Atomic Theory Models and Particles

Greek Idea “qualitative” Democritus (400 B.C.) Matter is made up of indivisible particles (atomos)

Dalton’s Atomic Theory 1808 All matter is composed of extremely small particles called atoms. Atoms of a given element are identical in size, mass, and other properties; different elements differ in size, mass, and other properties.

Dalton’s Atomic Theory Atoms cannot be subdivided, created, or destroyed Atoms of different elements combine in simple whole-number ratios to form chemical compounds.

Dalton’s Atomic Theory Law of definite proportions: A chemical compound contains the same elements in the same ratio no matter the source or size of the sample. Example : water H2O ratio of 2:1 And Law of Multiple proportions: The same elements can combine in different proportions to form different compounds. Example : water H2O and Hydrogen peroxide H2O2

Dalton’s Atomic Theory Law of conservation of mass – Matter (mass) cannot created nor destroyed, chemically nor physically. Example: Carbon + Oxygen = Carbon monoxide 12.00g + 16.00 g = 28.00g

1808 John Dalton’s model of the atom

Thomson’s Model 1897 Discovered electrons using the Cathode Ray Tube Atoms were made of positive stuff Negative electron floating around “Plum-Pudding” model or raisin bun

Thomson’s Experiment Voltage source - + Vacuum tube Metal Disks

Voltage source - +

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Voltage source - + Passing an electric current makes a beam appear to move from the negative to the positive end

Voltage source - + Passing an electric current makes a beam appear to move from the negative to the positive end

Voltage source - + Passing an electric current makes a beam appear to move from the negative to the positive end

Voltage source - + Passing an electric current makes a beam appear to move from the negative to the positive end

By adding an electric field Voltage source By adding an electric field

Voltage source + -

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Voltage source + - By adding an electric field he found that the moving pieces were negative

Other particles Later Thompson discovered the Proton – which is positively charged and 1840 times heavier than the electron Neutron - no charge but the same mass as a proton – by J. Chadwick

Ernest Rutherford 1911 Ernest Rutherford Experimented with gold foil He expected that the alpha particles would be deflected a small amount if Thomson’s model was correct.

He Expected The alpha particles to pass through without changing direction very much. Because…? …the positive charges were thought to be spread out evenly. Alone they were not enough to stop the alpha particles.

What he expected

Because

He thought the mass was evenly distributed in the atom

Since he thought the mass was evenly distributed in the atom

What he got

How he explained it Atom is mostly empty. Small dense, positive piece + Atom is mostly empty. Small dense, positive piece at center. Alpha particles are deflected by it if they get close enough.

Like Charges repel & unlike attract +

Rutherford’s Model Discovered dense positive piece at the center of the atom Named it the Nucleus Stated that Electrons moved around Mostly empty space

Bohr’s Model 1913 Electrons travel around the nucleus in orbits of certain energies. Like planets around the sun Amounts of energy separate one level from another.

Bohr’s Model In order to move from the original or ground state to an excited state or higher energy level an electron must absorb an energy quantum. and Must emit an energy quantum to move back toward the nucleus.

} Bohr’s Model Fifth Fourth Increasing energy Third Second First Further away from the nucleus means more (quanta) energy. There is no “in between” energy Energy Levels Fifth Fourth Third Increasing energy Second First Nucleus

Bohr’s Model Nucleus Electron Orbit Energy Levels The electron can absorb quanta to move from one level to other Energy Levels

The Quantum Mechanical Model Bohr’s model could not explain the behavior of atoms that have more than one electron. Bohr’s model could not account for the repulsion force between moving electrons

The Quantum Mechanical Model Energy is quantized and comes in chunks. A quanta is the amount of energy needed to move from one energy level to another. Since the energy of an atom is never “in between” there must be a quantum leap in energy. Electrons have properties of both waves and particles.

Heisenberg Uncertainty Principle We can never the know the exact location and velocity of an electron (particle) at the same time. Since we need light to see. When light is used the particle absorbs it and jumps to another location!

