Elements and chemical bonds

• Atomic number

  The number of protons in an atom of the element
• Atomic mass

  The average mass of all of the different isotopes of the element
• Period


  • A row of elements
• Group


  • A column of elements
• Metals

  • The elements arranged on the left side of the periodic table, except H
• Nonmetals


  • The elements on the right side of the periodic table, plus hydrogen
• Metalloids


  • The elements arranged in a stair-step pattern between metals and nonmetals
• Compounds


  Composed of atoms of two or more elements held together by chemical bonds
• Electron energy level and action in an atom


  1. The closer to the nucleus, the lower the energy level

  2. The farther from the nucleus, the higher the energy level
• Drawings should show 6 "+" and 6 "n" in the center circle, 2 on the inner dashed line, and 4 on the outer dashed line
• Valence electrons


  Involved in chemical bonding
• Valence electrons are the same for all elements in a group (with the exception of helium)
• Constructing and interpreting an electron dot diagram

  1. Identify the group number

  2. Identify the number of valence electrons (the last digit)

  3. Place one dot at a time on each side of the element symbol

  4. Use up the dots to determine how many bonds an atom can form
• Noble gases


  All noble gases (except helium) have eight valence electrons, so all of their valence electrons are paired
• Behavior of atoms with unpaired valence electrons

  1. The atom is chemically unstable

  2. The atom must gain, lose, or share unpaired electrons

  3. The atom forms bonds with other atom(s)

  4. When an atom's unpaired valence electrons become paired, the atom becomes stable
• Atoms of most elements are unstable because they have unpaired valence electrons, so they have a strong tendency to bond with other atoms and form compounds with completed electron pairs
• Compounds are chemical combinations of elements
• Compounds usually have different properties than the elements they are made from
• Covalent bonds are formed by sharing valence electrons, not by physical properties
• Covalent bond


  A chemical bond formed when two nonmetal atoms share one or more pairs of valence electrons
• Types of covalent bonds

  • Single covalent bond (2 atoms share 1 pair of valence electrons)
  • Double covalent bond (2 atoms share 2 pairs of valence electrons)
  • Triple covalent bond (2 atoms share 3 pairs of valence electrons)
• Covalent compounds


  • Low melting point
  • Low boiling point
  • Poor conductor of electricity or thermal energy
  • Usually gas or liquid at room temperature
• Molecule


  To a compound as an atom is to an element
• Polar molecule


  Unequal sharing of electrons, resulting in a partial positive end and a partial negative end
• Water molecules are polar because the negative electrons are more strongly attracted to the oxygen atom, leaving a slightly positive charge near the hydrogen atoms</b>
• Polar molecules


  Electrons are shared unequally
• Nonpolar molecules


  Electrons are shared equally
• "Like dissolves like"


  Polar compounds can dissolve in other polar compounds, and nonpolar compounds can dissolve in other nonpolar compounds, but polar and nonpolar compounds do not dissolve in each other
• Chemical formula


  A group of symbols and numbers that represent the elements and the number of atoms of each element that compose a compound
• The chemical formula for a molecule of water indicates two hydrogen atoms and one oxygen atom in every water molecule
• Types of molecular models

  • Ball-and-stick model
  • Space-filling model
  • Structural formula
• There are many more covalent compounds than there are pure elements because there are several possible combinations of ways that valence electrons can be shared, and unstable atoms have a tendency to bond in order to become more stable
• Ion


  An atom that has gained or lost one or more electrons, resulting in a positive or negative charge
• Formation of sodium chloride (NaCl)

  1. Sodium (Na) is a metal with 1 valence electron, which it loses to become a positive ion

  2. Chlorine (Cl) is a nonmetal with 7 valence electrons, which it gains to become a negative ion

  3. The oppositely charged ions attract one another and form an ionic compound
• Ionic compounds


  • Ions are strongly attracted to each other
  • Good conductors of electricity when in water
  • Usually solid at room temperature
  • Have high melting and boiling points
• Metallic bond


  Forms when many metal atoms share their pooled valence electrons
• Metallic compounds


  • Have high melting and boiling points
  • Are good conductors of thermal energy and electricity
  • Can be hammered into sheets or pulled into wires
• An ion is stable if its highest energy level contains the maximum number of paired electrons, and an atom is electrically neutral if it has the same number of protons and electrons, with the positive and negative charges balancing