4.1 Defining the Atom

4.1 Defining the AtomEarly Models

Democritus (about 400BC)-

matter composed of tiny, indivisible particles

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Dalton (about 1800)- - used scientific method

transformed Democritus’ ideas into theory

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4.1 Defining the Atom

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Atoms of element A are identical

Atoms of element B are identical, but different than element A

Atoms of element A and B can be physically mixed together

A and B can be chemically combined as a compound

Dalton Proposed1.All elements composed of tiny,

indivisible particles2. Atoms of an element are identical

& distinct from atoms of other elements

3. Atoms can form compounds, combining in simple whole-number ratios

4. Atoms can be rearranged (chemical reactions), but atoms of one element do not change into atoms of another element.

Sizing Up the Atom5x10-11 - 2x10-10

Atoms cannot be seen with a light microscopeA tunnelling electron microscope can make images of atoms like this:

Tunnelling Electron Microscope

Atoms cannot be seen with a light microscope





4.2 Structure of the Nuclear Atom

Subatomic Particles

Atoms not indivisible

Electrons

Protons

Neutrons

Electrons

Used cathode ray tube to show electrons are negatively charged


are needed to see this picture.Discovered 1897 by J.J.Thomson

Electrons

Discovered 1897 by J.J.Thomson


Deflected by magnet

Electrons



Deflected by electrical charge

Electrons


Calculated charge/mass ratio

The ratio of charge to mass was the same, regardless of gas in cathode tube.Conclusion? Electrons must be present and the same in all elements.

Electrons

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ElectronsRobert A Millikan 1916 oil drop experiment

Millikan calculated the mass of an electronMass or electron = 1/1840 mass of a hydrogen atomCharge = -1



ElectronsRobert A Millikan 1916 oil drop experiment

The actual apparatus used in the Millikan oil drop experiment



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Neutrons and Protons

Atoms are neutralWhen charges do exist, they are always in simple multiples of the charge on an electron (no fractions)Eugen Goldstein 1886 found cathode ray evidence of protonsProton- charge of +1

mass 1840 times mass of electron

(1 atomic mass unit)

Neutrons and Protons

NeutronsNot discovered until 1932

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Neutrons have no chargehave a mass similar to proton

Atomic Nucleus“Plum pudding,” modelElectron, “raisins,” stuck in a lump of positive, “dough.”

Disproved in 1911 by one of Thomson’s students, Ernest Rutherford




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Rutherford’s Gold Foil Experiment

Alpha particles shot at a thin piece of gold foil did not pass right through with slight deflection. Instead,

most passed straight through. Some bounced right back!

Rutherford’s Gold Foil Experiment

Rutherford concluded that •Most of the atom is empty space•All of the positive charge

and almost all of the mass isConcentrated in the tiny core, “nucleus.”

composed of protons and neutrons.An idea of the size: Atom = football stadium

Nucleus = marble

4.3 Distinguishing Atoms

Atomic number = number of protons

Defines the element!

Atoms are electrically neutral

therefore atomic number also is thenumber of electrons

Subatomic particles

Element Atomic # Protons Electrons

K 19 19

5

S 16

V 23

19

B 5 5

16 16

23 23

Copy and complete the table:

How many protons and electrons?

a.Fluorine

9

b. Calcium

20

c. Aluminium

13

number of protons + number of neutrons

= mass number

Therefore

mass number - number of protons = number of neutrons

Helium with a mass number of 4 has 2 protons. How many neutrons does it have?

(Atomic) Mass Number

2 neutrons

Au

Isotope Notation

197

79

mass number

atomic number

(number of protons+ neutrons)

(number of protons)

symbol

First letter always capitalizedSecond letter never capitalized

How many neutrons does an atom of gold-197 have?118 neutrons

How many neutrons p112 q17

O16 8

a. b. S3216

c. Ag108 47

Br8035

d. e. Pb20782

8 neutrons 16 neutrons 61 neutrons

45 neutrons 125 neutrons

Express in Isotope Notation p112 q18

C12 6a. Carbon-12

F19 9b. Fluorine-19

Be 9 4c. Beryllium-9

Express in Isotope Notation p112 q18

C12 6a. Carbon-12

F19 9b. Fluorine-19

Be 9 4c. Beryllium-9

Isotopes

Ne2110

Neon-20

Neon-21

Ne2010

Neon-22 Ne2210

10 neutrons

11 neutrons

12 neutrons

Isotopes have different # of neutrons,same # of protons and electrons

Q19 p113

O16 8

Use isotope notation to showOxygen-16 Oxygen-17 Oxygen 18

O17 8 O18

8

Q20How many neutrons in each?(Chromium’s atomic number=24)Chromium-50 Chromium-52Chromium-5326 neutrons 28 neutrons 29 neutrons

Atomic MassAtomic Mass Unit1/12 mass of an atom of carbon-12Since carbon 12 has 6 protons

and 6 neutrons,Mass of 1 proton is about 1amu

Why isn’t the atomic mass = atomic mass number?

Atomic MassWeighted average of the mass of all naturally occurring isotopes. To find atomic mass, calculate(Mass of isotope)•(%abundance as decimal)For each isotopeAnd add the results together

Atomic Mass

Carbon is98.89% Carbon-12 & 1.11% Carbon-13Its atomic mass would be(12amu•0.9889)+(13.03amu•0.0111) =

12.011 amu

Chemistry Warm UpYou CAN do this. Work out the example if you are stuck. It’s like this:Carbon is made up of98.89% Carbon-12 & 1.11% Carbon-13Its atomic mass would be(12amu•0.9889)+(13.03amu•0.0111) =

12.011 amu

Chemistry Warm Up

Try to answer these without your notes or the book,then go back and check your answers against your notes or the book.When you finish, complete the second version of

Reading & Processing Information from the Periodic Table (worksheet)

Periodic TableHorizontal row = periodAtomic number increases left to rightAtomic number increases as you go downProperties vary as you go across a periodVertical column = groupGroups have similar chemical properties

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4.1 Defining the Atom