Download atomos

Early Theories of Matter
The idea that matter is composed of tiny particles (which we now call atoms) did not even exist a
few thousand years ago. In fact, for more than a thousand years, great thinkers of their day argued
against the idea that atoms existed. The development of the concept of the atom and our
understanding of atomic structure are fascinating stories involving scores of great thinkers and
scientists.
Science as we know it today did not exist several thousands years ago. No one knew what a
controlled experiment was, and there were few tools for scientific exploration. In this setting, the
power of mind and intellectual thought were considered the primary avenues to the truth.
Curiosity sparked the interest of scholarly thinkers known as philosophers who considered the
many mysteries of life. As they speculated about the nature of matter, many of the philosophers
formulated explanations based on their own life experiences.
It wasn’t surprising then that many Greek philosophers thought matter was formed of air, earth,
fire, and water. They also associated properties with each of the four basic components of matter.
The pairings of opposite properties such as hot and cold, and wet and dry, mirrored the symmetry
and balance the philosophers observed in nature. These early nonscientific and incorrect beliefs
were not completely dispelled until the 1800’s
Democritus’s Ideas (460-370 B.C.) Greek philosopher
 first proposed that matter was not infinitely divisible
 matter was made up of tiny individual particles called atomos
 atoms could not be created, destroyed, or further divided
 matter was composted of empty space through which atoms move
 atoms are solid, homogeneous, indestructible, and indivisible
 different kinds of atoms have different sizes and shapes
 differing properties of matter are due to the size, shape, and movement of atoms
 apparent changes in matter result from changes in the groupings of atoms and not from changes
in the atoms themselves
* While a fair amount of Democritus’s ideas do not agree with modern atomic theory, his belief in the
existence of atoms was amazingly ahead of his time. Despite this, his ideas did not turn out to be a major
step toward our current understanding of matter. Over time, Democritus’s ideas were met with criticism
from other philosophers. “What holds the atoms together? They asked. Unable to answer the challenges
to his ideas, Democritus’s atomic theory was eventually rejected.
Aristotle (384-322 B.C.) Greek philosopher
* One of the most influential philosophers of his day. He wrote extensively on many subjects, including politics, ethics, nature, physics, and
astronomy.
 rejected Democritus’s atomic “theory” entirely because it did not agree with his own ideas on nature.
 did not believe that the “nothingness” of empty space could exist
 Aristotle’s ideas that denied the existence of atoms gained wide acceptance and went largely unchallenged for two thousand years
 Believed all substances were combinations of elements (fire, water, earth, air) and elemental qualities (hot, cold, wet, dry).
 To Aristotle, all material was made of atoms with the exception of the Prime Mover (God), and that earth and heaven were
subject to two different sets of laws.
Alchemists (400-1400)
* Alchemy was the search for a way to transform common metals into gold. “Scientists” who studied alchemy did not succeed in producing gold,
but their experiments revealed the properties of various metals and helped advance our understanding of matter.
 Because there were seven known heavenly bodies the alchemists believed that there were seven “base elements”
th
 during the 17 century some alchemists began focusing on identifying new compounds and reactions – these alchemists were
probably the first true chemists
th
 by the 20 century, scientists could transform atoms of some elements into atoms of other elements by bombarding them with
neutrons (even this process, however, was not successful in producing gold!)
John Dalton (1766-1844) English school teacher
* In the 19th century Dalton revised Democritus’s ideas based upon the results of scientific research he
conducted, resulting in Dalton’s Atomic Theory.
 All matter is composed of extremely small particles called atoms.
 All atoms of a given element are identical having the same size, mass, and chemical properties. Atoms
of a specific element are different from those of any other element.
 Atoms cannot be created, divided into smaller particles, or destroyed.
 Different atoms combine in simple whole number ratios to form compounds.
 In a chemical reaction, atoms are separated, combined, or rearranged.
* the advancements in science from Democritus’s day enabled Dalton to perform experiments that
allowed him to refine and verify his theories. Dalton studied numerous chemical reactions, making
careful observations and measurements along the way. He was able to accurately determine the
mass ratios of the elements involved in the reactions. Based on his research, he proposed his atomic
theory in 1803. Dalton’s convincing experimental evidence and clear explanation of the composition
of compounds and conservation of mass led to the general acceptance of his atomic theory.
