Unit 3 Atomic Structure

Atomic Theory

The Greeks History of the Atom In 400 B.C the Greeks tried to understand matter (chemicals) and broke them down into earth, wind, fire, and air. ~

Greek Model Democritus Greek philosopher Idea of ‘democracy’ “To understand the very large, we must understand the very small.” Democritus Greek philosopher Idea of ‘democracy’ Idea of ‘atomos’ Atomos = ‘indivisible’ ‘Atom’ is derived No experiments to support idea Continuous vs. discontinuous theory of matter Atomists; they argued for a completely materialistic universe consisting of atoms moving in a void. Since mere fragments of the ideas of Leucippus are known, his pupil, Democritus of Abdera (c 460 -370 B.C.) is considered the elaborator of this concept. Aaron J. Ihde The Development of Modern Chemistry, Dover Publishing, 1984 pg 6 It should also be noted that the Romans were not a scientific people and made almost no scientific contributions of their own. “To understand the very large, we must understand the very small.” -Democritus The world Reality to Democritus consists of the atoms and the void. Atoms are indivisible, indestructible, eternal, and are in constant motion. However, they are not all the same as they differ in shape, arrangement and position. As the atoms move they come into contact with other atoms and form bodies. A thing comes into being when the atoms that make it up are appropriately associated and passes away when these parts disperse. This leaves no room for the intelligent direction of things, either by human or divine intelligence, as all that exists are atoms and the void. Democritus stated, "Nothing occurs at random, but everything occurs for a reason and by necessity." The soul Although intelligence is not allowed to explain the organization of the world, according to Democritus, he does give place for the existence of a soul, which he contends is composed of exceedingly fine and spherical atoms. He holds that, "spherical atoms move because it is their nature never to be still, and that as they move they draw the whole body along with them, and set it in motion." In this way, he viewed soul-atoms as being similar to fire-atoms: small, spherical, capable of penetrating solid bodies and good examples of spontaneous motion. Democritus’s model of atom No protons, electrons, or neutrons Solid and INDESTRUCTABLE

Four Element Theory Plato was an atomist Thought all matter was composed of 4 elements: Earth (cool, heavy) Water (wet) Fire (hot) Air (light) Ether (close to heaven) ‘MATTER’ FIRE EARTH AIR WATER Hot Wet Cold Dry THE SCEPTICAL CHYMIST (1661) “The Greeks believed that earth, air, fire, and water were the fundamental elements that made up everything else. Writing in 1661, Robert Boyle (1627-1691) argued against this idea, paving the way for modern ideas of the elements. He defined an element accurately as a substance that could not be broken down into simpler substances.” Eyewitness Science “Chemistry” , Dr. Ann Newmark, DK Publishing, Inc., 1993, pg 18 Plato was Aristotle's student. It was Aristotle that suggested qualities of "hot, dry, cold, wet". Relation of the four elements and the four qualities Blend these “elements” in different proportions to get all substances

Some Early Ideas on Matter Anaxagoras (Greek, born 500 B.C.) Suggested every substance had its own kind of “seeds” that clustered together to make the substance, much as our atoms cluster to make molecules. Empedocles (Greek, born in Sicily, 490 B.C.) Suggested there were only four basic seeds – earth, air, fire, and water. The elementary substances (atoms to us) combined in various ways to make everything. Democritus (Thracian, born 470 B.C.) Actually proposed the word atom (indivisible) because he believed that all matter consisted of such tiny units with voids between, an idea quite similar to our own beliefs. It was rejected by Aristotle and thus lost for 2000 years. Aristotle (Greek, born 384 B.C.) Added the idea of “qualities” – heat, cold, dryness, moisture – as basic elements which combined as shown in the diagram (previous page). Hot + dry made fire; hot + wet made air, and so on. O’Connor Davis, MacNab, McClellan, CHEMISTRY Experiments and Principles 1982, page 26,

Who Was Right? did not experiment Aristotle was more famous He won! His ideas carried through middle ages. California WEB http://www.rightsleeve.com/images/large/RS_2265_lrg.jpg

Alchemy After that, chemistry was ruled by alchemy. They believed that that could take any cheap metals and turn them into gold. Alchemists were almost like magicians. elixirs, physical immortality

Alchemy Alchemical symbols for substances… . . . . . . . . . . . . . . . GOLD SILVER COPPER IRON SAND transmutation: changing one substance into another D In ordinary chemistry, we cannot transmute elements.

http://www.afternight.com/runes/a-alchem.gif Painting by David Ryckaert “The Alchemist”

Contributions of alchemists: Information about elements - the elements mercury, sulfur, and antimony were discovered - properties of some elements Develop lab apparatus / procedures / experimental techniques - alchemists learned how to prepare acids. - developed several alloys - new glassware

The Atomic Theory of Matter In 1803, Dalton proposed that elements consist of individual particles called atoms. His atomic theory of matter contains four hypotheses: 1. All matter is composed of tiny particles called atoms. 2. All atoms of an element are identical in mass and fundamental chemical properties. 3. A chemical compound is a substance that always contains the same atoms in the same ratio. 4. In chemical reactions, atoms from one or more compounds or elements redistribute or rearrange in relation to other atoms to form one or more new compounds. Atoms themselves do not undergo a change of identity in chemical reactions. Copyright © 2007 Pearson Benjamin Cummings. All rights reserved.

