Prentice Hall Physical Science
Chapter 4 Atomic Structure
4.1 Studying Atoms
A.
Ancient Greek Models of Atoms
Democritus believed all matter was made of extremely small particles that could not be divided
he called the particles atoms (from the Greek atomos meaning indivisible)
he thought there were different types of atoms with different properties (liquids had round, smooth atoms
and solids were rough and prickly)
B.
Dalton’s Atomic Theory
he proved atoms exist by discovering that compounds have a fixed composition
Dalton’s Theory:
all elements are composed of atoms
all atoms of the same element have the same mass, and atoms of different elements have different
masses
compounds contain atoms of more than one element
in a particular compound, atoms of different element always combine in the same way
he pictured atoms as solid spheres
in time it was found that not all of this was completely correct
C.
Thomson’s Model of the Atom
some objects have a positive or negative charge and objects with the same charge repel and objects with
opposite charges attract
Thomson used an electric current to learn more about atoms
he passed an electric current through a gas and a glowing beam appeared
the beam was repelled by a negatively charged metal plate (like charges repel and opposite charges attract)
Thomson concluded that the beam most be made of negatively charged particles, smaller than atoms.
since the atom was known to be electrically neutral, he developed a model in which negative charges were
evenly scattered throughout an atom filled with a positively charged pudding like material
called the “Plum Pudding” model (or chocolate chip cookie dough)
D.
Rutherford’s Atomic Theory
Rutherford discovered positively charged, fast moving particles called alpha particles
he shot some of these particles a piece of gold foil, thinking they would go straight through
when he actually performed the experiment, he found that the positively charged alpha particles were
greatly deflected by the gold (figure 7, p. 104)
he determined that the alpha particles must have come very close to another positively charged particle
causing them to be repelled and put off course
this led him to believe that the positive charge in an atom is not evenly distributed, but concentrated in a
very small central area he called the nucleus
his model of the atom is a small, positively charged nucleus surrounded by negatively charged particle
Prentice Hall Physical Science
Chapter 4 Atomic Structure
4.2
The Structure of the Atom
A.
Properties of Subatomic Particles
Protons (p+)
positively charged (1+)
found in the nucleus
mass of 1 amu (atomic mass unit)
Neutrons (no)
no charge (neutrally charged or neutral)
found in the nucleus
mass of 1 amu
Electrons (e-)
negatively charged (1-)
found in the electron cloud outside the nucleus
mass of 1/1836 amu
B.
Atomic Number, Mass Number, and Atomic Mass
Atomic Number
it is the number of protons in an atom
it is the WHOLE number in each square of the periodic table
all atoms of the same element have the same atomic number
since atoms are electrically neutral, the number of protons (positive charges) they have is equal to
the number of electrons (negative charges) they have
it is the number of protons in an atom that determine what element the atom is
Mass Number
it is the number of protons PLUS the number of neutrons in an atom
mass number = #protons + #neutrons
#neutrons = mass number - #protons
NOT on the periodic table
Atomic Mass
because protons are so small, it is hard to find their mass using traditional units, so scientists
developed the atomic mass unit (amu) to describe the mass of a proton
1 amu = 1/12 the mass of a carbon-12 atom
every atoms of one element does NOT have the same number of neutrons so every atom in one
element does not have the same mass
atomic mass is the weighted average of all of the possible atoms in an element
example:
chlorine comes in two forms: chlorine-35 and chlorine-37
75% is chlorine-35, 25% is chlorine-37
its atomic mass is then 25% (37) + 75% (35) = 35.5 amu
C.
Isotopes
atoms of the same element with the same number of protons, but different numbers of neutrons and
different mass numbers
example:
oxygen-16, oxygen-17, and oxygen-18
Prentice Hall Physical Science
Chapter 4 Atomic Structure
4.3 Modern Atomic Theory
A.
Bohr’s Model of the Atom
also called the planetary model
it focused on the arrangement of the electrons in the atom
Energy Levels
Bohr said that electrons move with constant speed in fixed orbits (energy levels) around the
nucleus
each electron in an atom has a specific amount of energy which is equal to the energy of one of the
energy levels in the atom
an electron must be in an energy level, it cannot be between
energy levels closest to the nucleus have the lowest amount of energy and the ones frthest away
has the most energy
if the atom loses or gains energy, the electrons lose or gain energy too and must, therefore, change
energy levels to match their energy
if the electron gains energy, the electron moves up to a higher E level
if the electron loses energy, the electron falls down to a lower E level
the amount of E gained (absorbed) or released can be measured
when E is released, the E is given off as light
since no two elements have the exact same E levels, each element has its own
unique color of light and this can be used to identify the element
electrons want to be in the lowest E state possible
B.
Electron Cloud Model
scientists now know that electrons don’t move like planets; they are MUCH less predictable
in this model, scientists state only where electrons probably are
the electron cloud represents where the electrons probably are outside the nucleus; the cloud is denser
where the probability of finding an electron is greater
example: fan blades spinning
C.
Atomic Orbitals
the electron cloud contains orbitals which are the regions around the nucleus where an electron is likely to
be found
each orbital can only hold 2 electrons
the lowest E level contains only 1 orbital, and can hold only 2 electrons
E level 2 = 4 orbitals and 8 electrons
E level 3 = 9 orbitals, 18 electrons
E level 4 = 16 orbitals, 32 electrons
D.
Electron Configurations
the arrangement of electrons in the orbitals of an atom
the most stable is the one with the electrons in the lowest possible E levels
this is the ground state