Chapter 3 Atomic Structure

Chapter 3 Atomic Structure 3-1 Early Models of the Atom 3-2 Discovering Atomic Structure 3-3 Modern Atomic Theory 3-4 Changes in the Nucleus

3-1 Early Models of the Atom What are atoms? What are the postulates of Dalton’s atomic theory?

Ancient Greek – 450BC • Proposed that all matter is composed of tiny, invisible particles called atoms • No one believed him during his lifetime • Including Aristotle • His beliefs were not accepted until the 17th and 18th centuries Democritus

Was not accepted until 2 discoveries were made • Lavoisier’s law of conservation of matter • Joseph Louis Proust’s law of constant composition • A compound will always contain the same proportions by mass of elements • Water will always have 88.9% oxygen (O) and 11.1% hydrogen (H) Acceptance

English school teacher • Studied past theories of atoms and laws of matter • Formed an atomic theory of matter John Dalton (1766 – 1844)

Ea element is composed of extremely small particles called atoms • All atoms of a given element are identical, but they differ from those of any other element • Atoms are neither created/destroyed in any chemrxn • A given compound always has the same relative #s and kinds of atoms Dalton’s Atomic Theory of Matter

The smallest particle of an element that retains the chemical identity of that element • There are 118 elements wh means there are 118 different kinds of atoms. Atoms

Atoms are like the words in these slides. • If we broke it all apart, separated and organized the letters, you would find only 26 piles. • But by taking letters from different piles we can create millions of very different words • Just like words can be separated into letters, matter can be separated into atoms. • These separated atoms are called elements • Think of all the words you could make with the letters A, D, and M…. Atoms

Produces images of atoms • Created in 1981 • Nickel Platinum Scanning Tunneling Microscope (STM)

Consumer Tip • “100 Percent Natural” Chemistry In Action (p93)

Macroscopic – looking a the whole picture • A tree • It is made of the leaves, branches, trunk, roots • Microscopic – the more detailed vision of an object and what makes it function • A leaf off a tree and the little veins that carry the nutrients through it Macroscopic vs Microscopic

Macroscopic vs Microscopic

Chemists make their observations in the macroscopic world • It is the world in wh we all live • In order to understand that world, the goal is to understand the atoms that the world is made of • Discoveries/Possibilities b/c of the study of atoms • Deciphering the genetic code • Designing plastics • Understanding the hole in the ozone • Imprinting data on silicon chips

3-2 Discovering Atomic Structure How is atomic structure related to electricity? What did cathode rays indicate about atoms? What did Rutherford conclude from his alpha-scattering experiment?

Scientists couldn’t figure out why atoms of one element acted differently than another element’s atoms • Michael Faraday (1791-1867) said that the structure of an atom was directly related to electricity Electric Charges

Atoms contain particles that have electrical charges Electric Charges

An object will either have a positive or negative charge • 2 like charges will repel • Positive w/ positive • Negative w/ negative • 2 opposite charges will attract • Positive w/ negative • Franklin didn’t know where these charges came from Benjamin Franklin

Electric current - A moving stream of electrical charges • Electricity from wall socket or battery • Studying electrical currents provide keys to understanding electrical charges • Mid-1800s, began studying electric currents in glass tubes w/ little air Cathode Rays and Electrons

Tube attached on ea end to a battery • Positive and negative • Negative = cathode • Positive = anode • Radiation travels from cathode to anode • b/c radiation came from cathode end, called cathode ray and the tube a cathode ray tube Cathode Rays and Electrons

Cathode Ray tube being effected by a magnet • http://www.youtube.com/watch?v=7YHwMWcxeX8&feature=related - + Battery Cathode Rays and Electrons

Negative particles within the atom • JJ Thompson (1856-1940) • Mass of 9.11 x 10 -28 gram • 0.000000000000000000000000000911 gram • Robert Millikan (1868-1953) Electrons

