Atomic Structure
Chapter 4
Sizing up the Atom
O Radii of most atoms: 5 x 10-11 m to 2 x 10-10 m
O Copper penny contains 2.4 x 1022 atoms. The
population of Earth is about 6 x 109 people.
There is about 4 x 1012 times as many atoms
in a coin than people on Earth.
O Line up 100,000,000 copper atoms side by
side, the line would be only 1 cm long
Subatomic particles
O Proton: positively charged subatomic particle (p+)
O Neutron: neutral subatomic particle (n0)
O Electron: negatively charged subatomic particle (e-)
O Protons and neutrons are the same size, but
electrons are 1840 times smaller.
O *All atoms are neutral*
O Nucleus: tiny central core of an atom and is
composed of protons and neutrons with an overall
positive charge.
Nuclear Atom
O Protons and neutrons are located in the
nucleus (positive charge)
O Electrons are distributed around the nucleus
and occupy almost all the volume of the
atom (negative charge)
O The positive nucleus is balanced out with
equal number or electrons. Atoms are
always neutral.
Atomic Number
O Elements are different because they contain
O
O
O
O
O
different numbers of protons
Atomic number: number of protons in nucleus of
atom
All atoms are neutral
# protons = # electrons
Blue number in upper right hand corner for each
element on periodic table
Nuclear charge is the charge of the nucleus and
is dependent on the number of protons.
Atomic Mass
O The weighted average mass of atoms in a
O
O
O
O
naturally occurring sample of the element
Reflects both the mass and the relative
abundance of the isotopes as they occur in
nature
Isotope: atoms that have same # of protons but
different # of neutrons (different mass numbers)
Neon-20, Neon-21, Neon-22
http://education.jlab.org/itselemental/iso006.h
tml
Mass Number
O Most of atom’s mass is concentrated in
nucleus and depends on # protons and
neutrons
O Mass number: total number of protons and
neutrons in nucleus of atom
O # neutrons = mass # – atomic #
O To determine the mass # you round the
atomic mass to the nearest whole #
Atomic Mass Unit (AMU)
O 1/12 the mass of a carbon -12 atom
O More useful to compare the relative masses
of atoms using a reference isotope as a
standard.
O Reference isotope: Carbon-12
O Helium mass = 4.00260 amu and is 1/3
the mass of carbon-12
O Table 4.3
To calculate atomic mass:
O Multiply the mass of each isotope by its
natural abundance, expressed as a decimal,
and then add the products.