Unit #3 Atomic Structure What is an Atom? Unit #3 Atomic Structure

Review from 8th Grade OBJECTIVES: Identify the subatomic particles Explain how the atomic number identifies an element. Use the atomic number and mass number of an element to find the numbers of protons, electrons, and neutrons Review key terms

Subatomic particles Relative mass Actual mass (g) Name Symbol Charge Electron e- -1 1/1840 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 kind of atom (since all protons are alike!) the same as the number of electrons in the neutral atom. Mass Number = the number of protons + neutrons. These account for most of mass

Counting the Pieces Protons: equal to atomic number Neutrons: Mass Number – Atomic Number Electrons: In a neutral atom equal to atomic number

Symbols Contain the symbol of the element, the mass number and the atomic number.

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 Complete symbol

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

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

What if Atoms Aren’t Neutral Ions: charged atoms resulting from the loss or gain of electrons

What if Atoms Aren’t Neutral Anion: negatively charged ion; result from gaining electrons Take the number of electrons in a neutral atom and add the absolute value of the charge Identify: Number of Protons Number of Neutrons Number of Electrons 81 1- Br 35

What if Atoms Aren’t Neutral Cation: positively charged ion; result from the loss of electrons Take the number of electrons in a neutral atom and subtract the value of the charge Identify: Number of Protons Number of Neutrons Number of Electrons 27 Al 3+ 13

Distinguishing Between Atoms OBJECTIVES: Explain how isotopes differ, and why the atomic masses of elements are not whole numbers.

Distinguishing Between Atoms OBJECTIVES: Calculate the average atomic mass of an element from isotope data.

Isotopes 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 We are more concerned with average atomic mass Average atomic mass is based on abundance of each element in nature. We don’t use grams because the numbers would be too small

Measuring Atomic Mass Unit is the Atomic Mass Unit (amu) It is 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 Convert the percent to a decimal (move the decimal 2 places to the left or divide by 100) Multiply the atomic mass by it’s percent (expressed as a decimal), then add the results.

Calculating Averages Calculating the Average Atomic Mass is similar to calculating your grade in this class Percent of Grade Student's Average Tests 35% 89 Quizzes 25% 95 Labs 20% 35 Homework 20% 100

Calculating Averages If Bromine-79 has an abundance of 50.69% and Bromine-81 has an abundance of 49.31% what is the average atomic mass of Bromine? Why is it better to use the mass of an individual isotope for the mass number rather than round the number from the periodic table?

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- 26 with a mass of 25.9826 amu. What is the 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.

History of the atom Not the history of atom, but the idea of the atom. Original idea Ancient Greece (400 B.C.) Democritus and Leucippus- Greek philosophers.

Democritus and Leucippus Leucippus was the first person to come up with the idea of the atom Democritus was a student of Leucippus and expanded on his idea

Democritus Democritus added: Matter is composed of atoms which move through empty space Atoms are solid, homogeneous, indestructible, and indivisible Different atoms have different shapes and sizes The size, shape, and movement of atoms determine their properties

Greek Aristotle - Famous philosopher All substances are made of 4 elements Fire - Hot Air - light Earth - cool, heavy Water - wet Blend these in different proportions to get all substances

Aristotle There was no scientific evidence to back up either Democritus or Aristotle, but people believed Aristotle based on reputation Aristotle’s theory persisted for 2,000 years

Leading to the modern theory Late 1700’s - John Dalton- England. Teacher- summarized results of his experiments and those of others. Dalton’s Atomic Theory Combined ideas of elements with that of atoms. Saw atoms as small solid spheres. Billiard Ball Model.

Dalton’s Atomic Theory All matter is made of tiny indivisible particles called atoms. Atoms of the same element are identical, those of atoms of different elements are different. Atoms of different elements combine in whole number ratios to form compounds. Chemical reactions involve the rearrangement of atoms. No new atoms are created or destroyed.

Laws Related to Atomic Theory Law of Conservation of Mass- Antoine Lavoisier; states that matter is neither created nor destroyed in chemical reactions Law of Definite Proportions- Joseph Proust; states that all compounds contain the same elements in the same ratio Law of Multiple Proportions- John Dalton; elements combine in small whole number ratios

Just How Small Is an Atom? Think of cutting a piece of lead into smaller and smaller pieces How far can it be cut? An atom is the smallest particle of an element that retains the properties of that element Atoms-very small

Structure of the Nuclear Atom OBJECTIVES: Distinguish among protons, electrons, and neutrons in terms of relative mass and charge. Describe the structure of an atom, including the location of the protons, electrons, and neutrons with respect to the nucleus.

Parts of Atoms J. J. Thomson - English physicist. 1897 Made a piece of equipment called a cathode ray tube. It is a vacuum tube - all the air has been pumped out.

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

Thomson’s Experiment Voltage source - +

Thomson’s Experiment Voltage source - +

Thomson’s Experiment Voltage source - +

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

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

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

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

Thomson’s Experiment Voltage source By adding an electric field

Thomson’s Experiment Voltage source + - By adding an electric field

Thomson’s Experiment Voltage source + - By adding an electric field

Thomson’s Experiment Voltage source + - By adding an electric field

Thomson’s Experiment Voltage source + - By adding an electric field

Thomson’s Experiment Voltage source + - By adding an electric field

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

Plum Pudding Model Proposed by JJ Thomson Said the atom had a uniform positive charge in which the negatively charged electrons resided

Other particles Proton - positively charged pieces 1840 times heavier than the electron – by E. Goldstein Neutron - no charge but the same mass as a proton – by J. Chadwick Where are the pieces?

Millikan’s Oil Drop JJ Thomson used the cathode ray to find the mass to charge ratio of an electron. He did not know the mass or charge, only the ratio Millikan’s Oil Drop experiment determined the charge of an electron

Millikan’s Experiment Atomizer Oil droplets - + Oil Telescope

Millikan’s Experiment X-rays X-rays give some electrons a charge.

Millikan’s Experiment From the mass of the drop and the charge on the plates, he calculated the mass of an electron Some drops would hover

Rutherford’s experiment Ernest Rutherford -English physicist. (1910) Believed in the plum pudding model of the atom Wanted to see how big they are. Used radioactivity. Alpha particles - positively charged pieces- helium atoms minus electrons Shot them at gold foil which can be made a few atoms thick.

Rutherford’s experiment When an alpha particle hits a fluorescent screen, it glows. Here’s what it looked like

Fluorescent Screen Lead block Uranium Gold Foil

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 at center. Alpha particles are deflected by it if they get close enough. +

+

Density and the Atom Since most of the particles went through, it was mostly empty space. Because the pieces turned so much, the positive pieces were heavy. Small volume, big mass, big density. This small dense positive area is the nucleus.

Bohr Model Bohr worked with the concepts of energy, wavelength and frequency Each color of light is associated with a different energy Each atom gives off its own unique color so.. Electrons of diiferent atoms have different energies

Bohr Model Each atom has its own specific electron arrangement Electrons are in Energy levels in the atom When an electron goes from an excited state back down to its ground state the atom emits light

The Bohr Ring Atom n = 4 n = 3 n = 2 n = 1

The Bohr Model Doesn’t work. Only works for hydrogen atoms. Electrons don’t move in circles. The quantization of energy is right, but not because they are circling like planets. Back to the drawing board

The Quantum Mechanical Model A totally new approach. De Broglie said matter could be like a wave, like standing waves. The vibrations of a stringed instrument. More on this in Unit # 4

Models of the Atom to Know Daltons Thomsons Rutherford Bohr Quantum