Chapter 5 Notes Atomic Theory As Seen Through History

Democritus—The First Model Democritus of Abdera (~400 B.C.) is the first person known to come up with the theory of the existence of particles called atoms. Democritus of Abdera (~400 B.C.) is the first person known to come up with the theory of the existence of particles called atoms. He said atoms were indivisible and indestructible particles that made up everything He said atoms were indivisible and indestructible particles that made up everything His theories were unable to be tested at the time. His theories were unable to be tested at the time. Statue of Democritus

John Dalton-the atom Englishman John Dalton is generally given credit for the first experiments to prove the nature of atoms Englishman John Dalton is generally given credit for the first experiments to prove the nature of atoms The experiments he did resulted in Dalton’s atomic theory. The experiments he did resulted in Dalton’s atomic theory. John.Dalton,

Dalton’s atomic theory 1)All elements are composed of tiny indivisible particles called atoms. 2)Atoms of the same element are identical. The atoms of any one element are different than those of any other element. 3)Atoms of different elements can physically mix together or chemically combine in simple, whole number ratios. 4)Chemical reactions occur when atoms are separated, joined or rearranged, but atoms of one element are never changed into another element as a result of chemical reaction.

After Dalton Much of Dalton’s theory still stands, but some has been changed. Much of Dalton’s theory still stands, but some has been changed. One change is that atoms are now known to be divisible. One change is that atoms are now known to be divisible. We will now learn about the three subatomic particles the atom can be broken down into. We will now learn about the three subatomic particles the atom can be broken down into.

J. J. Thompson-Electrons English physicist J.J. Thompson first discovered the electron in English physicist J.J. Thompson first discovered the electron in His experiments with cathode ray tubes led to the theory that particles much smaller than atoms existed, and that their charge was negative. His experiments with cathode ray tubes led to the theory that particles much smaller than atoms existed, and that their charge was negative. Today the electron carries exactly one unit of negative charge, and its mass is 1/1840 th of the mass of a hydrogen atom. Today the electron carries exactly one unit of negative charge, and its mass is 1/1840 th of the mass of a hydrogen atom. J.J.Thompson,

Goldstein and chadwick After the discovery of the electron, the race was on to find other particles; they knew they were there because the atom is electrically neutral. After the discovery of the electron, the race was on to find other particles; they knew they were there because the atom is electrically neutral. E. Goldstein is credited for discovery of the positively charged proton, which has a much greater mass than electrons. E. Goldstein is credited for discovery of the positively charged proton, which has a much greater mass than electrons. James Chadwick found yet another subatomic particle—the neutron. It has no charge, but the same mass as a proton. James Chadwick found yet another subatomic particle—the neutron. It has no charge, but the same mass as a proton. James Chadwick

Rutherford—The nucleus At this point in history, the prevalent thought was that the atom was mostly a positive substance with negative electrons throughout. At this point in history, the prevalent thought was that the atom was mostly a positive substance with negative electrons throughout. Rutherford sought to prove this by shooting particles through gold foil onto photo paper. Rutherford sought to prove this by shooting particles through gold foil onto photo paper. He expected to see the particles go through with slight deflection. He expected to see the particles go through with slight deflection. What he saw was that some particles went straight through, but some bounced back! What he saw was that some particles went straight through, but some bounced back!

Rutherford, cont’d. This gave rise to the theory that atoms mostly consist of empty space, with the majority of the mass in the center of the atom—called the nucleus. This gave rise to the theory that atoms mostly consist of empty space, with the majority of the mass in the center of the atom—called the nucleus. The protons and neutrons are located in the nucleus with the electrons orbiting around it. The protons and neutrons are located in the nucleus with the electrons orbiting around it.

The Bohr Model Niels Bohr introduced his model, which answered why electrons do not fall into the nucleus. He introduced the concept of energy levels, where the electrons orbited similar to the way the planets orbit the sun.

Bohr Model and Energy Levels In the Bohr model, electrons are in energy levels, or regions where they most probably are orbiting around the nucleus. The analogy is that energy levels are like the rungs of a ladder—you cannot be between rungs, just like an electron cannot be between energy levels. A quantum of energy is the amount of energy it takes to move from one energy level to the next.

Bohr Model and Energy Levels The Bohr model worked well for explaining the behavior of electrons in hydrogen, but for all other elements, the equations he used to predict the electron location did not work.

