Unit 2Atomic Structure
Atomic Theory
Greek Model
Greek philosopherIdea of ‘democracy’Idea of ‘atomos’
Atomos = ‘indivisible’ ‘Atom’ is derived
No experiments to support idea, his ideas were forgotten for thousands of years.
Democritus’s model of atom
No protons, electrons, or neutrons
Solid and INDESTRUCTABLE
Democritus
“To understand the very large,
we must understand the very small.”
AlchemyAfter 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
immortalityPhilosopher’s stone:
change lead to gold
Alchemy
. . . .
. . . . . . . .. . .
GOLD SILVER COPPER IRON SAND
Alchemical symbols for substances…
transmutation: changing one substance into another
In ordinary chemistry, we cannot transmute elements.
D
Contributionsof 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
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 indivisible particles called atoms.
2. All atoms of an element are identical in mass and 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 rearrange in to form one or more new compounds. Atoms themselves do not change of identity in chemical reactions.
Some parts of his theory were later proven wrong.
The Atomic Theory of Matter
Modern atomic theorySubatomic particlesElectrons:
Negatively charged subatomic particlesDiscovered by J.J. Thomson in 1897Discovered by observing deflection of cathode
rays.Occupy most of the volume of an atom YouTube - Thomson
Electron MicroscopyThe electron microscope is a type of
microscope that uses a beam of electrons to create an image of the specimen. It is capable of much higher magnifications and has a greater resolving power than a light microscope, allowing it to see much smaller objects in finer detail.
Typical light microscope magnifies up to 1000 x and an electron microscope magnifies over 100,000 xSEM - Image Gallery
Protons : positively charge particlesNeutrons: 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: http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther14.swf 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 AtomsElements 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
C6
Atomic number:Carbon has 6 protonsCarbon has 6 electrons
Learning Check: complete the following table
Element Symbol Atomic number
# protons
# electrons
1.
Tin
2.
16
3.
81
4.
76
5.
Gd
Mass numberMost 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.Mass number= protons + neutrons# neutrons= mass number – protons
Representing atoms:
Au 19779
Mass numberAtomic number
gold-197
Mass number
Learning check: determine the number of protons, neutrons and electrons for the following atoms.1. Carbon-12
2. Fluorine-19
3. Beryllium-9
IsotopesIsotopes 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 hydrogenHydrogen-2 or deuteriumHydrogen-3 or tritium
Classwork: isotope notation handout
Atomic MassMass of proton or neutron: 1.67x10-24gMass 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)
1. Divide the percentages by 100: 0.5069 and 0.4931
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%)
atomic mass= (0.5069x78.92) + (0.4931x80.92)=79.9062= 79.91 amu
Atomic mass (continued)Classwork p103 # 12,13; p 105 #33-34