Ch. 4 Sizing up the Atom Elements are able to be subdivided into smaller and smaller particles –...

Sizing up the Atom Elements are able to be subdivided into smaller and smaller particles these are the atoms, and they still have properties of that element If you could line up 100,000,000 copper atoms in a single file, they would be approximately 1 cm long Despite their small size, individual atoms are observable with instruments such as scanning tunneling (electron) microscopes

An STM image of nickel atoms placed on a copper surface. Source: IBM Research

Red ridge is a series of Cesium atoms

Image of a ring of cobalt atoms placed on a copper surface. Source: IBM Research

Atom - smallest particle making up elements One teaspoon of water has 3 times as many atoms as the Atlantic Ocean has teaspoons of water!

Think about the technological advances of the past 100 years! They have been nothing short of miraculous! RadiosCalculators TelevisionsComputers AutomobilesCell phones Jet airplanesIpods PlasticVelcro RefrigeratorsInternet (thanks, Al Gore) PenicillinCDs & DVDs Insulinand, of course - Electric guitars Sliced Bread!

Development of Atomic Theory This explosion of technology occurred once we had a better understanding of the atom and how it behaves!

Where did it all begin? The word atom comes from the Greek word atomos which means indivisible. The word atom comes from the Greek word atomos which means indivisible. The idea that all matter is made up of atoms was first proposed by the Greek philosopher Democritus in the 5th century B.C. The idea that all matter is made up of atoms was first proposed by the Greek philosopher Democritus in the 5th century B.C.

Then came the idea of The 4 Basic Elements Earth, Air, Fire, & Water After that came Alchemy. The change to real Chemistry didnt occur until the first true element was discovered! (1774) The first element discovered was

The discovery of oxygen is attributed to 3 scientists (working independently) Karl Scheele (1771) (German) Karl Scheele (1771) (German) first to prepare and describe oxygen first to prepare and describe oxygen Joseph Priestley (1774) (British) Joseph Priestley (1774) (British) isolated oxygen gas from mercuric oxide. isolated oxygen gas from mercuric oxide. observed accelerated burning observed accelerated burning Antoine Lavoisier (1784) (French) Antoine Lavoisier (1784) (French) made accurate measurements and interpreted Priestleys results made accurate measurements and interpreted Priestleys results

Carl Wilhelm Scheele beat Priestley to the discovery but published afterwards. Carl Wilhelm Scheele beat Priestley to the discovery but published afterwards. Too bad! So sad!

Priestley Medal Source: Roald Hoffman, Cornell University Priestley gets the main credit for discovering oxygen!

Priestley produced a gas (oxygen) by using sunlight to heat mercuric oxide kept in a closed container. The oxygen forced some of the mercury out of the jar as it was produced, increasing the volume about five times. Priestley produced a gas (oxygen) by using sunlight to heat mercuric oxide kept in a closed container. The oxygen forced some of the mercury out of the jar as it was produced, increasing the volume about five times. 2HgO (s) 2Hg (l) + O 2 (g)

Priestley: Scientific Contributions DISCOVERY OF 8 GASES Oxygen Oxygen Nitrogen Nitrogen Carbon Dioxide Carbon Dioxide Carbon Monoxide Carbon Monoxide Sulfur Dioxide Sulfur Dioxide Nitrous Oxide Nitrous Oxide Nitric Oxide Nitric Oxide Hydrogen Chloride Hydrogen Chloride

Priestley: Additional Scientific Contributions Discovered the interconnection between photosynthesis and respiration Discovered the interconnection between photosynthesis and respiration Discovered carbonated water Discovered carbonated water Discovered that India rubber removed graphite pencil marks - the first rubber eraser Discovered that India rubber removed graphite pencil marks - the first rubber eraser Now we can make mistakes!!