Antoine-Laurent Lavoisier (1743-1794) French Scientist
* recognized as the father of modern chemistry









While his fellow scientists tried to explain matter based on the elements fire, earth, air, and water, Lavoisier performed some of the
first quantitative chemical experiments.
Demonstrated with careful measurements that transformation of water to earth was not possible, but that the sediment observed
from boiling water came from the container.
The data and observations from his experimental decomposition of mercury(II) oxide led to the statement of the Law of
Conservation of Mass.
He also studied the nature of combustion and devised a system of naming elements
authored the first chemical textbook; it contained a list of elements, or substances that could not be broken down further, which
included oxygen, nitrogen, hydrogen, phosphorus, mercury, zinc and sulfur.
Invented the system of chemical nomenclature still largely in use today, including names such as sulfuric acid, sulfates, and sulfites.
Demonstrated that the “air” responsible for combustion was also the source of acidity; he named this portion oxygen (Greek for acid-
former), and the other azote (Greek for no life)
He also labeled the inflammable air hydrogen (Greek for water former), identifying that when combined with oxygen dew was
produced.
In the late 1790’s Lavoisier was able to compile a list of elements known at that time, there were 23.
Berzelius (1779-1848) Sweedish chemist
*disciple of Dalton
±
±
developed the concepts of the io and ionic compounds.
Developed a systematic program to try to make accurate and precise quantitative measurements and insure
the purity of elements.
±
Determined the exact elementary components of large numbers of compounds, resulting in the confirmation
of the Law of Definite Proportions.
±
±
Published a table of atoms weights in 1826 which was in good agreement with modern values.
Introduced the classical system of chemical symbols in 1811, in which elements are abbreviated by one or
two letters to make a distinct abbreviation from their Latin name.
Sir Joseph John Thomson (1850-1940) English physicist
é Discovered the electron using a cathode ray tube in 1897
é Was able to determine the charge-to –mass ratio of the electron, indicating that the mass of the charged particle was much less
than that of the hydrogen atom; thus revealing that there were subatomic particles to the atom.
é Proposed the plum pudding model of the atom.
Earnest Rutherford (1871-1937) New Zealander-English physicist
☼ Worked with alpha rays with both electric and magnetic fields
☼ Through his gold foil experiment (passing massive alpha particles through a thin sheet of
gold foil) he disproved Thomson’s plum pudding model of the atom and comprised his own
nuclear model of the atom
☼ He was the first to propose that the atom was “nuclear”
☼ Considering the properties of the alpha particles and the electrons, and the frequency of
the deflections, he calculated that an atom consisted mostly of empty space through
which the electrons move.
☼ He also concluded that there was a tiny, dense region, which he called the nucleus,
centrally located within the atom that contained all of the atom’s positive charge and
virtually all of its mass.
☼ In 1920 he refined the concept of the nucleus, concluding that the nucleus contained
positively charged particles called protons (subatomic particles carrying a charge equal
to but opposite that of an electron)
Robert Millikan (1868-1953) American physicist
‫ ﻈ‬Determined the charge of an electron in 1909.
‫ ﻈ‬His exceptional experimental setup and technique allowed him to identify the charge of the
electron to within 1% of the currently accepted value.
‫ ﻈ‬Knowing the electrons charge and using the known charge-to-mass ratio, he calculated the
mass of a single electron.
Mass of an electron = 9.1 x 10-28g = 1/1840 mass of a hydrogen atom
Niels Bohr (1885-1962) Danish physicist
‫ﺓ‬
‫ﺓ‬
Proposed a successful quantum model of the atom in 1913.
His model assumed that the electron exists at a precise distance from the nucleus, as long as an electron remains in one
location no energy is given off, electrons have circular orbits (this is only correct in s orbits), and the angular momenta
associated with allowed electron motion are integral multiples of h.
Henry Mosely (1887-1915) English scientist
ð Discovered that atoms of each element contain a unique positive charge in their nuclei, thus the number of protons
in an atom is referred to as the element’s atomic number.
James Chadwick (1891-1974) English physicist
º Showed that the nucleus also contained another subatomic a particle, a neutral
particle called the neutron (it has a mass nearly equal to that of a proton but carries
no electrical charge)