Atomic Structure

What is an atom?

Atom

An atom is the smallest physical particle of an element that still retains the properties of that element.

Atomic Theory First scientist to suggest

atoms: Democritus

“Everything that is must be eternal.”

Democritus believed that there were basic things that always exist, but can be rearranged; he called these “atomos”.

Atomic Theory

John Dalton exceptionally bright Quaker put in charge of the local school at

age 12 Color blind (was once called

Daltonism) First scientist to create a modern

definition of “atom”

Atomic Theory

“We might as well attempt to annihilate [a planet], as to create or destroy a particle of hydrogen.”

Determined relative weights of known elements by breaking apart compounds consisting of them.

Atomic Theory1. All elements are composed

of atoms.

2. Atoms of the same element are identical. Atoms of different elements are always different.

3. Atoms of different elements can chemically combine in fixed ratios to form compounds.

Atomic Theory4. Chemical reactions occur when atoms are joined,

separated, or rearranged. Atoms of one element are NEVER changed into another during a chemical reaction.

Atomic Size

At sea level, one cubic centimeter of air (size of a sugar cube, or marble) will have 45 billion billion atoms within it.

45,000,000,000,000,000,000

How many atoms would it take to fill a universe?

Count to 45,000,000,000,000,000,000

Fill Bishop McDevitt COMPLETELY with marbles.

Empty and re-fill it once a MINUTE (24 hours per day).

After 400,000 years, you would have 45,000,000,000,000,000,000.

Atomic Size

To see the atoms in a drop of water, you would need to enlarge the drop until…

Think of a line one millimeter long. If this line were blown up to the size of the empire state building, an atom would be…

Review of Atoms?

All atoms are identical.

Chemical reactions occur when atoms of one element are changed into atoms of another element.

Atomic Structure

What does an atom look like?

Proof of Atoms

One century after Dalton, Viennese scientist Ernst Mach wrote that “atoms cannot be perceived by the senses… they are things of thought.”

Three scientists that made contributions to atomic structure – J.J. Thompson, Ernest Rutherford, and Neils Bohr.

John Dalton’s Atom

Atoms are solid and indivisible.

J. J. Thompson’s Atom

Thompson passed electric current through gases in a glass tube. The result was a glowing beam flowing from the negative side to the positive side.

J. J. Thompson’s Atom

Glowing ray (cathode ray) was attracted to a positive magnet on the side of the tube.

Beam must be negatively charged.

J. J. Thompson’s Atom

Thompson hypothesized that all atoms had such negative charges within. (plum pudding model)

Electrons and Protons

Electrons – negatively charged portion of an atom

Protons – positively charged portion of an atom

James Chadwick’s Atom

Chadwick found that the mass of an atom is greater than the mass of protons plus electrons.

Neutrons – portion of an atom with no charge

Protons, neutrons, and electrons are called subatomic particles.

Ernest Rutherford’s Atom

Born in New Zealand. Terrible at

mathematics. Diligent problem solver.

First to discover the power of the atom, power to make bombs that could “make this old world vanish in smoke.”

Ernest Rutherford’s Atom

Shot radioactive particles through a sheet of gold foil.

Most went straight through, but others were knocked off course or bounced back.

“Like shooting a fifteen-inch shell at a sheet of paper and having it bounce off.”

Ernest Rutherford’s Atom

Ernest Rutherford’s Atom

Nucleus – compact center of the atom where protons and neutrons are located

Nucleus has very little of the atom’s volume, but contains most of the atom’s mass.

Niels Bohr’s Atom

Electrons orbit the nucleus in orbits, like a solar system.

Planetary Model

Atomic Properties

How do atoms differ?

Atomic Number

Elements differ because their atoms contain different numbers of protons!

Atomic Number – number of protons in the atom

Mass Number

Electrons have negligible mass compared to protons and neutrons.

Mass of a neutron is approximately equal to mass of a proton.

Mass Number - number of protons plus number of neutrons in an atom.

Names and Symbols

Gold – 118 neutrons Mass Number = ?

Beryllium – 5 neutrons Mass Number – ?

Sodium – 12 neutrons Mass Number – ?

Isotopes

Atoms of the same element with different number of neutrons

Chemically equivalent

Most elements occur as a mixture of isotopes

Isotopes of Hydrogen

Calculating Atomic Mass

Mass of a proton is more than 1800 times the mass of an electron

Spectrometer used to find masses of atoms

Masses of subatomic particles too small to work with, so a reference isotope is used

Calculating Atomic Mass

One amu (atomic mass unit) is defined as 1/12 the mass of a Carbon-12 atom

1 amu is approx. equal to the mass of a proton or neutron

Calculating Atomic Mass Atomic masses found on periodic table

are averages of all isotopes of that element

Calculating Atomic Mass

Chlorine is a mixture of Cl-35 and Cl-37.

76% Cl-35 (35) 24% Cl-37 (37)

0.76 x 35 amu = 26.50 amu 0.24 x 37 amu = 8.957 amu

Added together… 35.45 amu

Example #1

Carbon-12 98.89% Carbon-13 1.11%

Atomic Mass?

Example #2

Silicon-28 92.23% Silicon-29 4.67% Silicon-30 3.10%

Atomic Mass?

Self Quiz, Part 1

What is an atom?

What is the symbol for… Calcium? Phosphorus? Potassium? Copper? Silver? Tin? Mercury?

Self Quiz, Part 2

What is the atomic number of… Lithium? Lead? Gold? Neon?

Describe Dalton’s atomic model.

Name the four premises of Dalton’s atomic theory.

Self Quiz, Part 3

Explain the difference between atoms and molecules.

Describe Thompson’s experiments and his atomic model.

Describe Rutherford’s experiments and his atomic model.

What is an atomic number?

Self Quiz, Part 4

Compare and contrast protons, neutrons, and electrons.

Chlorine-35? Mass Number? Number of Protons? Number of Electrons?