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 Subatomic Particle Location Charge Mass neutron(no) nucleus 1.67x10-27 kg (1 amu) proton(p+) +1 electron(e-) electron cloud surrounding nucleus 1 9.11 x10-31 kg

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 mass is 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) = # of protons = 1 (same as Z) # of electrons = # 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. Avagadro’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

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.

Section 3: “The Modern Atomic Theory

Section 3: “The Modern Atomic Theory