Chapter 4: Atomic Structure
GA Standards
SCSh1. Students will evaluate the importance of curiosity, honesty, openness, and skepticism in science.
SCSh1.b. Recognize that different explanations often can be given for the same evidence.
SCSh1.c. Explain that further understanding of scientific problems relies on the design and execution of new
experiments which may reinforce or weaken opposing explanations.
SCSh7. Students will analyze how scientific knowledge is developed. Students recognize that:
SCSh7. c. From time to time, major shifts occur in the scientific view of how the world works. More often, however,
the changes that take place in the body of scientific knowledge are small modifications of prior knowledge.
Major shifts in scientific views typically occur after the observation of a new phenomenon or an insightful
interpretation of existing data by an individual or research group.
SCSh7. d. Hypotheses often cause scientists to develop new experiments that produce additional data.
SCSh7. e. Testing, revising, and occasionally rejecting new and old theories never ends.
SC3. Students will use the modern atomic theory to explain the characteristics of atoms.
SC3. a. Discriminate between the relative size, charge, and position of protons, neutrons, and electrons in the atom.
SC3. c. Explain the relationship of the proton number to the element’s identity.
SC3. d. Explain the relationship of isotopes to the relative abundance of atoms of a particular element.
4.1 defining the Atom
A. Early Models of the atom
 Atom is the smallest particle of an element that retains its identity in a chemical reaction
1. Democritus’s Atomic Philosophy
o Greek philosopher Democritus was first to suggest the existence of atoms
o Democritus believed that atoms were indivisible and indestructible
o His ideas did not explain chemical behavior and lacked experimental support.
2. Dalton’s Atomic Theory
o By using experimental method, Dalton transformed Democritus’s ideas on atoms into a
scientific theory
o Dalton’s atomic theory
1) All elements are composed of tiny indivisible particles called atoms
2) Atoms of the same element are identical. Atoms of anyone element are different from those
of any other element
3) Atoms of different elements can physically mix together or can chemically combine in
simple whole number ratio to form compounds.
4) Chemical reactions occur when atoms are separated, joined or rearranged. Atoms of one
element, however, are never changed into atoms of another element as a result of a chemical
reaction.
B. Sizing up the atom
 A copper coin the size of a penny contains about 2.4 x 1023 atoms
 Most atoms have a radius between 5 x 10-11 m to 2 x 10-10 m
 Despite their small size individual atoms are observable with instruments such as scanning tunneling
microscopes.
4.2 Structure of the Nuclear Atom
A. Subatomic Particles
 Three kinds of subatomic particles are electrons, protons, and neutrons.
1. Electrons
o 1897 J.J. Thomson discover electrons
o Electrons are negatively charged subatomic particles
o He passes electric current trough asses at low pressure
o
Cathode ray a stream of electrons produced at the negative electrode (cathode) of a tube containing a
gas at low pressure
o
o A cathode ray is deflected by electrically charged metal plates or magnets
2. Protons and Neutrons
o Atoms have not net electric charge, they are electrically neutral
o Electric charges are carried by particles of mater
o Electric charges always exist in whole-number multiples of a single basic units
o When a given number of negatively charged particles combines with an equal number of positively
charged particles
o Protons are positively charged subatomic particles
o Neutrons are subatomic particles with not charge but with a mass nearly equal to that of a proton
o
B. The Atomic Nucleus
 When subatomic particles were discovered, scientists wondered how these particle were put together in an
atom
 J.J Thompson suggested that the electron were evenly distributed throughout an atom filled uniformly with
positively charged material
 J.J. Thompson’s model was known as the ‘plum-pudding model”
1. Rutherford’s gold-foil Experiment
o 1911 Rutherford decided to test the current theory of atomic structure
o He shot alpha particles (positive particle) at thin sheet of gold
o The majority of the particles passed straight through
o A small fraction bounced off the gold foil at very large angels
2. The Rutherford Atomic Model
o Based on the experimental results Rutherford suggested a new theory of the atom
o He suggested that the atom is mostly empty space (explaining the lack of deflection of most alpha
particles)
o Also that all the positive charge and mass is concentrated in a small region
o Nucleus is the tiny central core of an atom and is composed of protons and neutrons.
o In the nuclear atom, the protons and neutrons are located in the nucleus. The electrons are
distributed around the nucleus and occupy almost all the volume of the atom
4.3 distinguishing Among Atoms
A. Atomic Number
 Elements are different because they contain different numbers of protons
 Atomic number of an element is the number of protons in the nucleus of an atoms in their element
 Remember that atoms are electrically neutral
 Because of that the number of electrons must equal the number of protons
B. Mass Number
 Mass number is the total number of protons and neutrons in an atom.
 If you know the mass number and the atomic number of any atom you can determine the number of neutrons
in the atom
 The number of neutrons in an atoms is the difference between the mass number and the atomic
number
 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑛𝑒𝑢𝑡𝑟𝑜𝑛𝑠 = 𝑚𝑎𝑠𝑠 𝑛𝑢𝑚𝑏𝑒𝑟 − 𝑎𝑡𝑜𝑚𝑖𝑐 𝑛𝑢𝑚𝑏𝑒𝑟
𝑚𝑎𝑠𝑠 #
197
 Short hand notation: 𝑎𝑡𝑜𝑚𝑖𝑐
#𝑎𝑡𝑜𝑚𝑖𝑐 𝑠𝑦𝑚𝑏𝑜𝑙 , 79𝐴𝑢 or gold-197
C. Isotopes
 Isotopes are atoms that have the same number of protons but different number of neutrons
 Because isotopes of an element have different numbers of neutrons, they also have different mass
numbers.
 Isotopes are chemically alike because they have identical numbers of protons and electrons (which are
responsible for chemical behavior)
D. Atomic Mass
 Atomic mass unit (amu) is defined as one twelfth of one mass of carbon-12 atom
 the mass of a single proton or a single neutrons is about one-twelfth of 12 am or about 1 amu
 in nature most elements occur as a mixture of two or more isotopes
 atomic mass of an element is a weighted 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 resulting sum is the weighted average mass
E. The Periodic table – a preview
 Periodic table is an arrangement of elements in which the elements are separate din to groups based on a set
of repeating properties
 A periodic table allows you to easily compare the properties of one element (or group of elements) to
another element (or group of elements)
 Period is the horizontal row of the periodic table
 Group (or family) is the vertical column of the periodic table