Antoine-Laurent Lavoisier Lavoisier: the Founder of Modern Chemistry Lavoisier continued the investigations of Priestly Quantitative experiments led to: Law of Conservation of Matter. He systematized the language of chemistry, its nomenclature and rhetoric. He was beheaded during the Reign of Terror for his role as a tax farmer prior to the Revolution (Priestley escaped to America!)

Lavoisier heated a measured amount of mercury to form the red mercuric oxide. He measured the amount of oxygen removed from the jar and the amount of red oxide formed. When the reaction was reversed, he found the original amounts of mercury and oxygen. Lavoisier heated a measured amount of mercury to form the red mercuric oxide. He measured the amount of oxygen removed from the jar and the amount of red oxide formed. When the reaction was reversed, he found the original amounts of mercury and oxygen. 2Hg (l) + O 2 (g) 2HgO (s)

Properties of Oxygen Colorless Colorless Odorless Odorless Tasteless Tasteless Gas at room temperature Gas at room temperature Slightly soluble in water Slightly soluble in water Inflammable (does NOT burn) Inflammable (does NOT burn) Only part of air that supports combustion Only part of air that supports combustion Physical Property or Chemical Property? P PPPPCCPPPPCC

These properties of oxygen were later used to determine the properties of other substances. By the late 18 th century, scientists finally came to the conclusion that Oxygen was truly an element (cant be broken down into simpler forms without losing its properties) Scientists began to search for & test other new elements.

Sometimes, when they tried to react substances together, nothing happened! Substances that DO NOT react are Inert They found that most materials will react to form new substances. These elements are said to be chemically active (reactive) Oxygen is very reactive, so is hydrogen which we will look at next! Increasing chemical reactivity inert Oxygenhydrogen

Discovery of Henry Cavendish (1766) Reacted various metals with acids producing a salt and hydrogen gas Acid + metal hydrogen gas + salt Zinc + sulfuric acid Hydrogen + zinc sulfate Zn (s) + H 2 SO 4(aq) H 2 (g) + ZnSO 4 (aq) While testing the properties of Hydrogen he While testing the properties of Hydrogen he found that water is a compound found that water is a compound (1731 1810) Word Equation Chemical equation

Hydrogen + Oxygen Water Hydrogen + Oxygen Water 2H 2 + O 2 2H 2 O

Antoine Lavoisier Named Priestlys newly discovered gas - oxygen - meaning acid former Named Priestlys newly discovered gas - oxygen - meaning acid former Named Cavendishs new gas hydrogen - meaning water former Named Cavendishs new gas hydrogen - meaning water former

Daltons Atomic Theory John Dalton (1766-1844) While his theory was not completely correct, it revolutionized how chemists looked at matter and brought about chemistry as we know it today (instead of alchemy) So, its an important landmark in the history of science.

Daltons Modern Atomic Theory (experiment based!) 3)Atoms of different elements combine in simple whole-number ratios to form chemical compounds 4)In chemical reactions, atoms are combined, separated, or rearranged but never changed into atoms of another element. 1)All elements are composed of tiny indivisible particles called atoms 2)Atoms of the same element are identical. Atoms of any one element are different from those of any other element.

Law of Definite Proportions n Each compound has a specific ratio of elements by mass. n Ex: Water is always 8 grams of oxygen for each gram of hydrogen.

Discovery of the Electron Began with the invention of the Crookes Tube (cathode ray tube) c. 1875

Cathode Ray Tube Electric current sent through gases sealed in tube at low pressure Anode- positive electrode Cathode- negative electrode Voltage source Metal Disks - electrodes - + gas

Modern Cathode Ray Tubes Cathode ray tubes pass electricity through a gas that is contained at a very low pressure. TelevisionComputer Monitor

In 1897, J.J. Thomson used a cathode ray tube to study gases.

n Passing an electric current makes a beam appear to move from the negative to the positive end so the beam was called a Cathode Ray Thomsons Experiment Voltage source + -

Thomsons Experiment n Thomson found that cathode rays were deflected from a negatively- charged plate. -

Voltage source Thomsons Experiment n and that cathode rays were attracted to plates with a positive charge n Does light bend like this? +

