Introduction to the Atom and Atomic Models

Atomic - Molecular Theory of Matter

The Atomic - Molecular Theory of Matter states that all matter is

composed of small, fast moving particles called atoms. These

atoms can join together to form molecules.

This theory is really thousands of individual ideas and models that provide evidence for the whole

theory.

MatterSince the atom is Since the atom is too small to be too small to be seen even with seen even with the most the most powerful powerful microscopes, microscopes, scientists rely scientists rely upon models to upon models to help us to help us to understand the understand the atom.atom.

Believe it or not this is a Believe it or not this is a microscope. Even with the microscope. Even with the world’s best microscopes world’s best microscopes we cannot see the we cannot see the structure or behavior of the structure or behavior of the atom.atom.

DEMOCRITUS400 BC

Ancient philosopher: Father of the Atom

Democritus believed all things consisted of tiny indivisible units.

He called these tiny units he called atomos. The Greek word

for “can not be cut” or “indivisible”

John Dalton (1799) Developed what is considered to be the 1st Atomic Theory

Was born into a modest Quaker family in England

Began lecturing in public at the age of 12

Dalton’s Model (1799)

• Dalton's model was that the atoms were tiny, indivisible, indestructible particles and that each one had a certain mass, size, and chemical behavior that was determined by what kind of element they were.

Dalton’s Model

Dalton’s model of the atom was similar to a tiny billiard ball.

Dalton’s model of the atom was solid and had no internal structure.

Dalton’s Atomic Theory

• elements consisted of tiny particles called atoms.1. all atoms of an element are identical2. atoms of each element are different from one another; they have different masses.3. compounds consisted of atoms of different elements combined together.4. chemical reactions involved the rearrangement of combinations of those atoms.

Flaws in Dalton’s Model

• Dalton’s falsely believed that the atom was the most fundamental particle.

– We now know the atom is made up of even smaller particles we call the proton, neutron and electron.

• Dalton’s theory could also not account for the formation of ions (charged particles)

Daltons Atomic Model Summary• Called: Billiard Ball Model• Could account for

– Atoms of different element have different atomic masses

– Elements consists of tiny particles called atoms

• Could NOT account for– Atom being the smallest particle– Atoms having an internal structure– Atoms having charged particles

John J. Thomson (1897)

• Discovered the electron using the Cathod Ray Tube (CRT)

• Thomson found that the beam of charge in the CRT was attracted to the positive end of a magnet and repelled by the negative end.

Thomson’s Hypothesis• Concluded that the cathode beam was a stream of

negative particles (electrons).

• He tested several cathode materials and found that all of them produced the same result.

• He also found that the charge to mass ratio was the same for all electrons regardless of the material used in the cathode or the gas in the tube.

• Thomson concluded that electrons must be part of all atoms.

Thomson’s atomic model

• Called the “plum-pudding” it was the most popular and most wildly accepted model of the time.

Thompsons atomic model could account for…..

• the atom having an internal structure

• Light given off by atoms • Atom with different

atomic masses

Thompsons atomic model could NOT account for…..

• Empty space (had atom filled with positive pudding)

• Formation of ions

Gold Foil Experiment

• Conducted by students of Rutherford.

• Proved that all atoms had a tiny, positively charged center.

• Confirmed that atom’s were mostly empty space.

Rutherford ~ early 1900s• α-particle interaction with matter

studied in gold foil experiment

Rutherford's Nuclear Model

• 1. The atom contains a tiny dense center – the volume is about 1/10 trillionth the

volume of the atom • 2. The nucleus is essentially the entire

mass of the atom• 3. The nucleus is positively charged

– the amount of positive charge of the nucleus balances the negative charge of the electrons

• 4. The electrons move around in the empty space of the atom surrounding the nucleus

Rutherford’s atomic model (1911)• Could account for:

– Empty space– Ions– Internal structure – Light given off when heated

to high temperature.

• Could not account for:– Stability

Planetary model• Planetary model used to explain

electrons moving around the tiny, but dense nucleus

• Nucleus contains– Protons- existence proposed in 1900s – Neutrons- existence proposed in 1930s

Bohr• Questioned ‘planetary model’ of

atom– Electrons located in specific levels from

nucleus (discontinuous model)

• Proposed electron cloud model based on evidence collected with H emission spectra

Bohr’s Atomic Model (1913)

• Bohr was a student of Rutherford.

