Chapter 4
Chemical Foundations:
Elements, Atoms,
and Ions
Elements
• Over 112 known: 88 found in nature, others
are man made.
• Each element has a unique one- or
two-letter symbol
Elements (cont.)
Elements (cont.)
Dalton’s Atomic Theory
• Elements are composed of tiny particles called atoms.
• All atoms of a given element are identical.
– All carbon atoms have the same chemical and physical
properties.
• Atoms of a given element are different from those
of any other element.
– Carbon atoms have different chemical and physical
properties than sulfur atoms.
Dalton’s Atomic Theory
• Atoms of different elements combine to form
compounds.
Dalton’s Atomic Theory
• Law of Constant Composition: all samples of a
compound contain the same proportions (by mass) of
the elements that form the compound.
• Atoms are indivisible by chemical processes.
– All atoms present at beginning are present at the end.
– Atoms are not created or destroyed, just rearranged in
chemical reactions.
– Atoms of one element cannot change into atoms of
another element.
• Cannot turn lead into gold by a chemical reaction
Formulas Describe Compounds
• A compound is a distinct substance that is
composed of atoms of two or more elements.
• Compounds are identified by the number and type
of each atom in the simplest unit of the compound.
– Molecules or ions
Molecular and Ionic Compounds
Writing Formulas of Compounds
• Each element is represented by its symbol.
• The number of each type of atom is indicated by a
subscript written to the right of the element
symbol.
• If only one atom is present, do not include a
subscript.
• If polyatomic groups are present in the molecule,
they are written inside parentheses if more than
one group is present.
• Examples: H2O, NaCl, KNO3, Mg(NO3)2
What Are Atoms Made Of?
• J.J. Thomson investigated a beam called a
cathode ray.
• He determined that the ray was made of tiny
negatively charged particles we now call electrons.
What Are Atoms Made Of?
The Electron
• Tiny, negatively charged particle
• Very light compared to mass of atom
– 1/1836th the mass of a H atom
• Move very rapidly within the atom
• Atoms of different elements all produce
electrons; therefore electrons are a
fundamental unit of atoms.
The Proton
• If negative electrons exist, must also
have positive particles to balance the
charge and make the atom electrically
neutral
• Assume “Plum Pudding Model” electrons suspended in a positively
charged electric field
Rutherford’s Gold Foil Expt
Rutherford’s Nuclear Model
• The atom contains a tiny dense center called
the nucleus.
– The volume is about 1/10 trillionth(萬億分之一)
the volume of the atom.
• The nucleus is essentially the entire mass
of the atom.
• The nucleus is positively charged.
– The amount of positive charge of the nucleus
balances the negative charge of the electrons.
• The electrons move around in the empty space
of the atom surrounding the nucleus.
Components of an Atom
The Modern Atom(原子)
• Atoms are composed of three main pieces:
protons(質子), neutrons(中子), and electrons(電
子).
• The nucleus(原子核) contains protons and
neutrons.
The Modern Atom (cont.)
Isotopes(同位素)
• All atoms of an element have the same
number of protons.
• The number of protons in an atom of a given
element is the same as the atomic
number(原子序數
原子序數).
原子序數
– Found on the periodic table(元素週期表)
• Atoms of an element with different numbers
of neutrons are called isotopes.
Isotopes (cont.)
• All isotopes of an element are chemically
identical.
– Undergo the exact same chemical reactions
• Isotopes of an element have different
masses(質量).
• Isotopes are identified by their mass numbers.
– Mass number = # protons + # neutrons
Isotopes of Carbon
•
12C
•
14C
has 6 protons (atomic number ) and 6
neutrons (atomic mass number = 6 + 6 = 12)
原子質量數
• 13C has 6 protons and 7 neutrons (atomic
mass number = 6 + 7 = 13)
has 6 protons and 8 neutrons (atomic
mass number = 6 + 8 = 14)
Elements (元素)
• Arranged in a pattern called the periodic
table
• Position on the table allows us to predict
properties of the element
Elements (cont.)
• Metals(金屬)
– About 75% of all the elements
– Lustrous(光澤), malleable(可塑性), ductile(韌
性), conduct heat and electricity
• Nonmetals(非金屬)
– Dull(無光澤), brittle, insulators
• Metalloids (類金屬)
– Also know as semi-metals
– Some properties of both metals & nonmetals
The Modern Periodic Table
• Elements with similar chemical and
physical properties are in the same column.
• Groups (Families族) and Periods(週期)
The Modern Periodic Table (cont.)
The Modern Periodic Table (cont.)
• Main group(主族)= representative elements(代表元
素)
– “A” columns
• Transition elements(過渡元素)
– All metals
• Bottom rows = inner transition elements = rare earth
elements(稀土元素)
– Metals
– Lanthanides(鑭系元素) and actinides (錒系元素)
Natural States of Elements
• Gases: monatomic gases such as noble
gases and diatomic gases such as O2
• Liquids such as Hg or Br2
• Solids such as metals or allotropes(同素
異形體) of carbon
Natural States of Elements (cont.)
Ions (離子)
• Cations(陽離子): ions that have a positive charge
– Form when an atom loses electrons
• Anions(陰離子): ions that have a negative charge
– Form when an atom gains electrons
• Ions with opposite charges attract
– Therefore cations and anions attract each other
Ions (cont.)
• Moving ions conduct electricity.
• Compound must have no total charge,
therefore we must balance the numbers of
cations and anions in a compound to get 0
total charge(總電荷).
Atomic Structures of Ions
• Metals form cations.
• For each positive charge the ion has 1 less
electron than the neutral atom.
– Na = 11 e-, Na+ = 10 e– Ca = 20 e-, Ca+2 = 18 e-
• Cations are named the same as the metal
sodium
Na → Na+ + 1e-
sodium ion
calcium
Ca → Ca+2 + 2e-
calcium ion
Atomic Structures of Ions (cont.)
• Nonmetals form anions.
• For each negative charge the ion has 1 more
electron than the neutral atom.
– F = 9 e-, F- = 10 e– P = 15 e-, P3- = 18 e-
• Anions are named by changing the ending
of the name to -ide
– Fluorine
– Oxygen
F + 1e- → FO + 2e- → O2-
fluoride ion
oxide ion
Ionic Charges and Compounds
• Charges on the ions of some elements can
be predicted from the periodic table.
• Cations and anions usually form
simultaneously to yield an ionic compound.
Ionic Charges and Compounds (cont.)
Formulas of Ionic Compounds
• The formula of an ionic compound can be
determined by balancing the positive charge
of the cation(s) with the negative charge of
the anion(s) to yield a net charge of zero.
Formation of Ionic Compounds