Atomic Theory and Atomic Structure

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Atomic Theory and Atomic Structure
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Democritus440 BC Greek Philosopher

• Universe was made of empty space and tiny bits of
  stuff
• Called these tiny bits of stuff atomos
• If you keep cutting something in half, eventually
  you would end up with a particle that could not
  be cut.
• Atom- not able to be divided
• He believed that atoms were indivisible and
  indestructible

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Democritus ( continued)

• Atoms are small hard particles.
• Atoms are made of a single material thats formed
  into different shapes and sizes.
• Atoms are always moving
• Atoms form different materials by joining
  together.

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Aristotle
Aristotle was a very famous Greek philosopher who
believed that matter could be divided into
smaller and smaller pieces forever.
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AristotleGreek Philosopher

• Matter is made of particles , which we call
  atoms.
• An atom is the smallest particle into which an
  element can be divided and still be the same
  substance.
• For example, the smallest particle of carbon is a
  single atom of carbon. If you divide it is no
  longer carbon anymore.

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Carbon
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John Dalton
British chemist and schoolteacher brings back
Democrituss idea of the atom. He performed many
experiments to study how elements join together
to form new substances He found that elements
combine in specific ratios and he supposed it was
because the elements are made of atoms.
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Daltons Fixed Ratio
Remember Daltons Theory says all matter is
made of individual particles (atoms) which cannot
be divided. Main points All elements are
composed of atoms. All atoms of same element
have same mass, and atoms of different elements
have different masses. Compounds contain atoms
of more than one element In a compound, atoms of
different elements always combine in same way in
fixed ratios. Theory was revised with later
discoveries
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John DaltonLate 1700s- British Chemist Teacher

• Daltons Atomic Theory ( published in 1803 )
• 1. All elements are made of atoms.
• 2. Atoms are small particles that cannot be
  divided, created, or destroyed.
• 3. Atoms of the same element are exactly alike,
  and atoms of different elements are different.
• Atoms of same element have the same mass.
• 4. Atoms join with other atoms to make new
  substances.
• 5. Chemical reactions involve the rearrangement
  of atoms to make new compounds.

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J.J. Thomson
J.J. Thomson discovered the existence of
electrons. He discovered that atoms are made of
smaller negatively-charged particles called
electrons.
Thomsons discovery was the result of doing
experiments with cathode ray tubes
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Thomson
Atom is neutral (no charge). Must have positive
charges because he found negative charges. These
negative charges are later called electrons
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Thomson Discovery of the Electron
In 1897, J.J. Thomson used a cathode ray tube to
deduce the presence of a negatively charged
particle the electron.
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Modern Cathode Ray Tubes

• Cathode ray tubes pass electricity through a gas
  that is contained at a very low pressure.

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J.J. Thomson1897- British Scientist

• There are small particles inside the atom.
• Created the plum-pudding model. (Looks like
  chocolate chip ice cream)
• Atom made of a positively charged material with
  the negatively charged electrons scattered
  through it.

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Thomson believed that the electrons were like
plums embedded in a positively charged
pudding, thus it was called the plum
pudding model.
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Thomsons Plum Pudding Model
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Ernest Rutherford1909- A Student of Thomson

• Gold Foil Experiment
• Atoms are made of mostly empty space.
• A tiny part of an atom is made of dense matter.
• 1911- Revised Theory- New Model
• The center of the atom, the nucleus, is extremely
  dense and has a positive charge.
• Rutherford is responsible for
• discovering the positively charged
• nucleus.

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Ernest Rutherford

• Shot a beam of particles at a thin sheet of gold.
• He predicted particles would travel straight
  through with a little deflection (very small
  change in direction).
• Believed atoms mass and charge were too small to
  stop the particles (like shooting a bullet at a
  tissue).

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Gold Foil Experiment

• Alpha particles are helium nuclei - The alpha
  particles were fired at a thin sheet of gold foil
• Particles that hit on the detecting screen
  (film) are recorded

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Gold Foil Experiment
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Rutherfords Findings

• Most of the particles passed right through
• A few particles were deflected
• VERY FEW were greatly deflected

Like howitzer shells bouncing off of tissue
paper!
CONCLUSIONS

1. The nucleus is small
2. The nucleus is dense
3. The nucleus is positively charged

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Rutherfords Atomic Model
Based on his experimental evidence The atom is
mostly empty space. All the positive charge, and
almost all the mass is concentrated in a small
area in the center. He called this center of
the atom ? a nucleus The nucleus is composed of
protons and neutrons (they make the nucleus!) The
electrons distributed around the nucleus, and
occupy most of the volume. His model was called a
nuclear model
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Niels Bohr1913- Danish Scientist

• Electrons move around the nucleus in paths or
  energy levels.

