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