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.