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Bohrs Model

• Added on to Rutherfords model
• Described the electrons in energy levels (like
  planets orbiting the sun)
• Each energy level has a specific energy (closer
  to nucleus lower energy)
• (farther from the nucleus higher energy )
• Electrons can move from one level to another, but
  cannot be between levels (think of steps on a
  staircase)

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Bohrs Model
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Energy Levels

• When an electron gains energy, it moves up to a
  higher energy level
• This is called exciting the electron
• When an electron loses energy, it moves down to a
  lower energy level AND releases this energy
• Energy released can be visible light

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Erwin Schrodinger (Austrian Physicist) and Werner
Heisenberg (German Physicist)

• Electrons do NOT travel in specific paths.
• They travel in Electron Clouds (a region around
  the nucleus where electrons are found)

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Electron CloudsEnery Levels
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What is an atom made of?
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What are Protons?

• Much larger and heavier than electrons
• Protons have a positive charge ()
• Located in the nucleus of the atom 
• Atomic Mass Unit (amu)- SI Unit used to express
  the mass of parts of atoms
• 1 proton 1 amu

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What are Neutrons?

• Large and heavy like protons
• Neutrons have no electrical charge
• Located in the nucleus of the atom
• 1 neutron 1 amu

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What are Electrons?

• Tiny, very light particles
• Have a negative electrical charge (-)
• Move around the outside of the nucleus
• Mass is almost 0 amu

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How do we describe atoms ?

• Atoms are composed of identical protons,
  neutrons, and electrons
• How then are atoms of one element different from
  another element?
• Elements are different because they contain
  different numbers of PROTONS
• The atomic number of an element is the number
  of protons in the nucleus and identifies the type
  of atom.

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How do we describe atoms?

• Atomic Number equals the number of protons in the
  nucleus.
• In a neutral atom, the number of protons equals
  the number of electrons.
• Atomic Mass Number is equal to the number of
  protons plus neutrons.
• Atomic Weight is the average mass of the
  naturally occurring isotopes of an element.

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Atomic Number

• Atomic number (Z) of an element is the number of
  protons in the nucleus of each atom of that
  element.

Element of protons Atomic (Z)
Carbon 6 6
Phosphorus 15 15
Gold 79 79
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Mass Number

• Mass number is the number of protons and neutrons
  in the nucleus of an isotope Mass p n0

Nuclide p n0 e- Mass
Oxygen - 18 8 10 8 18
Arsenic - 75 33 42 33 75
Phosphorus -31 15 16 15 31
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Atomic Symbol

• Contain the symbol of the element, the mass
  number and the atomic number.
• Superscript ? Mass number
• Subscript ? Atomic number

X
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Atomic Symbol

• Find each of these
• number of protons
• number of neutrons
• number of electrons
• Atomic number
• Mass Number

Br
80
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What are Ions?

• An atom of an element without equal numbers of
  electrons and protons.
• An atom of an element that carries an electrical
  charge is called an ion
• If the atom loses electrons, the atom becomes
  positively charged (because the number of
  positively charged protons will be more the
  number of electrons) ion
• If an atom gains electrons, the atom becomes
  negatively charged (more negative charges than
  positive charges) - ion

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What are Ions?

• The number of protons does not change in an ion
• The number of neutrons does not change in an ions
• So, both the atomic number and the atomic mass
  remain the same.

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What are Isotopes?

• An atom that has the same number of protons
  (atomic number) as other atoms in the same
  element BUT a DIFFERENT amount of neutrons
  therefore, it has different atomic masses.
• Some are unstable- the atoms nucleus will change
  over time.
• Some are considered Radioactive- When they fall
  apart, they will give off smaller particles and
  energy.
• Identify Isotopes by mass number. (Sum of the
  protons and neutrons.)

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What are Isotopes?
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What holds atoms together?

• Gravitational Force- mass attraction
• Electromagnetic Force- charge attraction
• Strong Force- keeps the nucleus together
• Weak Force- Radioactive Atoms Only.