Light doesnt bend so the cathode ray must be made of particles rather than Light! Since they are attracted to a positive plate & repelled by a negative one the particles arent neutral What charge must they have? Thats right! NEGATIVE!! Thomson called these negative particles ELECTRONS

Mass of the Electron 1916 Robert Millikan determined the mass of the electron: 1/1840 the mass of a hydrogen atom; and, has one unit of negative charge The oil drop apparatus Mass of the electron is 9.11 x 10 -28 g

Conclusions from the Study of the Electron: a)Cathode rays have identical properties regardless of the element used to produce them. Therefore, all elements must contain identically charged electrons. b)Atoms are neutral, so there must be a positive substance in the atom to balance the negative charge of the electrons c)Electrons have so little mass that atoms must contain other particles that account for most of their mass

Thomsons Atomic Model Thomson believed that the electrons were like plums embedded in a positively charged pudding, thus it was called the plum pudding model. J. J. Thomson

Plum-Pudding Model Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 56

In 1903, An important discovery leading to further understandings of atomic structure happened by accident. Henri Becquerel discovered radioactivity Radioactivity is the spontaneous emission of energy from an object 1903: Shared a Nobel Prize with Pierre andNobel PrizePierre Marie CurieMarie Curie for discovering radioactivity.

Ernest Rutherford (1871-1937) The Nobel Prize in Chemistry 1908 Studied under J. J. Thomson

3 Types of Radiation discovered by Ernest Rutherford Alpha ( ) a positively charged helium nucleus 4 2 He +2 Alpha ( ) a positively charged helium nucleus 4 2 He +2 Beta ( ) fast-moving electrons - eBeta ( ) fast-moving electrons - e Gamma ( ) like high-energyGamma ( ) like high-energyx-rays

Ernest Rutherfords Gold Foil Experiment - 1911 Shot alpha particles at a thin sheet of gold foil Particles that hit on a detecting screen (film) were recorded

Lead block Polonium Gold Foil Flourescent Screen

He Expected: The alpha particles to pass through the foil without changing direction very much. The alpha particles to pass through the foil without changing direction very much. Because Because The positive charges were spread out evenly (according to Thomsons atomic theory). Alone they were not enough to stop the alpha particles. The positive charges were spread out evenly (according to Thomsons atomic theory). Alone they were not enough to stop the alpha particles.

What he expected

Again, because he thought the mass was evenly distributed in the atom

What he got

Rutherfords Observations Most of the particles went straight through the foil (what he expected) Most of the particles went straight through the foil (what he expected) A few particles were slightly deflected A few particles were slightly deflected Still fewer actually bounced back towards the source! Still fewer actually bounced back towards the source! Astonishing!!! Astonishing!!! Rutherford said it was like firing a Howitzer shell at a piece of tissue paper & having it bounce back & hit you! Rutherford said it was like firing a Howitzer shell at a piece of tissue paper & having it bounce back & hit you! Like howitzer shells bouncing off of tissue paper!

Rutherfords Conclusions Since most of the particles went through the foil - atoms are mostly empty space. Since most of the particles went through the foil - atoms are mostly empty space. Because a few particles were deflected they must have come close to a positively charged core. Because a few + particles were deflected they must have come close to a positively charged core. Since a very few particles were deflected straight back, the positively-charged core must be very dense. Since a very few particles were deflected straight back, the positively-charged core must be very dense. This small dense positive area is the nucleus. This small dense positive area is the nucleus. +

The Rutherford Atomic Model Based on his experimental evidence: Based on his experimental evidence: The atom is mostly empty space The atom is mostly empty space All the positive charge, and almost all the mass is concentrated in a small area in the center. He called this a nucleus All the positive charge, and almost all the mass is concentrated in a small area in the center. He called this a nucleus The electrons are distributed around the nucleus, and occupy most of the volume The electrons are distributed around the nucleus, and occupy most of the volume His model was called a nuclear model His model was called a nuclear model

Discovery of Protons Eugen Goldstein in 1886 observed particles with a positive charge passing through a perforated cathode.