Quantum Mechanical Model In 1926, Erwin Schrodinger used the new quantum theory to write and solve mathematical equations to describe electron location.

The Quantum Mechanical Model Today’s model comes from the solutions to Schrodinger’s equations. Previous models were based on physical models of the motion of large objects. This model does not predict the path of electrons, but estimates the probability of finding an electron in a certain position. There is no physical analogy for this model!

Synopsis The atom is the smallest part of an element that retains its properties.The atom is the smallest part of an element that retains its properties. It is made of mostly empty space,with the majority of the mass concentrated in the middle (the nucleus).It is made of mostly empty space,with the majority of the mass concentrated in the middle (the nucleus). The nucleus contains the positively charged protons and the chargeless neutrons.The nucleus contains the positively charged protons and the chargeless neutrons. The electrons are situated outside of the nucleus in the empty space in energy levelsThe electrons are situated outside of the nucleus in the empty space in energy levels

Atomic number, mass number, atomic mass & isotopes

Atomic Number The atomic number of an element tells you the number of protons in the nucleus.The atomic number of an element tells you the number of protons in the nucleus. Because atoms are electrically neutral, the number of protons will be the same as the number of electrons.Because atoms are electrically neutral, the number of protons will be the same as the number of electrons. Each element has a different atomic number.Each element has a different atomic number. Atomic number

Answer the following What element has atomic number 32? What element has atomic number 32? How many protons does helium have? How many protons does helium have? What element has 79 protons? What element has 79 protons? What is uranium’s atomic number? What is uranium’s atomic number? How many electrons does calcium have? How many electrons does calcium have?

Mass number Most of the mass of an atom is in the nucleus; so much that the electrons can be neglected. Most of the mass of an atom is in the nucleus; so much that the electrons can be neglected. The mass number is the number of protons and neutrons in the nucleus. The mass number is the number of protons and neutrons in the nucleus. If you know the mass number and the atomic number, you can determine the composition of an atom. If you know the mass number and the atomic number, you can determine the composition of an atom. # of neutrons = mass number – atomic number # of neutrons = mass number – atomic number

Mass number, cont’d. Composition can be written in shorthand by putting the symbol for the element with the mass number and atomic number to the left. Composition can be written in shorthand by putting the symbol for the element with the mass number and atomic number to the left. C 12 6 Or, the mass number and chemical element can designate atoms, such as in: Or, the mass number and chemical element can designate atoms, such as in: Carbon-12

How many neutrons are in potassium-39? How many neutrons are in potassium-39? How many protons are in lead-208? How many protons are in lead-208? If an element has 22 protons and 26 neutrons, what is its mass number? If an element has 22 protons and 26 neutrons, what is its mass number? What element is in the previous question? What element is in the previous question? How many neutrons are in ? How many neutrons are in ? Answer the following Hg

Isotopes Atoms of the same element always have the same number of protons, but can have different numbers of neutrons. Atoms of the same element always have the same number of protons, but can have different numbers of neutrons. An atom with the same number of protons but different number of neutrons are called isotopes. An atom with the same number of protons but different number of neutrons are called isotopes. Isotopes are chemically alike, because it is the protons which are responsible for the chemical behavior. Isotopes are chemically alike, because it is the protons which are responsible for the chemical behavior.

What is the difference between iodine-127 and iodine-125? What is the difference between iodine-127 and iodine-125? Is an element with 25 protons and 30 neutrons an isotope of one with 26 protons and 30 neutrons? Why or why not? Is an element with 25 protons and 30 neutrons an isotope of one with 26 protons and 30 neutrons? Why or why not? Answer the following

Atomic mass Because there can be several isotopes of an element, there is another number, the atomic mass, that is the weighted average mass of all an element’s isotopes. Carbon-12 was used as a reference to measure mass of atoms, and the carbon-12 atom was said to have a mass of exactly 12 amu’s. Atomic mass

Finding Weighted Avg. Take into account both the mass of all isotopes as well as their relative abundances. For example: There are two known stable isotopes of carbon; carbon-12 and carbon-13. If 99.0% is carbon-12 and the rest is carbon-13, what is the average atomic mass of carbon?

Finding Weighted Avg. Zinc has five naturally occurring isotopes; zinc-64 (48.89%), zinc-66 (27.81%), zinc-68 (18.57%), zinc-67 (4.11%) and zinc-70 (0.62%). Based on this, what is the average atomic mass of zinc?