Chapter 19: Properties of Atoms and Periodic Table 

Section 1: Structure of the Atom

• Scientific Shorthand

  • Chemical symbols consist of one capital letter or a capital letter plus one or two lowercase letters.
  • Elements have been named in a variety of ways. Some elements are named to honor scientists, for places, or for their properties. Other elements are named using rules established by an international committee.

• Atomic Components

  • Atom: the smallest piece of matter that still retains the property of the element.
  • Atoms are composed of particles called protons, neutrons, and electrons
  • The nucleus of the atom contains protons and neutrons that are composed of quarks. The proton has a positive charge and the neutron has no charge. A cloud of negatively charged electrons surrounds the nucleus of the atom.
  • Nucleus: a small, positively-charged center of the atom
  • Protons: particles with an electrical charge of 1+
  • Neutrons: neutral particles that do not have an electrical charge.
  • Electrons: particles with an electrical charge of 1-.
  • Atoms of different elements differ in the number of protons they contain.

• Quarks: Even Smaller Particles

  • Scientists hypothesize that electrons are not composed of smaller particles and are one of the most basic types of particles.
  • Quarks: smaller particles that make up protons and neutrons
  • Scientists theorize that an arrangement of three quarks held together with the strong nuclear force produces a proton.
  • To study quarks, scientists accelerate charged particles to tremendous speeds and then force them to collide with—or smash into—protons.
  • Bubble chambers can be used by scientists to study the tracks left by subatomic particles.
  • Scientists found five quarks and hypothesized that a sixth quark existed. However, it took a team of nearly 450 scientists from around the world several years to find the sixth quark.

• Models - Tools for Scientists

  • Scientists and engineers use models to represent things that are difficult to visualize or picture in the mind.
  • Scaled-down models allow you to see either something too large to see all at once or something that has not been built yet.
  • To study the atom, scientists have developed scaled- up models that they can use to visualize how the atom is constructed.
  • John Dalton’s atomic model was a simple sphere.
  • Electron Cloud: the area around the nucleus of an atom where its electrons are most likely found.

Section 2: Masses of Atoms

• Atomic Mass

  • The nucleus contains most of the mass of an atom because protons and neutrons are far more massive than electrons.
  • An electron’s mass is so small that it is considered negligible when finding the mass of an atom.
  • The unit of measurement used for atomic particles is the atomic mass unit (amu).
  • The atomic mass unit is defined as one-twelfth the mass of a carbon atom containing six protons and six neutrons.
  • The number of protons tells you what type of atom you have and vice versa.
  • Atomic Number: The number of protons in an atom is equal to a number
  • Mass Number: the sum of the number of protons and the number of neutrons in the nucleus of the atom.'
  • number of neutrons = mass number - atomic number
  • Atoms of the same element with different numbers of neutrons can have different properties.

• Isotopes: Atoms of the same element that have different numbers of neutrons

  • Boron- 10 and boron-11 are two isotopes of boron. These two isotopes differ by one neutron.
  • Average Atomic Mass: the weighted-average mass of the mixture of its isotopes.

Section 3: The Periodic Table

• Organizing the Elements

  • Periodic means “repeated in a pattern.”
  • Mendeleev discovered that the elements have a periodic pattern in their chemical properties.
  • Chemical properties found in lighter elements could be shown to repeat in heavier elements.
  • Periodic Table: the elements are arranged by increasing atomic number and by changes in physical and chemical properties.
  • In 1913, the work of Henry G. J. Moseley, a young English scientist, led to the arrangement of elements based on their increasing atomic numbers instead of an arrangement based on atomic masses. This new arrangement seemed to correct the problems that had occurred in the old table. The current periodic table uses Moseley’s arrangement of the elements.

• The Atom and the Periodic Table

  • Objects often are sorted or classified according to the properties they have in common.
  • groups: families
  • Scientists have found that electrons within the electron cloud have different amounts of energy.
  • Energy levels in atoms can be represented by a flight of stairs. Each stair step away from the nucleus represents an increase in the amount of energy within the electrons.
  • Elements that are in the same group have the same number of electrons in their outer energy levels.
  • Energy levels are named using the numbers one to seven.
  • A complete and stable outer energy level will contain eight electrons.
  • In elements in periods three and higher, additional electrons can be added to inner energy levels, although the outer energy level contains only eight electrons.
  • One proton and one electron are added to each element as you go across a period in the periodic table.
  • The first row has hydrogen with one electron and helium with two electrons both in energy level one.
  • The second row begins with lithium, which has three electrons, two in energy level one and one in energy level two.
  • The row ends with argon, which has a full outer energy level of eight electrons.
  • Elements that are in the same group have the same number of electrons in their outer energy levels.
  • Electron Dot Diagram: uses the symbol of the element and dots to represent the electrons in the outer energy level.
  • Electron dot diagrams also are used to show how the electrons in the outer energy levels are bonded when elements combine to form compounds.
  • Electron dot diagrams show the electrons in an element’s outer energy level.
  • All halogens have seven electrons in their outer energy levels.
  • Not all elements will combine readily with other elements.

• Regions on the Periodic Table

  • Periods: The horizontal rows of elements on the periodic table
  • Metalloids are located along the green stair-step line. Metals are located to the left of the metalloids. Nonmetals are located to the right of the metalloids.

• Elements in the Universe

  • Using the technology that is available today, scientists are finding the same elements throughout the universe.
  • Atoms join together within stars to produce elements with atomic numbers greater than 1 or 2, the atomic numbers of hydrogen and helium.
  • Scientists think that some elements are found in nature only within stars.
  • Many scientists believe that supernovas have spread the elements that are found throughout the universe.

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