The Atomic Theory of Matter Dalton’s atomic theory is essentially correct, with four minor modifications: 1. Not all atoms of an element must have precisely the same mass. 2. Atoms of one element can be transformed into another through nuclear reactions. 3. The composition of many solid compounds are somewhat variable. 4. Under certain circumstances, some atoms can be divided (split into smaller particles: i.e. nuclear fission). Copyright © 2007 Pearson Benjamin Cummings. All rights reserved.

Dalton’s Symbols John Dalton 1808 Jons Jakob Berzelius (1799 -1848) Swedish chemist who invented modern chemical symbols. Berzelius discovered the elements silicon, selenium, cerium, and thorium. John Dalton 1808

Subatomic particles Electrons: Negatively charged subatomic particles Discovered by J.J. Thomson Discovered by observing deflection of cathode rays. National High Magnetic Field Laboratory: Electromagnetic Deflection in a Cathode Ray Tube (I) Tutorial

Protons : positively charge particles Neutrons: no charge particles, mass similar to protons. The atomic nucleus: Protons and neutrons are located in the center of the atom Through the gold-foil experiment, Rutherford determined: YouTube - Rutherford's Experiment: Nuclear Atom The atom is mostly empty space Positive charge and most of atom’s mass is concentrated in a small region, called the nucleus (composed of protons and neutrons)

Distinguishing Among Atoms Elements are different because they contain different number of protons. Atomic number: indicates the number of protons in the nucleus of an element. Since atoms are electronically neutral: # protons= # electrons Atomic number: Carbon has 6 protons Carbon has 6 electrons 6 C

Electron Microscopy Images of objects are produced by using an electron beam. Typical light microscope magnifies up to 1000 x Electron microscope magnifies over 100,000 x SEM - Image Gallery

Learning Check: complete the following table Element Symbol Atomic number # protons # electrons 1. Tin 2. 16 3. 81 4. 76 5. Gd

Learning Check: complete the following table Element Symbol Atomic number # protons # electrons 1. Tin Sn 50 2. Sulfur S 16 3. Thallium Tl 81 4. Osmium Os 76 5. Gadolinium Gd 64

Au Mass number gold-197 Mass number 197 79 Most of the mass of an atom is concentrated in its nucleus and depends on the number of protons and neutrons. Mass number: total number of protons and neutrons in an atom. # neutrons= mass number – atomic number Representing atoms: Mass number Mass number Au 197 79 gold-197 Atomic number

Learning check: determine the number of protons, neutrons and electrons for the following atoms. Carbon-12 Fluorine-19 Beryllium-9

Learning check: determine the number of protons, neutrons and electrons for the following atoms. Carbon-12 p+= 6 e-=6 no= 6 Fluorine-19 p+= 9 e-=9 no= 10 Beryllium-9 p+= 4 e-=4 no= 5

Hg 200 80 4. 5. 207 Pb 82

Hg 200 80 4. p+= 80 e-=80 no= 120 5. 207 Pb 82 p+= 82 e-=82 no= 125

Isotopes Isotopes are atoms of the same element that have the same number of protons, but different number of neutrons. Isotopes of an element have the same atomic number but different number of neutrons, thus have different mass numbers. Hydrogen has 3 isotopes: Hydrogen-1 or simply hydrogen Hydrogen-2 or deuterium Hydrogen-3 or tritium

Learning check: write the symbol (including atomic number and mass number )for the following isotopes. oxygen-17, oxygen-18 Chromium-50, chromium-52, chromium-53

Learning check: write the symbol (including atomic number and mass number )for the following isotopes. oxygen-17, oxygen-18 O O Chromium-50, chromium-52, chromium-53 Cr Cr Cr 17 18 8 8 50 52 53 24 24 24

Atomic Mass Mass of proton or neutron: 1.67x10-24g Mass of electron : 9.11x10-28g These values are impractical to work with, so scientists compare relative masses of atoms using a reference isotope : carbon-12 An atomic mass unit (amu) is defined as 1/12 the mass of carbon-12

Atomic mass (continued) The atomic mass of an element is not a whole number because the isotopes of an element and its natural abundance is taken in consideration. The atomic mass of an element is a weighted average mass of the atoms in a naturally occurring sample of the element.

Atomic mass (continued) Ex. 1 The atomic mass of copper is 63.546 amu. Which of copper’s two isotopes is more abundant: copper-63 or copper-65? Since 63.546 is closer to 63 than 65, the most abundant isotope is copper-63.

Atomic mass (continued) Ex. 3 There are 3 isotopes of silicon; they have mass numbers of 28, 29, and 30. The atomic mass of silicon is 28.086 amu. Comment on the relative abundance of these 3 isotopes. Silicon-28 must be the most abundant.

Atomic mass (continued) Ex. 2 Calculate the atomic mass of bromine. The two isotopes of bromine have atomic masses and relative abundance of 78.92 amu (50.69%) and 80.92 amu (49.31%) 1. Divide the percentages by 100: 0.5069 and 0.4931 atomic mass= (0.5069x78.92) + (0.4931x80.92) =79.9062 = 79.91 amu (remember units and sig figs)

Atomic mass (continued) Classwork p103 # 12,13; p 105 #33-34