Atomic Structure Defining the Atom Defining the Atom Essential Question How did the concept of the atom move from philosophy to science? Defining the Atom • An atom is the smallest particle of an element that retains its identity in a chemical reaction. Philosophy or Science? • Democritus believed that matter consisted of tiny, indivisible, unchangeable particles called atoms. • Until recent centuries the existence of the atom was a philosophical question rather than a scientific one. Philosophy to Science • Greater accuracy in measurement allowed for experiments which indicated the existence of atoms. • Law of Conservation of Mass, Law of Definite Proportions, and Law of Multiple Proportions were clues to the existence of atoms. Dalton’s Atomic Theory • 1) All elements are composed of tiny indivisible particles called atoms. • 2) Atoms of one element are identical. The atoms of one element are different from those of any other element. Dalton’s Theory (cont.) • 3) Atoms of different elements can physically mix together, or can chemically combine in simple wholenumber ratios to form compounds. • 4) Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of one element, however, are never changed into atoms of another element as a result of a chemical reaction. Sizing Up the Atom • A copper penny contains about 2.4 x 1022 atoms. • Earth’s population measures about 6 x 109 people. • There are about 4 x 1012 times as many atoms in a penny as people on the earth. Sizing Up the Atom • A line of 100,000,000 copper atoms side by side would be only 1 cm long. Sizing Up the Atom • Sizes of atoms are expressed in picometers (trillionths of a meter). • 1 cm is the same fractional part of 600 miles as 100 picometers is of 1 cm! Sizing Up the Atom • Dry air contains 0.002% Neon. • Yet there are about 5 x 1017 atoms of Neon in every breath you inhale! • That’s 500,000,000,000,000,000 atoms! Sizing Up the Atom • In Summary: • Atoms are itty-bitty. Atomic Structure Structure of the Nuclear Atom Structure of the Nuclear Atom Essential Question What are the three principle sub-atomic particles and what part do they play in the structure of the atom? The Structure of the Nuclear Atom • Atoms are known to be divisible. • Sub-atomic particles include neutrinos, quarks, baryons, hadrons, fermions, bosons, mesons, leptons, photons, gravitons, gluons, electrons, protons and neutrons. • Not to mention futons, croutons, Nissans and morons. The Structure of the Nuclear Atom We will be most concerned with protons, neutrons and electrons. Subatomic Particles The Electron • Discovered by Thomson • Negatively charged particles • Relative charge = 1– • Symbol = e– • 1 / 1840 the mass of a proton • Reside outside of the nucleus Subatomic Particles The Proton • Discovered by Goldstein • Positively charged particles • Relative charge = 1+ • Symbol = p+ • Mass of 1 atomic mass unit (amu) Subatomic Particles The Neutron • Discovered by Chadwick • Neutrally charged particles • Relative charge = 0 • Symbol = n0 • Mass of 1 atomic mass unit (amu) The Atomic Nucleus • Discovered by Rutherford (Gold Foil Experiment) • The tiny central core of an atom • Composed of protons and neutrons The Atomic Nucleus • The nucleus is highly compacted and extremely dense. • Most of the mass of atoms is in the nucleus. • Nuclei have a density of about 2 x 108 metric tons / cm3 ! Atomic Structure Distinguishing Among Atoms Distinguishing Among Atoms Essential Question What part do protons, neutrons and electrons play in the identity and characteristics of atoms? Atomic Number • The number of protons determines the identity of the element. • The number of protons is indicated by the atomic number. Mass Number • The mass number indicates the number of protons AND neutrons in the nucleus. • How do you find the number of neutrons from the mass number of a given element? Isotopes • Isotopes are atoms of that have the same number of protons (the same element) but have different numbers of neutrons. • They, therefore, have different mass numbers and different masses. Atomic Mass • Masses of atoms are incredibly small and impractical to express in grams • It is more practical to compare the masses of atoms to a different standard. Atomic Mass Unit • An AMU (atomic mass unit) is defined as 1/12 the mass of a Carbon-12 atom. • Since the Carbon-12 atom is made up of 6 protons and 6 neutrons, an AMU is approximately equal to the mass of one proton or one neutron. Atomic Mass • An element’s atomic mass is a weighted average mass of all the atoms in a naturally occurring sample of the element. • To calculate atomic mass, multiply the mass of each isotope by its percent abundance (expressed as a decimal) and add the products.