Chapter 5 Notes Atomic Theory As Seen Through History.

Chapter 5 NotesAtomic Theory As Seen Atomic Theory As Seen Through HistoryThrough History

Democritus—The First Model

• Democritus of Abdera (~400 B.C.) Democritus of Abdera (~400 B.C.) is the first person known to come is the first person known to come up with the theory of the existence up with the theory of the existence of particles called atoms.of particles called atoms.

• He said He said atomsatoms were were indivisible and indestructible indivisible and indestructible particles that made up particles that made up everythingeverything

• His theories were unable His theories were unable to be tested at the time.to be tested at the time.

Statue of Democritus

John Dalton-the atom•Englishman John Englishman John

Dalton is generally Dalton is generally given credit for the given credit for the first experiments to first experiments to prove the nature of prove the nature of atoms atoms

•The experiments he did The experiments he did resulted in resulted in Dalton’s Dalton’s atomic theoryatomic theory..

John.Dalton,1766-1844

Dalton’s atomic theory1)1) All elements are composed of tiny All elements are composed of tiny

indivisible particles called atoms.indivisible particles called atoms.2)2) Atoms of the same element are Atoms of the same element are

identical. The atoms of any one element identical. The atoms of any one element are different than those of any other are different than those of any other element.element.

3)3) Atoms of different elements can Atoms of different elements can physically mix together or chemically physically mix together or chemically combine in simple, whole number ratios.combine in simple, whole number ratios.

4)4) Chemical reactions occur when atoms Chemical reactions occur when atoms are separated, joined or rearranged, but are separated, joined or rearranged, but atoms of one element are never atoms of one element are never changed into another element as a changed into another element as a result ofresult of chemical chemical reactionreaction..

After Dalton•Much of Dalton’s theory still Much of Dalton’s theory still

stands, but some has been stands, but some has been changed. changed.

•One change is that atoms One change is that atoms are now known to be are now known to be divisible.divisible.

•We will now learn about the We will now learn about the three subatomic particles three subatomic particles the atom can be broken the atom can be broken down into.down into.

J. J. Thompson-Electrons• English physicist J.J. English physicist J.J.

Thompson first discovered the Thompson first discovered the electronelectron in 1897. in 1897.

• His experiments with cathode His experiments with cathode ray tubes led to the theory ray tubes led to the theory that particles much smaller that particles much smaller than atoms existed, and that than atoms existed, and that their charge was negative.their charge was negative.

• Today the electron carries Today the electron carries exactly one unit of negative exactly one unit of negative charge, and its mass is charge, and its mass is 1/18401/1840thth of the mass of a of the mass of a hydrogenhydrogen atom. atom.

J.J.Thompson,1856-1940

Goldstein and chadwick

• After the discovery of the After the discovery of the electron, the race was on to find electron, the race was on to find other particles; they knew they other particles; they knew they were there because the atom is were there because the atom is electrically neutral.electrically neutral.

• E. Goldstein is credited for E. Goldstein is credited for discovery of the positively discovery of the positively charged charged protonproton, which has a , which has a much greater mass than much greater mass than electrons.electrons.

• James Chadwick found yet James Chadwick found yet another subatomic particle—the another subatomic particle—the neutronneutron. It has no charge, but . It has no charge, but the the same mass as asame mass as a proton proton..

James Chadwick1891-1974

Rutherford—The nucleus• At this point in history, the prevalent At this point in history, the prevalent

thought was that the atom was thought was that the atom was mostly a positive substance with mostly a positive substance with negative electrons throughout.negative electrons throughout.

• Rutherford sought to prove this by Rutherford sought to prove this by shooting particles through gold foil shooting particles through gold foil onto photo paper.onto photo paper.

• He expected to see the particles go He expected to see the particles go through with slight deflection. through with slight deflection.

• What he saw was that some particles What he saw was that some particles went straight through, but went straight through, but some bounced back!some bounced back!

Rutherford, cont’d.

• This gave rise to the theory This gave rise to the theory that atoms mostly consist of that atoms mostly consist of empty space, with the majority empty space, with the majority of the mass in the center of of the mass in the center of the atom—called the the atom—called the nucleusnucleus..

• The protons and neutrons are The protons and neutrons are located in the nucleus with the located in the nucleus with the electrons orbiting around it.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 The atom is the smallest part of an element

that retains its properties.that retains its properties.• It is made of mostly empty space,with the It is made of mostly empty space,with the

majority of the mass concentrated in the majority of the mass concentrated in the middle (the nucleus).middle (the nucleus).

• The nucleus contains the positively The nucleus contains the positively charged protons and the chargeless charged protons and the chargeless neutrons.neutrons.

• The electrons are situated outside of the The electrons are situated outside of the nucleus in the empty space in nucleus in the empty space in energy levelsenergy levels

Atomic number, mass number,atomic mass & isotopes

Atomic Number• The atomic number of an element The atomic number of an element

tells you the number of protons in the tells you the number of protons in the nucleus.nucleus.

• Because atoms are electrically Because atoms are electrically neutral, the number of protons will be neutral, the number of protons will be the same as the number of electrons.the same as the number of electrons.

• Each element has a different atomic Each element has a different atomic number.number. Atomic

number

Answer the following

•What element has atomic What element has atomic number 32?number 32?

•How many protons does helium How many protons does helium have?have?

•What element has 79 protons?What element has 79 protons?•What is uranium’s atomic What is uranium’s atomic

number?number?•How many electrons does How many electrons does

calcium have?calcium have?

Mass number• Most of the mass of an atom is in the Most of the mass of an atom is in the

nucleus; so much that the electrons nucleus; so much that the electrons can be neglected.can be neglected.

• The mass number is the number of The mass number is the number of protons and neutrons in the nucleus.protons and neutrons in the nucleus.

• If you know the mass number and the If you know the mass number and the atomic number, you can determine atomic number, you can determine the composition of an atom.the composition of an atom. # of neutrons = mass number – # of neutrons = mass number –

atomic numberatomic number

Mass number, cont’d.• Composition can be written in Composition can be written in

shorthand by putting the symbol for shorthand by putting the symbol for the element with the mass number and the element with the mass number and atomic number to the left.atomic number to the left.

C126

• Or, the mass number and Or, the mass number and chemical element can designate chemical element can designate atoms, such as in:atoms, such as in:

Carbon-12

•How many neutrons are in How many neutrons are in potassium-39?potassium-39?

•How many protons are in lead-How many protons are in lead-208?208?

• If an element has 22 protons and If an element has 22 protons and 26 neutrons, what is its mass 26 neutrons, what is its mass number?number?

•What element is in the previous What element is in the previous question?question?

•How many neutrons are in ?How many neutrons are in ?

Answer the following

HgHg202011 8080

Isotopes• Atoms of the same element Atoms of the same element

always have the same number of always have the same number of protons, but can have different protons, but can have different numbers of neutrons.numbers of neutrons.

• An atom with the same number of An atom with the same number of protons but different number of protons but different number of neutrons are called neutrons are called isotopesisotopes..

• Isotopes are chemically alike, Isotopes are chemically alike, because it is the protons which because it is the protons which are responsible for the chemical are responsible for the chemical behavior.behavior.

•What is the difference What is the difference between iodine-127 and between iodine-127 and iodine-125?iodine-125?

• Is an element with 25 protons Is an element with 25 protons and 30 neutrons an isotope and 30 neutrons an isotope of one with 26 protons and of one with 26 protons and 30 neutrons? Why or why 30 neutrons? Why or why not?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?