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Chapter 4
Atomic Structure
4.1.) Early Models of the Atom
Atom – the smallest particle of an element that retains its identity
in a chemical reaction
- Democritus, Greek philosopher, believed that atoms were
indivisible and indestructible
o No experimental support
- Robert Dalton’s theory, using experimental methods,
proposed:
o All elements are composed of tiny indivisible particles
called atoms
o Atoms of the same element are identical
o Atoms of different elements can physically mix or
chemically combine in simple whole-number ratios to
form compounds
o Chemical reactions occur when atoms are separated,
joined, or rearranged, but they are never changed into
another element
- The radii of most atoms is between 5 x 10-11 m to 2 x 10-10 m
o Individual atoms can be observed with instruments
like a scanning tunnel microscope
4.2.) Structure of the Nuclear Atom
- Atoms are divisible into three kinds of fundamental
particles called electrons, protons, and neutrons
- J.J. Thomson discovered the electron, a negatively charged
subatomic particle, by deflecting a cathode ray through
magnets or electrically charged plates
o Thomson knew that opposite charges attract and like
charges repel, so he sent a stream of negatively
charged particles (originally called corpuscles) at high
speed. The particles were deflected toward the positive
plate
- Robert Millikan found the charge on an electron to be one
negative unit. Its mass is 1/1840th the mass of a hydrogen
atom
- Eugene Goldstein named the positively charged subatomic
particles protons, with a mass about 1840 times that of an
electron
- James Chadwick confirmed the existence of the neutron as
a particle with no charge but a mass slightly larger than a
proton’s
The Atomic Nucleus
- Thomson’s atomic model, the “plum-pudding model,” had
the electrons stuck into a lump of positive charge
- Ernest Rutherford tested atomic structure theory by
directing a narrow beam of alpha particles (helium atoms
that have lost their electrons) at a thin sheet of gold foil.
The particles should have been deflected but most were
not.
o Rutherford proposed that the atom is mostly empty
space with most of the mass concentrated in the
nucleus
- Known as the nuclear model
4.3.) Distinguishing Among Atoms
Elements are different because they contain different numbers of
protons
- Atomic Number: the number of protons in the nucleus of
that element
o In electrically neutral atoms, the number of protons
equals the number of electrons
- Mass number is the total number of protons and neutrons
in an atom
o The number of neutrons in an atom is the difference
between the mass number and the atomic number
o Shorthand notation uses the chemical symbol of the
element with the atomic number written as the
subscript and the mass number as the superscript
197
79
Au
o Atoms can also be referred to using the mass number
and the element name; i.e. gold-197
Isotopes- atoms that have the same number of protons but
different numbers of neutrons
- Because isotopes have different numbers of neutrons, they
also have different mass numbers
o Three known isotopes of hydrogen:
- Hydrogen (hydrogen-1) has one proton and no
neutrons
- Deuterium (hydrogen-2) has one proton and one
neutron
- Tritium (hydrogen-3) has one proton and two
neutrons
- Atomic Mass is determined by a mass spectrometer
o Using carbon-12 as the standard, an atomic mass unit
(amu) is defined as one twelfth of the mass of a
carbon-12 atom
o In nature, most elements occur as a mixture of two or
more isotopes
o Atomic mass is the average mass of the atoms in a
naturally occurring sample of the element
- To calculate the atomic mass of an element,
multiply the mass of each isotope by its natural
abundance, expressed as a decimal, and then add
the products
The Periodic Table – A Preview
- A periodic table is an arrangement of elements in which the
elements are separated into groups based on a set of
repeating properties
- A periodic table allow you to easily compare the properties
of one element (or group of elements) to another element
(or group of elements)
o Each horizontal row is called a period
- Seven periods with repeating patterns in each
period
- Each vertical column is called a group or family, having
similar chemical and physical properties