University of the Philippines Manila How did our present-day concept of the atom evolve?
Biology 124 – Radiation Biology • 1897 – British chemist J.J. Thompson showed that the atom was
Prof. Marla Endriga not indivisible after all, but was made up of even smaller parts
The Atom o Performed experiments using a cathode-ray tube
o Discovered the existence of particles that have negative
Transcribed by: Alyanna Ysabel L. Salvador
electric charges
Atom § called the negatively charged particles “corpuscles” (now
• Atoms are the basic units of matter and the defining structure of electrons)
elements. o created a model (Raisin Pudding Theory) where he
described the atom as a positively charged sphere with
Molecule electrons embedded in it as far apart as possible
§ mass of positive charge = pudding
• Combining atoms of the same kind or different kinds makes
electrons = raisins
molecules.
• 1911 – British physicist Ernest Rutherford carried out
• The kind of molecules depends on which atoms combine.
experiments to test whether atoms are solid
o Bombarded a thin sheet of gold foil with tiny positively
Element
charged particles called alpha particles (later found to be
• The molecule contains only one kind of atoms
helium nuclei)
• E.g. hydrogen, carbon, gold and oxygen gas o Examined the fluorescent screen placed behind the foil
o Found that most of the alpha particles passed right through
Compound the gold foil. While a few were deflected
• Molecule contains two or more kinds of atoms, and are actually o Led to conclusion that the atom is mostly empty space with
linked together in a chemical bond a core or nucleus where all the positive charges are
• E.g. water concentrated. Electrons travel around the nucleus like
planets around the sun, but their exact places cannot be
Mixture described.
• The molecules are simply jumbled together (and are not • 1913 – Danish physicist Niels Bohr proposed a model of the
chemically combined) atom which adopted Rutherford’s nuclear atom
• E.g. air (a mixture of oxygen and nitrogen molecules and other o Determined that electrons are not randomly located around
gases) the nucleus but that they travel around the nucleus in specific
shells or layers
What is inside the atom? § Shells or layers are found at certain fixed distance from
• Proton – positive electrical charge; approximately 1837 times the nucleus
heavier than the electron o Also said that atoms absorb or give off energy as a photon
• Electron – negative electric charge; approximately 1837 times of light or radiation when the electrons move from one shell
lighter than a neutron or proton to another
• Neutron – neutral particle; has no electrical charge; has about • 1932 – British physicist James Chadwick discovered the neutron
the same mass of a proton and an electron combined o Has about the same size as the proton but with no electrical
charge
Historical Perspective of the Atom o Discovery explained why atoms were heavier than the total
• Ancient Greek philosophers: mass of their protons and electrons
o Wondered what would happen if a piece of matter – like a • Modern model of an atom
stone – was split into smaller and smaller particles o A result of the work of many scientists from the 1920s to the
o No matter how tiny the particles became, it would be possible present
to break them up into smaller particles o Current model says that electrons do not travel in definite
o Democritus: argued that no matter how hard or long you paths and the exact path of a moving electron cannot be
divide a piece of matter; you will eventually come to a piece predicted
so very small that it could not be divided or split further o There are regions inside the atom where electrons are likely
§ Called this indivisible particle an atom to be found. These regions are called electron clouds.
§ Atom – from the Greek word atomos meaning “can’t be
cut” or “indivisible” Atomic Number (Z)
• For thousands of years, no one had anything new or important • The number of protons in the nucleus of an atom
to say about the atom • This number determines the chemical identity of an atom
• 1803 – British chemist and physicist John Dalton introduced his
atomic theory based on experiments Atomic Mass/Mass Number (A)
• Dalton’s Atomic Theory • Equal the total number of protons (Z) and neutrons (N) in the
o All substances are made of atoms; atoms are small particles nucleus of an atom
that cannot be created, divided nor destroyed
o Atoms join with other atoms to make new substances Nuclide
o Atoms of the same element are exactly alike, while atoms of • Any atomic species characterized by the number of protons and
different elements vary in weight and other properties number of neutrons
• Dalton imagined atoms as tiny solid balls.
Isotopes Half-life
• Atoms of the same element having different number of neutrons, • The time it takes for an original number of atoms in a sample of
in combination with the fixed number of protons radioactive material to decay or disintegrate to one half this
• Practically all elements have more than one isotope number
• Isotopes of a given element have the same chemical properties, • Range from less than a second to billions of years
but they may differ in their nuclear properties • Unaffected by the physical and chemical state of the radioactive
material, temperature, pressure and other physical changes
Classification
• Stable
o Do not spontaneously emit energy in the form of rays or
particles
• Unstable or Radioisotope
o Unstable nuclei stabilize themselves by emitting or shooting
energy rays called gamma rays, similar to x-rays
o Others may emit particles from their nuclei and change to
different elements
o Source of instability of atoms is the unbalanced number of
protons and neutrons in their nuclei.
§ If the nucleus of an atom has too many neutrons or
protons, as compared to the optimum ratio of neutrons
to protons, then this atom becomes unstable.
Half-Lives of Some Radioactive Elements
Element Half-Life Element Half-Life
Polonium-216 0.6 seconds Californium- 2.2 years
252
Classification of Radioisotopes Iridium-131 4.9 seconds Cobalt-60 5.26 years
• Natural Sulfur-38 2 hrs 52 mins Krypton-85 10.6 years
o E.g. carbon-14, potassium-40 Technetium- 6.01 hours Hydrogen-3 12.26 years
• Artificial 99m (Tritium)
o Most produced artificially in special nuclear devices such as Sodium-24 15 hours Lead-210 22.3 years
a nuclear reactor by neutron bombardment Gold-198 2.7 days Cesium-137 30 years
Radioisotope production Radon-22 3.82 days Plutonium- 87.74 years
• Radioisotope is produced by neutron bombardment 238
Iodine-131 8 days Americium- 432.7 years
241
• Cerenkov Effect – cobalt 60 has gamma rays. When underwater, Radium-223 11.43 days Radium-226 1,600 years
it has a different velocity compared to when in air – thus Phosphorus- 14.3 days Carbon-14 5,730 years
exhibiting glow. 32
Strontium-90 28.8 days Chlorine-36 400,000 years
Do radioisotopes remain unstable or radioactive all the time? Iridium0192 74 days Uranium-235 710 million
• Unstable (radioactive) atoms undergo spontaneous decay into a years
more stable form (usually a different element) by emitting or Sulfur-35 87.2 days Potassium-40 1.28 billion
shooting out gamma rays or particles from their nucleus. years
• Ionizing radiation = rays and particles Calcium-45 164 days Uranium-238 4.5 billion
• Radioactivity – the property of radioisotopes which emit radiation years
spontaneously Cadmium-199 463 days Thorium-232 1.41 x 1010
• Radioactive Material – any material that exhibits the property of years
radioactivity
• Radioactive decay – the process of emitting ionizing radiation