Henry Becquerel (1852-1908) • Placed uranium on photo paper and an image appeared • Uranium was emitting radiation • Radioactivity: spontaneous emission of radiation from an element • Marie Curie and husband Pierre discovered the elements of radium and polonium were also radioactive Radioactivity

Thompson said there were electrons in the atom (neg charge) • Why is the atom neutral then? • Rutherford’s Gold Foil Experiment • http://www.youtube.com/watch?v=5pZj0u_XMbc • Called this center the nucleus • Has a positive charge • Very small • If the atom was the size of a football stadium, the nucleus would be smaller than a dime sitting in the middle • Electrons would be smaller than Franklin Roosevelt’s eye on the dime The Nuclear Atom

The Nuclear Atom http://www.ndt-ed.org/EducationResources/HighSchool/Magnetism/reviewatom.htm

How far are the electrons from the nucleus? If the earth was the nucleus, the electrons would cover an area as large as the distance b/w the earth and nearest stars

3-3 Modern Atomic Theory What are the names and properties of the 3 subatomic particles? How can you determine the # of protons, neutrons, and electrons in an atom/ion? What is an isotope? What is atomic mass?

We know atoms are made from protons, neutrons, and electrons • Recently scientists have found even smaller particles • Quarks, Gluons, Mesons, Muons, and others • They don’t seem to impact any Chemistry so chemists ignore • Physicists study them Subatomic Particles

Nucleus • Contains the protons and neutrons • Protons = positive – p+ • Have the same but opposite charge as electrons • Neutrons = neutral/no charge – n0 • Electrons • Negatively charged – e- • Move in the space outside nucleus – e- cloud • Very small compared to p+ • 2000 e- = 1 p+ The Structure of the Atom

Mass • Too small for normal measurements • Has own unit - atomic mass unit (amu) • P+ and n0 = 1 amu, e- = 0 amu b/c so small • Length • Diameter = 0.100 – 0.500 nanometer • Nanometer = nm = 10-9 meter • If you drew a line across a penny (1.9 cm), you would touch 810 million copper atoms • If you lined up all 810 million nuclei, you would only have a line 4 x 10-6 meter long • 4 millionths of a meter Size of Subatomic Particles

Henry Moseley (1887-1915) • Student of Rutherford • Discovered atoms of ea element contained differ positive charges • Lead to the idea that an atom’s identity comes from the # of p+ in nucleus • Call this # atomic number Atomic Numbers

The # of protons • Ea element has a unique atomic # • Can tell an element’s atomic # from periodic table Atomic Number

The p+ are positive • The e- are negative • The atom is neutral • This means, the p+ must equal the e- • For N, atomic # = 7 • Means p+ = 7 • Means e- = 7 Neutral Atom

How many protons and electrons in: • Oxygen (O) • 8 p+ and e- • Magnesium (Mg) • 12 p+ and e- • Silicon (Si) • 14 p+ and e- • What element has 11 protons? • Sodium Examples

When an atom gains/loses e-, it will have a charge • When an atom has a charge, called ion • Charge of ion = #p+ - #e- • If a magnesium atom loses 2 e-, ionic form has a charge of: • #p+ - #e- = 12 – 10 = +2 • It is important to add the plus (+) sign into the answer • Also possible to have a negative (-) • Some people write the charge with the +/- after the # (2+) • After you have calculated the charge, to write it with the element symbol, add it as a subscript • For our magnesium example: Mg+2 Ions

Write the chemical symbol for the ion w/: • 9 p+ and 10 e- • F- • 13 p+ and 10 e- • Al+3 • 7 p+ and 10 e- • N-3 • How many p+ and e- are present in: • S-2 ion • 16 p+ and 18 e- • Li+ ion • 3 p+ and 2 e- • Write the chemical symbol for the ion w/: • 12 p+ and 10 e- • Mg+2 • 74 p+ and 68 e- • W+6 Examples

