Transcript Slide 1 - Images

Inside the Atom
Chapter 2
Section 1: Models of the Atom
Section 2: The Nucleus
Models of the Atom
• Greek philosophers devised a theory of atoms,
or tiny particles.
John Dalton
• Combined the idea of elements with the Greek
theory of the atom.
• Matter is made up of atoms.
• Atoms cannot be divided into smaller pieces.
• All atoms of an element are exactly alike.
• Different elements are made of different atoms.
• Dalton’s theory was tested by William Crookes
and his cathode-ray tube experiment.
J.J. Thompson
• Discovered negatively charged particles,
electrons, which are a part of every atom.
• Thomson revised Dalton’s model to include a
sphere with a positive charge and negatively
charged electrons spread evenly within the
positive charge.
• The negatively charged electrons and the
positive charge in the sphere neutralized each
other.
Earnest Rutherford
• Tested Thompson’s model, which was found to
be an inaccurate model of the atom
An atomic model (Bohr’s model)
• With a nucleus was developed.
• The positively charged proton is located in a
very small space at the center of an atom.
• Most of an atom is empty space occupied by
nearly massless electrons.
• Electrically neutral particles, neutrons, are
also located in the nucleus.
• The number of electrons equals the number
of protons in an atom.
The Nucleus
• Atomic number – number of protons in the
nucleus of an atom
• Isotopes of an atom have the same number of
protons but different numbers of electrons
• Mass number is the number of neutrons plus the
number of protons
• Average atomic mass – the average mass of the
mixture of an elements isotopes
• The strong nuclear force holds tightly packed
protons together in a nucleus
Electron Cloud Model
• Explains the unpredictable wave behavior of
electrons, which could be anywhere in the
area surrounding the nucleus.
Electron shells
Electrons move rapidly around the
nucleus in areas called shells.
Each shell can hold up to 8 electrons
except the first shell which can only
hold 2 electrons
The number of the period (row) an
element is in, is the same as the
number of shells it has
• The first shell needs to be full before the
second shell gets any electrons.
• The last, or outer, shell is called the VALENCE
SHELL.
• The electrons in the valence shell are called
valence electrons
Electron Shell Diagrams
(Also called Bohr diagrams after the man who
started them.)
• The atomic number tells you the number of
protons an element has.
• In its natural state, each element has the same
number of electrons as protons.
• So: We use the atomic number to know how
many electrons we need to draw. Then we
start filling up shells from the inside out.
Reminders
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We write the element symbol in the center.
The first shell is full at 2.
Every other shell is full at 8.
We always draw 4 single electrons before we
start to pair them up.