Atomic Theory

Mr. MonteroChemistry

Dr. Michael M. Krop High School

Early Atomic Theory

• Material World is made up of tiny indivisible particles.

• A-tomos (Not divisible)

• There was one unique atom for every substance

• No proofDemocritus (460 – 370 BC)

Early Atomic Theory

Aristotle (384 – 322 BC)

Democritus is WRONG!!!Matter is continuous. The

essence of matter is called hyle. However, I offer no proof.

Everyone believed me but it turns out I WAS WRONG.

The Law of Conservation of Mass

Total mass of reactants

=

Total mass of products

Antoine Lavoisier (1734-1794)

Mass is neither created nor destroyed during chemical or physical

reactions.

The Law of Definite Proportions

Joseph Louis Proust (1754 -1826)

In a pure compound, the elements combine in

definite proportions to each other

9 g H2O = 1 g H + 8 g O

18 g H2O = 2 g H + 16 g O

Water is Oxygen and Hydrogen combined in a 8:1 mass ratio

Dalton’s Atomic Theory

• Elements are composed of very small indivisible particles called Atoms

• Atoms of a given element are identical to each other. Atoms of different elements are different from each other.

John Dalton (1766-1844)

Dalton’s Atomic Theory

• Compounds are formed when atoms of more than one element combine; a given compound always has the same relative number and kind of atoms.

• Atoms of an element are not changed during a chemical reaction. Reactions involve rearranging of atoms.

John Dalton (1766-1844)

Rearrangement of Atoms in a Chemical Reaction

Combustion of methane

STM image of the surface of gallium arsenide (GaAs).

Gallium atoms (blue spheres), Arsenic atoms (red spheres).

DEFINITION OF THE ATOM

Basic building block of matter.

Smallest particle of an element that

retains the chemical identity of the

element.

Is the atom truly indivisible?

Is the atom indivisible like a solid ball?

NO!!!Atoms are composed of electrically charged particles called (Subatomic Particles)

Law of Electrostatic attraction

Like charges repel each other.

Unlike charges attract each other.

Cathode Rays

What is traveling from the Cathode (-) to the Anode (+)?Why is the ray bent by a magnet?

Discovery of the Electron

J.J. Thomson (1856-1940)

Cathode Rays are actually particles (since

light does not bend) with a “negative” charge.

ELECTRONS!

Thomson’s Atomic Model

Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model. (Think of a Chocolate Chip Cookie)

Mass of the Electron

• Millikan finds the mass of the electron in 1909

• The mass of the electron is 9.11 x 10-31 kg

• That is 2000 times lighter than Hydrogen

Robert Millikan (1868-1953)

Millikan’s Oil Drop Experiment

The Oil Drop Apparatus

Millikan’s Oil Drop Experiment

Conclusions from the study of the Electron

• Cathode rays have identical properties regardless of the element used to produce them. All elements must contain identically charged electrons.

• Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electrons

• Electrons have so little mass that atoms must contain other particles that account for most of the mass

Radioactivity

Henri Becquerel (1852-1908)

I discovered that Uranium gives of Radioactivity. It spontaneously emits radiation.

I suggested to Marie and Pierre Curie to work on this phenomenon

Radioactivity

Pierre Curie (1859-1906) Marie Curie (1867-1934)

We are the most famous couple in

Science

We worked on Radioactivity. An

element was named after us (Curium Cm)

Radioactive Particles α, β, γ

• Alpha particles (α) are the nucleus of Helium (2 protons, 2 neutrons) +2 charge

• Beta particles (β) are high-speed electrons. -1 charge.

• Gamma Rays (γ) are high energy radiation, not particles. Zero Charge

Rutherford’s Gold Foil Experiment

Rutherford’s Gold Foil Experiment

• Most Particles pass right through

• Some particles are slightly deflected

• Very seldom a particle will bounce right back

• This experiment demolishes J.J. Thomson’s Plum-Pudding Model

Rutherford’s Findings

• The nucleus is Dense• The nucleus is Small• The nucleus is positively

charged

“Like Howitzer shells bouncing off of tissue

paper!”

Ernest Rutherford (1871-1937)

How dense is the Nucleus?

• A pea with the mass of 250 million tons

• The mass of all cars in the US in an object that would easily fit in a teaspoon

How small is the nucleus?

• If the diameter of the atom is the size of a football field how big is the nucleus?

The size of a Cherry!

Other Subatomic Particles

• Protons were discovered by Rutherford in 1919

• Neutrons were discovered by Chadwick in 1932

Subatomic ParticlesSubatomic Particles

Particle Charge Mass (kg) Location

Electron -1 9.109 x 10-31 Electron cloud

Proton +1 1.673 x 10-27 Nucleus

Neutron 0 1.675 x 10-27 Nucleus

How big is an atom?

The diameter of a US Penny is 19 mm. The diameter of a copper atom is 2.6 Å. How many atoms of copper would fit side by side in a straight line across the diameter of a penny?

1 Å = 10-10 m

Answer: 7.3 x 107 Cu atoms

Atomic Definitions

Atomic Number (Z): The number of protons in an atom

Mass Number (A): The number of nucleons (protons and neutrons)

Atomic Mass: Mass of the atom (expressed in atomic mass units or “u”)

Atomic Symbols

Isotopes

• Dalton predicted that all atoms of the same element are identical. This is not true.

• Isotopes are atoms that have the same number of protons but different number of neutrons (different mass number).

Isotopes of Carbon

SymbolNumber of

ProtonsNumber of Electrons

Number of Neutrons

11C 6 6 5

12C 6 6 6

13C 6 6 7

14C 6 6 8

D2O is water that

has Deuterium (2H)

instead of Protium (1H )

Deuterium is heavier

than hydrogen since

it contains an extra

neutron.

Heavy Ice

Atomic Mass Unit (amu)

• We need a more convenient unit to measure the mass of atoms.

• Atomic mass of Carbon-12 is assigned an exact value of 12 amu

• 1 amu = 1.66054 x 10-24g

Particle Mass (amu)

Proton 1.0073

Neutron 1.0087

Electron 5.486 x 10-4

Adding the mass of protons and neutrons

The mass of one 12C atom is EXACLTY 12 amu.

12C has 6 protons and 6 neutrons.

6 x (1.0073) + 6 x (1.0087) = 12.096 u

Where is the mistake????

Measuring the mass of isotopes

Mass Spectrometer

Calculating Atomic Mass

• Same atoms have different masses (Isotopes)

• In nature some isotopes are more abundant than others.

• It is possible to take an average of the atomic masses of all existing isotopes.

• The average atomic mass is the number reported in the periodic table

Calculating Atomic Mass

• The atomic mass depends on the relative abundance of the isotope. Therefore, it is a weighted average

Finding the atomic mass of Carbon. Naturally occurring Carbon is Composed of 98.892% 12C and 1.108% 13C. Their masses are 12 u (exact) and 13.00335 u respectively. What is the average atomic mass?

(0.98892)(12 u) + (0.01108)(13.00335 u) = 12.011 u