Ch. 4 ATOMIC STRUCTURE

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! Radios Calculators Televisions Computers Automobiles Cell phones Jet airplanes Ipods Plastic Velcro Refrigerators Internet (thanks, Al Gore) Penicillin CD’s & DVD’s Insulin and, 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 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 didn’t occur until the first true element was discovered! (1774) The first element discovered was

Karl Scheele (1771) (German) first to prepare and describe oxygen The discovery of oxygen is attributed to 3 scientists (working independently) Karl Scheele (1771) (German) first to prepare and describe oxygen Joseph Priestley (1774) (British) isolated oxygen gas from mercuric oxide. observed accelerated burning Antoine Lavoisier (1784) (French) made accurate measurements and interpreted Priestley’s results Discovery of Oxygen gas is attributed to three persons Karl Scheele who first prepare and discribed oxygen Joseph Priestly isolated oxygen gas from HgO. Collected the gas and observed that accelerated burning and increased activity of mice. Connected oxygen with combustion and plant and animal metabolism Also prepared many gases hydrogen chloride, ammonia, sulfur dioxide Founder of the ACS Antoine Lavoiser who made accurate measurements and interpreted Priestly’s results. He determined that mass of oxygen release from HgO is equal to mass loss from HgO. And correctly calculated the change in mass of oxygen durning a combustion reactions. (1784 wrote the Law of Conservation of Mass)

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

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

2HgO(s) → 2Hg(l) + O2 (g) 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: Scientific Contributions DISCOVERY OF 8 GASES Oxygen Nitrogen Carbon Dioxide Carbon Monoxide Sulfur Dioxide Nitrous Oxide Nitric Oxide Hydrogen Chloride

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

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!) Antoine-Laurent Lavoisier

2Hg(l) + O2 (g) → 2HgO(s) 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.

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

These properties of oxygen were later used to determine the properties of other substances. By the late 18th century, scientists finally came to the conclusion that Oxygen was truly an element (can’t 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! Oxygen hydrogen inert Increasing chemical reactivity

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) + H2SO4(aq) → H2 (g) + ZnSO4 (aq) While testing the properties of Hydrogen he found that water is a compound (1731 – 1810) Word Equation Chemical equation

Hydrogen + Oxygen Water 2H2 + O2 2H2O

Antoine Lavoisier Named Priestly’s newly discovered gas - “oxygen” - meaning “acid former” Named Cavendish’s new gas “hydrogen” -meaning “water former”

Dalton’s 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, it’s an important landmark in the history of science.

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

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

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

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

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

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

- + Thomson’s Experiment Voltage source 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”

Thomson’s Experiment Voltage source - Thomson found that cathode rays were deflected from a negatively-charged plate.

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

Light doesn’t ‘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 aren’t neutral – What charge must they have? That’s right! NEGATIVE!! Thomson called these negative particles – ELECTRONS

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

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

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

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

Henri Becquerel discovered radioactivity 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 and Marie Curie for discovering radioactivity.

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

3 Types of Radiation discovered by Ernest Rutherford Alpha (ά) – a positively charged helium nucleus 42 He+2 Beta (β) – fast-moving electrons -e Gamma (γ) – like high-energy x-rays

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

Flourescent Screen Lead block Polonium Gold Foil

He Expected: The alpha particles to pass through the foil without changing direction very much. Because… The positive charges were spread out evenly (according to Thomson’s 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

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

+

Rutherford’s Conclusions + 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. 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.

The Rutherford Atomic Model Based on his experimental evidence: 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” The electrons are distributed around the nucleus, and occupy most of the volume 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”

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

Subatomic Particles Particle Charge Mass (g) Location Electron (e-) -1 9.11 x 10-28g (virtually 0) outside nucleus Proton (p+) (H+) +1 1 amu (1.7 x 10-24g) in nucleus Neutron (no) (1.67 x 10-24g)

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

Henry Moseley (1887 – 1915) 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".

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

Since all atoms are neutral - the # protons in an atom = # electrons Atomic Number Henry Moseley – used x-ray spectra & came up with the idea of the 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

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

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

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

Isotopes 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. 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). Boron-10 (B-10) has 5 p and 5 n Boron-11 (B-11) has 5 p and 6 n 10B 11B

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. Radio-carbon dating of fossils 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) Mass number X Superscript → Element symbol Atomic number Subscript → REMEMBER! number of electrons = number of protons So all atoms are neutral!

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

Br Nuclear Symbols 80 35 Find each of these: number of protons number of neutrons number of electrons Atomic number Mass Number 80 Br 35

Nuclear 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 Write the complete symbol 78 34 Se 34 44 34

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

ISOTOPES Isotope p+ n0 e- Mass # Oxygen - 10 - 33 42 - 31 15 18 8 8 Arsenic 75 33 75 Phosphorus 16 15 31

The element hydrogen has 3 isotopes Protons Electrons Neutrons Nucleus Hydrogen–1 (protium) 1 Hydrogen-2 (deuterium) Hydrogen-3 (tritium) 2

Examples of Isotopes

Learning Check – Counting Naturally occurring carbon consists of three isotopes, 12C, 13C, and 14C. State the number of protons, neutrons, and electrons in each of these carbon atoms. 12C 13C 14C 6 6 6 #p+ _______ _______ _______ #no _______ _______ _______ #e- _______ _______ _______

Answers 12C 13C 14C 6 6 6 #p+ 6 6 6 #no 6 7 8 #e- 6 6 6

Learning Check An atom has 14 protons and 20 neutrons. A. Its atomic number is 1) 14 2) 16 3) 34 B. Its mass number is C. The element is 1) Si 2) Ca 3) Se D. Another isotope of this element is 1) 34X 2) 34X 3) 36X 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 don’t 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) It is defined as one-twelfth the mass of a carbon-12 atom. Carbon-12 chosen because of its isotope purity. Each isotope has its own mass number, so we determine the average atomic mass from the element’s percent abundance.

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

Composition of the nucleus Atomic Mass is the weighted average of all the naturally occurring isotopes of an element. on the Periodic Table Isotope Symbol Composition of the nucleus % in nature Carbon-12 C-12 6 protons 6 neutrons 98.89% Carbon-13 C-13 7 neutrons 1.11% Carbon-14 C-14 8 neutrons <0.01% (98.89 x 12) + (1.11 x 13) + (0.01 x 14) 100 = 12.011

D. Average Atomic Mass EX: Calculate the avg. atomic mass of oxygen if its abundance in nature is 99.76% 16O, 0.04% 17O, and 0.20% 18O. Avg. Atomic Mass 16.00 amu

Sub-atomic Particles - Summary Protons p+ - positive charge, in nucleus , mass of 1 amu Neutrons n0 – no charge, in nucleus, mass of 1 amu Electrons - e- negative charge, orbiting nucleus, “no mass”