In 1920, Rutherford studied these particles & called them protons. They have a charge of positive 1 and a mass of 1.7 x 10- 24 grams. This is not a handy number to work with so we use a mass of 1 amu. Amu stands for atomic mass unit

1932 James Chadwick confirmed the existence of the neutron a particle with no charge, but a mass nearly equal to a proton (1 amu). Discovery of the Neutron Rutherford predicted the existence of the neutron in 1920. Twelve years later, his assistant found it! So now we have a more complete picture of an atom!

Subatomic Particles ParticleCharge Mass (g) Location Electron (e - ) (e - ) 9.11 x 10 -28 g (virtually 0) outside nucleus Proton (p + ) (H + ) (H + )+1 1 amu (1.7 x 10 -24 g) in nucleus Neutron (n o ) (n o )0 1 amu 1 amu (1.67 x 10 -24 g) (1.67 x 10 -24 g) in nucleus

Elements are the new building blocks Hydrogen Nitrogen-7 Oxygen-8 Carbon-6

Between 1912 and 1914, the physicist H.G.J. Moseley conducted a series of experiments where he bombarded targets made out of different kinds of metals with cathode rays. Each metal he studied emitted X-rays of a characteristic frequency, almost like a set of "fingerprints". Henry Moseley (1887 1915)

The pattern that emerged when the observed X-rays were organized in order of increasing frequency suggested to Moseley a regular increase in the positive charge on the nuclei of the atoms. He called this positive nuclear charge- the Atomic Number of the element

Atomic Number Elements are different because they contain different numbers of PROTONS The atomic number of an element is the number of protons in the nucleus Since all atoms are neutral - the # protons in an atom = # electrons Henry Moseley used x-ray spectra & came up with the idea of the Atomic Number

Atomic Number, Z All atoms of the same element have the same number of protons in the nucleus, Z 13 Al 26.981 Atomic number Atom symbol AVERAGE Atomic Mass

Mass Number Mass number is the number of protons and neutrons in the nucleus of an isotope: Mass # = # protons + # neutrons

Subatomic Particles Most of the atoms mass. NUCLEUS ELECTRONS PROTONS NEUTRONS NEGATIVE CHARGE POSITIVE CHARGE NEUTRAL CHARGE ATOM Atomic Number equals the # of... equal in a neutral atom

Isotopes Frederick Soddy (1877-1956) in 1912 (worked with Rutherford) Frederick Soddy (1877-1956) proposed the idea of isotopes in 1912 (worked with Rutherford) Isotopes are atoms of the same element having different mass numbers, due to varying numbers of neutrons. Isotopes are atoms of the same element having different mass numbers, due to varying numbers of neutrons. Soddy won the Nobel Prize in Chemistry in 1921 for his work with isotopes and radioactive materials. Soddy won the Nobel Prize in Chemistry in 1921 for his work with isotopes and radioactive materials.

Isotopes Atoms of the same element (same Z) but different mass number (A). Atoms of the same element (same Z) but different mass number (A). Boron-10 (B-10) has 5 p and 5 n Boron-11 (B-11) has 5 p and 6 n Boron-10 (B-10) has 5 p and 5 n Boron-11 (B-11) has 5 p and 6 n 10 B 11 B

Isotopes Radioisotopes (radioactive isotopes) - Radioisotopes (radioactive isotopes) - unstable isotopes that spontaneously decay emitting radiation They play an important part in the technologies that provide us with food, water and good health. They play an important part in the technologies that provide us with food, water and good health. Radio-carbon dating of fossils Radio-carbon dating of fossils In medicine, diagnosis, treatment, and research In medicine, diagnosis, treatment, and research Sterilization of meat Disinfestation of grain and spices Increasing shelf life (eg, fruits)

Nuclear Symbols Contain the symbol of the element, the mass number and the atomic number (represent isotopes of elements) Contain the symbol of the element, the mass number and the atomic number (represent isotopes of elements) X Mass number Atomic number Subscript Superscript Element symbol REMEMBER! number of electrons = number of protons So all atoms are neutral!

