Development of the Atomic Model 6 Major Contributors to the Atomic - - PowerPoint PPT Presentation

Development of the Atomic Model

6 Major Contributors to the Atomic Model Theory

• 1. The Greeks
• 2. Dalton
• 3. Thomson’s Plum Pudding
• 4. Rutherford’s Gold-Foil

Experiment

• 5. Bohr’s Planetary Model
• 6. Chadwick’s Quantum Mechanical

The Greeks (430 BCE)

Long long ago, the Greeks believed that everything was made of atoms and that all atoms were

• identical. They believed that

matter was formed of small pieces that could not be cut into smaller parts. Used the word atomos which means “uncuttable”.

“The ancient Greeks did not prove the existence of atoms because they did not do experiments. In science, ideas are just ideas unless they can be

• tested. The idea of atoms began to develop again in

the 1600’s. This time, however, people did do

• experiments. As a result, atomic theory began to

take shape. Atomic theory grew as a series of models that developed from experimental

• evidence. As more evidence was collected, the

theory and models were revised.”

John Dalton (early 1800’s)

• All elements are composed of atoms that cannot

be divided.

• All atoms of the same element are exactly alike

and have the same mass. Atoms of different elements are different and have different masses.

• An atom of one element cannot be changed into

an atom of a different element. Atoms cannot be created or destroyed in any chemical change (or reaction), only rearranged.

• Dalton thought that atoms were like smooth,

hard balls that could not be broken into smaller pieces.

• With only a few changes, Dalton’s atomic theory

is still accepted today.

Thomson’s Plum Pudding Model (1897)

Protons & electrons are randomly spread throughout the atom (like plums in pudding, apparently… think of raisin bran cereal- it’s all mixed together). Atoms do not contain a nucleus.

“Thomson described an atom that consisted of negative charges scattered throughout a ball of positive charge. The negatively charged particles later became known as electrons.”

Rutherford’s Gold Foil Experiment (1911)

Through his experiment (aiming a beam of positively charged particles at a thin sheet of gold foil), Rutherford discovered that most of the mass of an atom is in the nucleus and that the rest of the atom is empty space

Niels Bohr’s Planetary Model (1913)

Niels Bohr was a student of both Thomson &

• Rutherford. Bohr theorized that the electrons

are in fixed energy levels that orbit around the nucleus.

Nucleus

** Still use this model when diagramming an element

Chadwick’s Quantum Mechanical (1932)

Current Theory!!!

Instead of being in fixed energy levels orbiting the nucleus, the QMT says that the electrons orbit in an electron cloud. “The modern model describes an atom as consisting of a nucleus that contains protons & neutrons, surrounded by a cloudlike region of moving electrons.”

(Although when we draw an atomic model, we still use Bohr’s theory in

• rder to see the energy levels)

TED Talk

• Click the link above and watch the short clip

with headphones or LOW volume.

• If the link doesn’t work, it’s OK! - you can

still summarize your learning from the notes at the bottom of your worksheet 