Atomos: Not to Be Cut

The History of Atomic Theory

Atomic Models A model uses familiar

ideas to explain unfamiliar facts observed in nature.

A model can be changed as new information is collected.

The atomic model has changed throughout the centuries, starting in 400 BC, when it looked like a billiard ball →

Democritus This is the Greek

philosopher Democritus -

His theory: Matter could not be divided into smaller and smaller pieces forever, eventually the smallest possible piece would be obtained.

400 BC

Atomos This piece would be

indivisible. He named the smallest

piece of matter “atomos,” meaning “not to be cut.”

This theory was ignored and forgotten for more than 2000 years!

Why?The eminent

philosophers of the time, Aristotle and Plato, had a more respected, (and ultimately wrong) theory.

Aristotle and Plato favored the earth, fire, air and water approach to the nature of matter. Their ideas held sway because of their eminence as philosophers. The atomos idea was buried for approximately 2000 years.

Dalton’s Model In the early 1800s,

the English Chemist John Dalton performed a number of experiments that eventually led to the acceptance of the idea of atoms.

Dalton’s Theory All elements are made of tiny,

indivisible particles called atoms. All atoms of the same element are

identical. All atoms of different elements are different (unique).

Atoms combine with different atoms to form compounds. Atoms unite in whole number ratios (H2O, CO2).

Atoms are not created or destroyed, simply rearranged in chemical reactions.

.

This theory became one of the foundations of modern chemistry.

Thomson’s Plum Pudding Model

In 1897, the English scientist J.J. Thomson provided the first hint that an atom is made of even smaller particles.

Thomson ModelThomson studied

the passage of an electric current through a gas.

As the current passed through the gas, it gave off rays of negatively charged particles.

Thomson ModelHe proposed -“Plum

Pudding” model.

Atoms were made from a positively charged substance with negatively charged electrons scattered about, like raisins in a pudding.

Thomson Model

This surprised Thomson, because the atoms of the gas were uncharged. Where had the negative charges come from?

Where did they come from?

Rutherford’s Gold Foil Experiment

In 1908, the English physicist Ernest Rutherford.

Rutherford’s experiment Involved firing a stream of tiny positively charged particles at a thin sheet of gold foil (2000 atoms thick)

Gold Foil Experiment

Gold Foil Experiment Most of the positively

charged “bullets” passed right through the gold atoms in the sheet of gold foil without changing course at all.

Some of the positively charged “bullets,” however, did bounce away from the gold sheet as if they had hit something solid. He knew that positive charges repel positive charges.

This could only mean atoms were mostly open space.

Rutherford concluded that an atom had a small, dense, positively charged center that repelled his positively charged “bullets.”

He called the center of the atom the “nucleus”

The nucleus is tiny compared to the atom as a whole.

Bohr ModelIn 1913, the

Danish scientist Niels Bohr proposed that electrons were in a specific energy level.

Bohr Modelelectrons move in

definite orbits around the nucleus, much like planets circle the sun.

These orbits, or energy levels, are located at certain distances from the nucleus.

Wave Model

The Wave Model

Today’s atomic model is based on the principles of wave mechanics.

electrons do not move about an atom in a definite path, like the planets around the sun.

In fact, it is impossible to determine the exact location of an electron. The probable location of an electron is based on how much energy the electron has.

The Wave Model According to the modern atomic model, at atom

has a small positively charged nucleus surrounded by a large region (electron cloud) in which there are enough electrons to make an atom neutral.

Electron Cloud:

Depending on their energy they are locked into a certain area in the cloud.

Electrons with the lowest energy are found in the energy level closest to the nucleus

Electrons with the highest energy are found in the outermost energy levels, farther from the nucleus.

The AtomCHAPTER 4, PART I

http://www.gpb.org/chemistry-physics/chemistry/302

Relative Sizes

What is the relative distance from the nucleus of an atom to the electrons? If the nucleus of an atom were the size of a marble how far away would the first electron be?

Answer

Subatomic ParticlesProtonNeutronElectron

FYI:Quarks

Up / DownCharm / StrangeTop / Bottom

ProtonSymbol = P+ Charge = PositiveMass = 1 atomic mass unit (amu)Location = In the nucleus

NeutronSymbol = n0 Charge = NeutralMass = 1 atomic mass unit (amu)Location = In the nucleus

ElectronSymbol = e- Charge = NegativeMass = 0 atomic mass unit (amu)Location = Orbiting the nucleus

Atomic Number, (Z) = # of P+ in an atom’s nucleus

Mass Number, (A) = Sum of the # of N0 and # of P+ in a given nucleus.

  AZ

A = mass #, Z = atomic #, X = symbolX

Atomic Calculations1) Atomic Number = Number of Protons

Z = Atomic Number = # p+

2) If neutral, then protons must equal electrons

3) Atomic Mass = Number of Protons + Neutrons

A = Atomic MassA = Z + n0

Atomic Calculations Game

http://education.jlab.org/elementmath/index.html

Isotopes – atoms of the same element with the same # of P+, but different # of N0 and different mass numbers.Ex.

 Nuclide – a particular atom containing a

definite number of protons and neutrons.Ex. Carbon-12 Iron-56

Nucleons – particles that make up the atomic nucleusEx. protons and neutrons.

 

Atoms vs Ions

C B-1 C+1

Ion – an atom that has lost or gained electrons and now has a charge

 Number of protons = Z (atomic number)Number of neutrons = A – Z (atomic mass –

atomic number)Number of electrons = Number of

protons - charge

Average Atomic MassAMU – atomic mass unit, 1/12 the mass of

a carbon-12 atom.Mass on periodic table is a weighted

average mass of all isotopes of an element.

Example

Neon-12 relative abundance – 98%Carbon-14, relative abundance – 2%

0.98 (12 amu) + 0.02 (14 amu) = 12.04 amu

Example 2What is the atomic mass of silicon if

92.21% of its atoms have mass of 27.977 amu, 4.70% have mass of 28.976 amu, and 3.09% have a mass of 29.974 amu?

28.09 amu