All atoms of the same element, have the same # of p+ • They may not have the same # of n0 • If atoms have the same # of p+ but different # of n0 , we call them isotopes • Most elements have at least 1 isotope • 1 usually more frequent than another • In nature, it is usually a mixture • To tell isotopes apart, we use the mass # Isotopes

Mass # = #p+ + #n0 • An atom w/ 17p+ and 18n0 would have an mass # of 35 • Mass # = 17 + 18 = 35 • b/c 17 p+, tells us it is a chlorine atom • Chlorine – 35 • A way to write the element symbol w/ atomic and mass #s would be: mass # 37 Cl element symbol atomic # 17 Mass Number

How many protons, neutrons, and electrons are in the following ions? • Fe+2 • 26 p+, 24 e-, and 30 n0 • Al+3 • 13 p+, 10 e-, and 14 n0 • Se-2 • 34 p+, 36 e-, and 45 n0 • Write the complete chemical symbol for the ion w/ • 21 p+, 24 n0, and 18 e- • Sc+3 • 53 p+, 74 n0, and 54 e- • I- 56 26 27 13 79 34 45 21 127 53 Examples

The average mass of all the isotopes of an element • Listed in the periodic table Atomic Mass

Practice Problems # 1-30

3-4 Changes in the Nucleus What changes accompany nuclear reactions? What is radioactivity?

Change the composition of an atom’s nucleus • Produces alpha, beta, or gamma radiation • Alpha and beta radiation comes from radiation emitted from the nucleus Nuclear Reactions

Almost all atoms have stable nuclei • Not radioactive • Radioactivity could have harmful effects – good its rare to find in nature • Why are some more stable than others? • # of p+ and n0 in the nucleus • Some combinations cause instability Nuclear Stability

In nucleus, p+ and n0 are packed together in a very small space • How do p+ stay together in the small space if like charges repel? • Held there by strong nuclear force • Can only be found in this situation • Neutrons act like a net to hold the p+ in along with the strong nuclear force Nuclear Stability

Pattern of stability • Atomic # 1-20 – nuclei stable, = # of p+ and n0 • Beyond 20 p+ - more n0 needed to keep stable • Atomic # above 83 – radioactive nuclei • No # of n0 will make it stable • Atoms unstable if too many or too few neutrons • Atoms w/ too many emit beta radiation Nuclear Stability

Alpha (α) • Alpha particles have 2 p+ and 2 n0 • Identical to Helium – 4 nucleus • Travel only a few cm • Easily stopped by paper or clothing • Usually doesn’t pose a health threat unless actually enters the body 4 4 4 +2 He α He 2 2 2 Types of Radioactive Decay

Beta (β) • High speed electrons (not the ones around the nucleus) • Comes from charges inside a nucleus • A neutron changes into a p+ and e- • p+ stays in nucleus • e- (beta particle) is propelled out of nucleus at high speed • 100 times more penetrating than alpha • Able to penetrate 1-2 mm of solid material • Able to pass through clothing and damage skin 0 - 0 0 e e β -1 -1 -1 Types of Radioactive Decay

Gamma (γ) • Very energetic form of light our eyes can’t see • Doesn’t have any particles • More penetrating than others • Able to penetrate deep into solid material • Body tissue • Stopped only by heavy shielding • Concrete or lead 0 γ 0 Types of Radioactive Decay

When an atom emits radiation, it undergoes radioactive decay • Called decay b/c nucleus is decomposing to form a new nucleus • The best way to understand the decay is w/ a nuclear equation 226 4 α Ra 222 Rn + 88 86 2 Types of Radioactive Decay

Look at Figure 3-30 on p115 and answer the questions • Would this protective suit protect the worker from alpha radiation? • Why would a person working w/ alpha radiation also need to be concerned w/ gamma radiation? • Would protective clothing such as this stop gamma radiation from penetrating the worker’s skin? Partner Activity

Chapter 3 Atomic Structure