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Atomic Structure - Two Observable
Facts made in the 1800’s
• Law of Conservation of Mass
– Matter is neither created nor destroyed in a
chemical reaction
• Law of Constant Composition
– The elements present in a compound are
present in a fixed and exact proportion by mass
Dalton’s Atomic Theory
• From 1803 to 1808 an English schoolmaster
named John Dalton came up with a very
good Atomic Theory.
– All matter is composed of atoms (Dalton
thought they were indestructible)
• Atoms are the building blocks of matter
– Atoms of one element are identical
Dalton’s Atomic Theory
– Atoms of different elements can be
distinguished by their different masses
– Compounds are combinations of atoms of
different elements and possess properties
different from those of their component
– In chemical reactions, atoms are neither created
nor destroyed but only exchanged between
starting compounds to form new compounds
Questions: How is a Hydrogen
atom different from a Helium
• Is there something that makes them
• We need to look at their structure to
understand the difference
The Structure of Atoms
• Atoms aren’t the smallest particles of matter
• They are made up of three types of particles
– Proton - a positively charged particle
– Neutron - a neutral particle
– Electron - a negatively charged particle (much
lighter than a Proton or Neutron)
A picture of the Atom
• The nuclease is at the center of the atom and
contains the protons and neutrons - it is very
small compared to the atom
– A pea at the center of the WSU football stadium
is about the same ratio
• The electrons occupy the rest of the space
of the atom
The Charge of the Atom
• The charge of a proton is exactly opposite
of the charge of an electron
– The proton is +1 and the electron is -1
• The number of protons in an atom is unique
to each element and is called the atomic
• An atom having a net electrical charge is
called an ion.
– A positively charged atom is a cation
– A negatively charged atom is an anion
Atomic Mass
• Each atom has a fixed mass
• Because the carbon 12 isotope is stable and
common we use it as a standard and assign
it a mass of exactly 12.0000.
• Everything else is compared to this carbon12 atom
– Hydrogen is 1.008 on this relative scale
– Atoms of the same element have the same
number of protons (atomic number)
– The number of neutrons in an atom can vary
• a family of elements with atoms having the same
chemical properties but slightly different masses.
• These atoms with differing masses are called
– An isotope is identified by its mass number: the
sum of the number of protons and the number
of neutrons it contains
• Write the element (with atomic mass and
atomic number) and charge for each
4 protons, 5 neutrons, 2 electrons
26 protons, 29 neutrons, 26 electrons
9 protons, 10 neutrons, 10 electrons
80 protons, 121 neutron, 78 electrons