• Improved Rutherford’s model by proposing electrons are found only in specific fixed orbits.

• These orbits have fixed levels of energy

• This explained how electrons could give off light (gain or lose energy)

BOHR MODEL• Electrons are placed in energy levels

surrounding the nucleus

Nucleus

(p+ & n0) 2e-

8e-

8e-

Bohr’s Atomic Model

• Could account for– Internal structure– Atoms of different masses– Atom being mostly empty space– Light given off– Formation of positive ions

• Flaws– Only really worked for Hydrogen

Chadwick (1932)

• Discovered the neutron by bombarding Be with beta radiation.

• Nuclear fission released a neutron.

chart

Review

• Describe each of the 6 different atomic models. Give the– Scientist Name– Name of model– What they could account for– What they could not account for (flaws)

Subatomic particle summaryParticle Discovery by Year experiment

Proton Rutherford 1911 Gold Foil Experiment

Electron Thompson 1897 The response of cathode ray tube to a magnetic and electric fields

Neutrons Chadwich 1932 Bombarded Be with beta radiation and a neutron was released

Subatomic Particles

Electron

Proton

Neutron

Name Symbol Charge

Relative mass

Actual mass (g)

e-

p+

n0

-1

+1

0

1/1840

1

1

9.11 x 10-28

1.67 x 10-24

1.67 x 10-24

Subatomic Particles (cont.)

• All atoms of an element have the same # of protons protons identify an atom protons identify an atom atomic # atomic #

• Atoms are electrically neutral #p = #e#p = #e--

• Only neutrons and protons contribute to an atoms mass

#n + #p = atomic mass#n + #p = atomic mass

Subatomic Particles

POSITIVECHARG E

PROTONS

NEUTRALCHARG E

NEUTRON S

NUCLEU S

NEGATIVE CHARGE

ELECTRONS

ATO M

Most of the atom’s mass.

NUCLEUS ELECTRONS

PROTONS NEUTRONS NEGATIVE CHARGE

POSITIVE CHARGE

NEUTRAL CHARGE

ATOM

QUARKSAtomic Numberequals the # of...

equal in a neutral atom

ISOTOPES= = atoms with the same atoms with the same

number of protons number of protons but DIFFERENT but DIFFERENT numbers of neutronsnumbers of neutrons

Atomic Number

Mass Number

Element Symbol

Ex. Ex. Na-23 or Sodium-23Na-23 or Sodium-23

C-14 or Carbon-14C-14 or Carbon-14

F-19 or F-19 or

C. Johannesson

B. Isotopes

• Atoms of the same element with different mass numbers.

C126Mass #

Atomic #

Nuclear symbol:

Hyphen notation: carbon-12

C. Johannesson

B. Isotopes

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C. Johannesson

B. Isotopes

• Chlorine-37

– atomic #:

– mass #:

– # of protons:

– # of electrons:

– # of neutrons:

17

37

17

17

20

Cl3717

C. Johannesson

C. Relative Atomic Mass

• 12C atom = 1.992 × 10-23 g

1 p = 1.007276 amu

1 n = 1.008665 amu

1 e- = 0.0005486 amu

© Addison-Wesley Publishing Company, Inc.

atomic mass unit (amu)

1 amu = 1/12 the mass of a 12C atom

C. Johannesson

D. Average Atomic Mass

• weighted average of all isotopes• on the Periodic Table• round to 2 decimal places

100

(%)(mass(mass)(%) )

Avg.AtomicMass

Avg.AtomicMass

D. Average Atomic Mass

• EX: Calculate the avg. atomic mass of oxygen if its abundance in nature is 99.76% 16O, 0.04% 17O, and 0.20% 18O.

100

(18)(0.20)(17)(0.04))(16)(99.76 16.00amu

C. Johannesson

Avg.AtomicMass

D. Average Atomic Mass

• EX: Find chlorine’s average atomic mass if approximately 8 of every 10 atoms are chlorine-35 and 2 are chlorine-37.

10

(37)(2)(35)(8)35.40 amu