Democritus

460 BC – 370 BC

First scholar to suggest

that atoms existed.

Believed that atoms

were indivisible and

indestructible.

https://reich-chemistry.wikispaces.com/file/view/Democritus.jpg

Democritus

460 BC – 370 BC

Problems with theory:

1. Did not explain chemical behavior

2. No experimental support

John Dalton

1766 – 1844

‘Dalton’s Atomic Theory’

1.All elements are composed

of tiny indivisible particles

called atoms.

2. Atoms of the same

element are identical.

http://upload.wikimedia.org/wikipedia/commons/e/e9/John_Dalton.jpg

All element A

atoms are

identical.

All element B

atoms are

identical, but

they are different

than element A

John Dalton

1766 – 1844

‘Dalton’s Atomic Theory’

3. Atoms of different

Elements can physically mix

together or can chemically

combine in simple whole-

number ratios to form

compounds.

http://upload.wikimedia.org/wikipedia/commons/e/e9/John_Dalton.jpg

John Dalton

1766 – 1844

‘Dalton’s Atomic Theory’

4. Chemical reactions occur

when atoms are separated,

joined, or rearranged. Atoms

of one element, however,

are never changed into atoms

of another element as a result

of a chemical reaction.

http://upload.wikimedia.org/wikipedia/commons/e/e9/John_Dalton.jpg

Cathode Ray Tube

(CRT)

Developed by Sir William Crooke

Cathode Ray Tube

(CRT)

Used by J.J. Thomson

http://dwb4.unl.edu/Chem/CHEM869Z/CHEM869ZLinks/www.chem.uiuc.edu/demos/cath.map

J.J. Thomson

1856-1940

Discovered the ‘corpuscle’ (later known

as the ‘electron’) in 1897.

Used a CRT and electric plates

in his experiment.

Found the charge-to-mass ratio

of the electron in the cathode ray.

Reached the conclusion that electrons

must be parts of the atoms of all elements.

J.J. Thomson

1856-1940

Called his atomic theory the

‘Plum pudding model’

Plum-pudding Model

Electrons are evenly

distributed throughout

an atom filled uniformly

with positively charged

material.

Robert Millikan

1868 - 1953

Oil drop experiment

Found the charge on

an electron.

Used that value to

calculate the

mass of an electron.

The mass of an electron is

9.11 x 10-28

gram

http://www.nndb.com/people/771/000091498/robert-millikan-1.jpg

Eugen Goldstein

(1850 – 1930)

Observed a CRT and noticed rays

traveling in the direction opposite

to that of the cathode rays.

He called the rays ‘canal rays’.

These ‘canal rays’ were

made up of positively

charged particles which

became known as ‘protons’

http://images.absoluteastronomy.com/images/encyclopediaimages/f/fr/francis_william_aston.jpg

James Chadwick

(1891 – 1974)

Discovered the neutron in 1932.

Was the assistant of Marie Curie.

http://images.google.com/imgres?imgurl=http://www.osaarchivum.org/guide/rip/10/pic/WhoIsWho/James_Chadwick.gif&imgrefurl=http://www.osaarchivum.org/guide/rip/10/whoiswho.html&usg=__TxivZWkS-KebLp_x0_tSAxSYar0=&h=198&w=140&sz=13&hl=en&start=9&um=1&tbnid=r_f0zprCIWjhkM:&tbnh=104&tbnw=74&prev=/images?q=james+chadwick+atomic+theory&hl=en&rlz=1T4DMUS_enUS266US266&sa=N&um=1

Ernest Rutherford

1871-1937

Improved on the Atomic Theory

through his gold-foil experiment.

The atom is made up of

mostly empty space.

99.9% of the mass of the

atom is in the nucleus which

is where the protons are.

The electrons surrounded the nucleus.

http://www.gutenberg.org/files/20417/20417-h/images/image388a.jpg

Ernest Rutherford

1871-1937

Gold-foil

experiment

• Alpha particles scatter from the gold foil.

Atomic Number – number of protons

in the nucleus of the atom.

Mass Number = protons + neutrons

Atomic Mass – the average mass

of all the atoms in a naturally

occurring sample of the element.

Isotope – atoms that have the

same number of protons but

different numbers of neutrons.

Lithium - 6

Lithium - 7

Li

Li

6

3

7

3

Nuclear Notation

₃Li ⁷ ₃Li ⁶

₁₀Ne ₁₀Ne ₁₀Ne ₂₀ ₂₁ ₂₂

The 3 isotopes of neon

Ions – atoms that have the same

number of protons but a different

number of electrons.

Ca2+

40

20 F1-

19

9

N3-

13

7 Na

1+ 22

11

Atomic Mass is defined as

one-twelfth of the mass of a

carbon-12 atom.

It is a weighted average mass of

the atoms in a naturally occurring

sample of the element.

Weighted Grades

A student has the following points

in chemistry class:

Homework 8 / 10 9 / 15 1 / 5 7 / 10

Labs 19 / 20 9 / 10 4 / 10

Quizzes/

Tests

20 / 25 47 / 50

Homework is worth 10%, Labs

are worth 20% and Tests are 70%.

Homework average percentage: 62.5%

Lab average percentage: 80.0%

Quiz/Test average percentage: 89.3%

Homework is 10% so 62.5% x .10 = 6.25%

Labs are 20% so 80.0% x .20 = 16.0%

Quiz/Test are 70% so 89.3% x .70 = 62.5%

Of all chlorine atoms, 75.771% are chlorine-35. Chlorine-35 atoms have a mass of 34.96885 amu. All other chlorine atoms are chlorine-37 and these have a mass of 36.96590 amu. What is the average atomic mass of chlorine?

Neon has three different isotopes. 90.51% of neon atoms have a mass of 19.992 amu. 0.27% of neon atoms have a mass of 20.994 amu. 9.22% of neon atoms have a mass of 21.991 amu. What is the average atomic mass of neon?