Elements Molecules and Ions

Chapter 4 Elements Molecules and Ions

• All of the materials in the universe can be chemically broken down into about 100 different elements.

• Nine elements account for about 98% of the earth’s crust, oceans, and atmosphere.

• In the human body, oxygen, carbon, hydrogen, and nitrogen are the most abundant elements.

• Each element has a name and a symbol.

  • The symbol usually consists of the first one or two letters of the
    element’s name.

  • Sometimes the symbol is taken from the element’s original
    Latin or Greek name.

• The law of constant composition states that a given compound always contains the same proportion by mass of the elements of which it is composed.

• Dalton’s atomic theory states:

  • All elements are composed of atoms.

  • All atoms of a given element are identical.

  • Atoms of different elements are different.

  • Compounds consist of the atoms of different elements.

  • Atoms are not created or destroyed in a chemical reaction.

• A compound is represented by a chemical formula in which the number and type of atoms present are shown by using the element symbols and subscripts.

• Experiments by J. J. Thompson and Ernest Rutherford showed that atoms have internal structure.

  • The nucleus, which is at the center of the atom, contains protons (positively charged)
    and neutrons (uncharged).

  • Electrons move around the nucleus.

    • Electrons have a small mass (1/1836 of the proton mass).

    • Electrons have a negative charge equal and opposite to that of the proton.

• Isotopes are atoms with the same number of protons but a different number of neutrons.

• A particular isotope is represented by the symbol

• ZAX

• , in which Z represents the number of protons (atomic number) and A represents the total number of protons and neutrons (mass number) in the nucleus.

• The periodic table shows all of the known ele- ments in order of increasing atomic number; the table is organized to group elements with similar properties in vertical columns.

• Most elements have metallic properties (the metals) and appear on the left side of the periodic table.

• Nonmetals appear on the right side of the periodic table.

• Metalloids are elements that have some metallic and some non-
  metallic properties.

• Atoms can form ions (species with a charge) by gaining or losing electrons.

  • Metals tend to lose one or more electrons to form positive ions called cations; these are generally named by using the name of the parent atom.

  • Nonmetals tend to gain one or more electrons to form negative ions called anions; these are named by using the root of the atom name followed by the suffix -ide

• The ion that a particular atom will form can be predicted from the atom’s position on the periodic table.

  • Elements in Group 1 and 2 form 1+ and 2+ ions, respectively.

  • Group 7 atoms form anions with 1- charges.

  • Group 6 atoms form anions with 2- charges.

• Ions combine to form compounds. Compounds are electrically neutral, so the sum of the charges on the anions and cations in the compound must equal zero.