The Quantum Mechanical Model 1924 -1927 to present Things that are very small behave differently from things big enough to see. The quantum mechanical model is a mathematical solution It is not like anything you can see.

The Quantum Mechanical Model Has energy levels for electrons. Orbits are not circular. They are a region in space. An electron cloud. It can only tell us the probability of finding an electron a certain distance from the nucleus.

The Quantum Mechanical Model A area where there is a chance of finding an electron. Is the electron cloud. The cloud is broken down into four quantum numbers

Quantum Mechanical Model

Subatomic particles Relative mass Actual mass (g) Name Symbol Charge Electron e- -1 9.11 x 10-28 Proton p+ +1 1 1.67 x 10-24 Neutron n0 1 1.67 x 10-24

Counting the Pieces Atomic Number = number of protons in the nucleus # of protons determines the kind (name) of atom # of protons is the same as the number of electrons in the neutral atom.

Counting the Pieces Mass Number = the number of Protons + Neutrons. These account for most of mass

Nuclear Symbols Contain the symbol of the element, the mass number and the atomic number. Mass number X Atomic number

F Symbols 19 9 Find the number of protons number of neutrons number of electrons Atomic number Mass Number 19 F 9

Br Symbols 80 35 Find the number of protons number of neutrons number of electrons Atomic number Mass Number 80 Br 35

Symbols if an element has an atomic number of 34 and a mass number of 78 what is the number of protons number of neutrons number of electrons Nuclear symbol

Symbols if an element has 91 protons and 140 neutrons what is the Atomic number Mass number number of electrons Nuclear symbol

Symbols if an element has 78 electrons and 117 neutrons what is the Atomic number Mass number number of protons Nuclear symbol

Isotopes Dalton was wrong. Atoms of the same element can have different numbers of neutrons. different mass numbers. called isotopes.

Naming Isotopes We can also put the mass number after the name of the element. carbon- 12 carbon -14 uranium-235

Atomic Mass How heavy is an atom of oxygen? There are different kinds of oxygen atoms. More concerned with average atomic mass. Based on abundance of each element in nature. Don’t use grams because the numbers would be too small.

Measuring Atomic Mass Unit is the Atomic Mass Unit (amu) One twelfth the mass of a carbon-12 atom. Each isotope has its own atomic mass, thus we determine the average from percent abundance.

Calculating averages Multiply the atomic mass of each isotope by it’s abundance (expressed as a decimal), then add the results. Sample

Atomic Mass Calculate the atomic mass of copper if copper has two isotopes. 69.1% has a mass of 62.93 amu and the rest has a mass of 64.93 amu.

Atomic Mass Magnesium has three isotopes. 78.99% magnesium 24 with a mass of 23.9850 amu, 10.00% magnesium 25 with a mass of 24.9858 amu, and the rest magnesium 25 with a mass of 25.9826 amu. What is the average atomic mass of magnesium? ____________ If not told otherwise, the mass of the isotope is the mass number in amu

Atomic Mass Is not a whole number because it is an average. are the decimal numbers on the periodic table.

IONS Ions - atoms or groups of bonded atoms that has a positive or negative charge How does an atom get a positive or negative charge?

X IONS By losing or gaining electrons. It is the electron that can move. X Mass Number Charge Atomic Number

IONS Li +1 F -1 19 7 3 9 Metals lose electrons. Nonmetals gain electrons. 19 7 Li +1 F -1 3 9

IONS If an element has 17 protons, 18 electrons, what is the charge on the ion? Write the symbol. If an element has 20 protons, 18 electrons, what is the charge on the ion? Write the symbol.

SUMMARY Atomic Number is made up of ________________. Atomic Mass is made up of _______________ and _____________. Isotopes have different numbers of _____________________.

Summary Ions have different numbers of ___________________. A positive ion ____________ electrons. A negative ion ___________

Isotopes Revisited What is an isotope? What subatomic particle is the same in isotopes? What subatomic particle is different in isotopes?