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Chapter 5. Atomic Structure. Democritus. Greek teacher, who first suggested the existence of atoms, around 400 B.C. He believed atoms were indivisible and indestructible. He was unable to test his theory due to the time in history. Modern Atomic Structure John Dalton (1766-1844).
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Chapter 5 Atomic Structure
Democritus • Greek teacher, who first suggested the existence of atoms, around 400 B.C. • He believed atoms were indivisible and indestructible. • He was unable to test his theory due to the time in history.
Modern Atomic Structure John Dalton (1766-1844) • An English school teacher, who studied the ratios in which elements combine in chemical reactions. • Based on his results he formulated several hypotheses.
Dalton’s Atomic Theory • All elements are composed of tiny indivisible particles called atoms. • Atoms of the same element are identical. • Atoms of different elements can physically mix or chemically combine. • Chemical reactions occur when atoms are separated, joined, or rearranged.
How small is the atom? • 100,000,000 copper atoms could be placed side by side and be no longer than 1 cm. • A pure copper coin (pre-1982) contains 2.4 x 1023 atoms. Compare that to the number of people on earth today – 6 x 109
J.J. Thomson, English Physicist(1856-1940) • He discovered the electron in 1897, by passing an electric current through a gas at low pressure. • These gases were sealed in a cathode ray tube.
The glass tube contained a gas at low pressure and on each end of the tube was a metal disk called an electrode. When electricity was passed between the electrodes, a green beam of light was formed, which traveled from the cathode (positive electrode) to the anode (negative electrode). This was a beam of electrons.
Don’t forget to study for the test this Friday! A television is a special type of cathode ray tube.
Robert A. Millikan (1868-1953) • His experiments allowed him to find the quantity of charge carried by an electron. • He also was able to accurately calculate the relative mass of an electron. (1/1840 the mass of a Hydrogen atom).
After a hydrogen atom loses an electron, what is left? • Atoms are electrically neutral (If they were not, you would get shocked every time you touched an object!!). • Electric charges are carried by particles of matter. • Electric charges always exist in whole number ratios. • When negative and positive particles combine, a neutral particle remains. • Can you figure out what is left over?
E. Goldstein in 1886 • Studied cathode ray tubes and noticed that some rays were traveling in an opposite direction. He concluded that they were positive particles that we know as protons.
James Chadwick • In 1932, this English physicist confirmed the existence of the neutron. These particles have the same mass as a proton, but have no charge.
Ernest Rutherford • In 1911, Rutherford tested the theory of atomic structure. • Until this time, scientists were questioning how the subatomic particles were put together in an atom.
Rutherford aimed a beam of alpha particles at a sheet of gold foil surrounded by a fluorescent screen. They found that most of the particles passed through the screen with no deflection. However, a few particles were deflected greatly, even directly back at the source. Rutherford concluded that most of an atom is empty space, with a very dense positive nucleus.
Rutherford’s new model • He proposed that most of the atom is empty space, and all of the positive charge and mass are concentrated in a small region in the center.
If an atom were the size of this stadium, then it’s nucleus would be about the size of a marble!!
Atomic Number • Atoms differ from each other by the number of protons contained in the nucleus. • The number of protons in an atom is equal to the atomic number. • Since atoms are electrically neutral, the number of protons equals the number of electrons.
Mass Number • The number of protons and neutrons in an atom is equal to it’s mass number. • If you know the mass number and atomic number of an atom, you can determine the atom’s composition.
An Atom’s Composition • By subtracting the atomic number from the mass number, one can determine the number of neutrons. • By knowing the atomic number, one can determine the number of protons and electrons.
Shorthand! • Atoms can be represented like this in order to show their mass number and atomic number. • How many neutrons does this atom of Lithium have?
It is possible for atoms of the same element to have a different number of neutrons. They are called isotopes. There are three known isotopes of hydrogen. The first has a mass number of one, and has no neutrons. The second, Deuterium, has a mass number of two, with one neutron. The third, known as Tritium has two neutrons, and a mass number of three.
Atomic Mass It is possible to measure the actual mass of an atom, however it is more useful to compare the relative masses of atoms using a reference. Carbon-12 was used as the standard and assigned exactly 12 atomic mass units, due to its six protons and six neutrons. From this, you may think that the atomic mass of elements would always be a whole number. This would be an incorrect assumption. Most elements occur as a mixture of two or more isotopes. Also, elements do not have the same number of each isotope in nature. Some isotopes are more abundant than others. Atomic Mass is a weighted average of all the isotopes.
Calculating Atomic Mass • Chlorine-35 is 75.77% abundant and has a mass of 34.969, and chlorine-37 is 24.23% abundant and has a mass of 36.966. What is the average atomic mass? • (34.969 x .7577) + (36.966 x .2423) = 35.453
One for you to try!! Oxygen has 3 known isotopes. Oxygen-16 has a mass of 15.995 and is 99.759% abundant; Oxygen-17 has a mass of 16.995 and is 0.037% abundant; Oxygen-18 has a mass of 17.999 and is 0.204% abundant. What is the average atomic mass of Oxygen?
Dmitri Mendeleev • Mendeleev was the first to logically arranged the known elements into a table. • He used increasing atomic mass along with other physical and chemical properties to order the elements. • He even left space for elements that had not been discovered yet, and predicted their properties!
Henry Moseley • In 1913, Moseley determined the atomic numbers of the atoms of the elements. • He then arranged the elements by increasing atomic number instead of atomic mass, which is how the periodic table is today.
The Modern Periodic Table There are seven rows of the periodic table, and they are known as periods. Each vertical column of the table is called a group or family. The table in this order gives rise to periodic law, meaning that the elements exhibit a repeating pattern of their physical and chemical properties.
The Periodic table has been divided into three broad categories, known as the metals, nonmetals, and metalloids.
Metals • Metals have a high electrical and thermal conductivity, a high luster, are ductile (drawn into wires), and malleable (beaten into thin sheets). • About 80% of all elements are metals.
Nonmetals • Nonmetals are generally nonlustrous, poor conductors, brittle, and may exist as solids, liquids, or gases at normal conditions. • Can you guess which elements are shown here?
Metalloids • Metalloids usually exhibit some properties of both metals and nonmetals. • Silicon and germanium are two very important metalloids, used for computer chips and solar cells.
Alkali Metals (Group 1A) • These metals are soft and very reactive. They must be stored in a substance such as kerosene or oil because they will react very quickly in air, and even more violently in water.
Alkaline Earth Metals (Group 2A) • These metals are also very reactive. • 2 electrons in their outer energy level
The Transition Metals • The 38 elements in groups 3 through 12 of the periodic table are called "transition metals". • Iron, cobalt, and nickel are the only elements known to produce a magnetic field.
Halogens (Group 7A) • 7 electrons in their outer energy level. • Halogens easily combine with metals to form salts. • Most reactive of all nonmetals. Iodine
Noble Gases • 8 electrons in their outer energy level. • Because of their electron arrangement Noble Gases are almost complete inactive, "inert". • All members of the family are colorless gases. • Argon is the most abundant Noble Gas, making up almost one percent of the atmosphere.
Rare-earth metals • Within the rare-earth group, there are two sub-groups: Lanthanoid series - elements #57-71 • Soft, malleable metals with a high luster and conductivity. Actinoid series - elements #89-103 • All these metals are radioactive.
