons in the nucleus of an atom. Each element has a unique number of protons. • Mass number of an atom is the sum of the numbers of protons and neutrons in the nucleus of an atom. • Atomic massis the average weighted mass of an element ’s naturally occurring differences in the number of neutrons.
Section 2: “The Structure of Atoms” • Atomic Number (Z) = 6 • Mass Number (A) = 12
Section 2: “The Structure of Atoms” • Atomic Number (Z) = 1 • Atomic Mass (A) = 1.00794 amu • Mass Number (A) = 1 • # of protons = 1 (same as Z) • # of electrons = 1 (same as Z) • # of neutrons = 0 (# of n0 = A – Z)
Section 2: “The Structure of Atoms” • An isotope is an atom that has the same number of protons as other atoms of the same element do but that has a different number of neutrons (and a different atomic mass) • Because isotopes have the same number of protons and electrons, they generally have similar chemical properties.
Section 2: “The Structure of Atoms” • On the periodic table, the atomic mass displayed is an average of the atomic masses of all of that elements isotopes. • It is a weighted average so the mostly commonly found isotopes have the most impact on the mass.
Section 2: “The Structure of Atoms” • A mole is the SI base unit that describes the amount of a substance. • Avogadro’s constant is the number of particles in 1 mole of a substance and is equal to 602213670000000000000000 or 6.022 x 1023.
Section 2: “The Structure of Atoms” • The molar mass is the mass in grams of 1 mol of a substance. • This is equal to the atomic mass of atoms measured in grams rather than amu’s. • Example: The atomic mass of magnesium is equal to 24.30 amu. The molar mass of magnesium is equal to 24.30 g.
Section 3: “The Modern Atomic Theory • Electrons determine charge in an atom.
Section 3: “The Modern Atomic Theory • Energy levels are areas where electrons can exist around the nucleus of an atom.
Section 3: “The Modern Atomic Theory • The more energy an electron has the higher the energy level they can exist on. • Electrons move from one level to the next by gaining or losing energy.
Section 3: “The Modern Atomic Theory • Each energy level holds a specific maximum number of electrons: 1st = 2 2nd = 8 3rd = 18 4th = 32
Section 3: “The Modern Atomic Theory • Valence electrons are electrons that are found in the outermost shell of an atom and that determine the atom’s chemical properties and reactivity. • The octet rule says that once an atom has 8 valence electrons, it will no longer be reactive (with the exception of the first energy level which only needs 2).
Section 3: “The Modern Atomic Theory • An orbital is a region in an atom where there is a high probability of finding electrons. • Niels Bohr proposed that an atom’s electrons move around an orbital in a fixed path like planets around the sun. • Modern atomic theory says that rather than moving in fixed paths, electrons move like waves on a vibrating string where its exact location cannot be determined.
Section 3: “The Modern Atomic Theory • An “s” orbital is shaped like a sphere and can hold a maximum of 2 electrons. • Each “p” orbital is shaped like a bar bell. There are 3 different types that can each hold 2 electrons. The “p” orbital, therefore, can hold up to 6 electrons. • “d” and “f” orbitals are more complex. There are 5 types of “d” orbitals and 7 types of “f” orbitals each that hold 2 electrons each. Therefore, in total they hold 10 and 14 electrons respectively.