Rhenium Re 186 75 Protons: 75 Neutrons: 111 Electrons: 75

Nuclear Symbols n Find each of these: a) number of protons b) number of neutrons c) number of electrons d) Atomic number e) Mass Number Br 80 35

Nuclear Symbols n If an element has an atomic number of 34 and a mass number of 78, what is the: a) number of protons b) number of neutrons c) number of electrons d) Write the complete symbol Se 78 34 44 34

Naming Isotopes We can name isotopes by placing the mass number after the name of the element: We can name isotopes by placing the mass number after the name of the element: carbon-12 carbon-12 carbon-14 carbon-14 uranium-235 uranium-235 Mass numbers

ISOTOPES Isotope p+p+p+p+ n0n0n0n0 e-e-e-e- Mass # Oxygen - 10 -3342 - 31 - 3115 8 8 18 Arsenic 7533 75 Phosphorus 15 31 16

IsotopeProtonsElectronsNeutronsNucleus Hydrogen1 (protium) (protium)110 Hydrogen-2(deuterium)111 Hydrogen-3(tritium)112 The element hydrogen has 3 isotopes

Examples of Isotopes

Learning Check Counting Naturally occurring carbon consists of three isotopes, 12 C, 13 C, and 14 C. State the number of protons, neutrons, and electrons in each of these carbon atoms. Naturally occurring carbon consists of three isotopes, 12 C, 13 C, and 14 C. State the number of protons, neutrons, and electrons in each of these carbon atoms. 12 C 13 C 14 C 6 6 6 6 6 6 #p + _______ _______ _______ #n o _______ _______ _______ #e - _______ _______ _______

Answers 12 C 13 C 14 C 6 6 6 6 6 6 #p + 6 6 6 #n o 6 7 8 #e - 6 6 6

Learning Check An atom has 14 protons and 20 neutrons. A.Its atomic number is 1) 142) 163) 34 B. Its mass number is 1) 142) 163) 34 C. The element is 1) Si2) Ca3) Se D.Another isotope of this element is 1) 34 X 2) 34 X 3) 36 X 16 14 14 16 14 14

Atomic Mass How heavy is an atom of oxygen? It depends, because there are different kinds of oxygen atoms. We are more concerned with the average atomic mass. This is based on the abundance (percentage) of each variety (isotope) of that element in nature. We dont use grams for this mass because the numbers would be too small

Measuring Atomic Mass Instead of grams, the unit we use is the Atomic Mass Unit (amu) Instead of grams, the unit we use is the Atomic Mass Unit (amu) It is defined as one-twelfth the mass of a carbon-12 atom. It is defined as one-twelfth the mass of a carbon-12 atom. Carbon-12 chosen because of its isotope purity. Carbon-12 chosen because of its isotope purity. Each isotope has its own mass number, so we determine the average atomic mass from the elements percent abundance. Each isotope has its own mass number, so we determine the average atomic mass from the elements percent abundance.

To calculate the average atomic mass: Multiply the mass of each isotope by its abundance, then add the results. Multiply the mass of each isotope by its abundance, then add the results. Abundance may be expressed as a decimal or a %, (Divide by 100 if using %s) Abundance may be expressed as a decimal or a %, (Divide by 100 if using %s) Avg. Atomic Mass

IsotopeSymbol Composition of the nucleus % in nature Carbon-12C-12 6 protons 6 neutrons 98.89% Carbon-13C-13 6 protons 7 neutrons 1.11% Carbon-14C-14 6 protons 8 neutrons

Ch. 4 Sizing up the Atom Elements are able to be subdivided into smaller and smaller particles –...

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Transcript of Ch. 4 Sizing up the Atom Elements are able to be subdivided into smaller and smaller particles –...