Atomic Theory & Structure

Early Greek Theories Democritus 400 B.C. – Democritus thought matter could not be divided infinitely Called nature’s basic particle the atom. This led to the idea of atoms in a void 350 B.C. – Aristotle said matter was continuous Modified an earlier theory that matter was made of four “elements”: Earth, air, fire, water He was wrong, but his opinion was accepted for 2000 years Neither view was supported by evidence Aristotle

Foundation of the Atomic Theory In the 1790s, scientists began to focus on quantitative analysis, which led to several discoveries: Law of Conservation of Mass = Mass is neither created nor destroyed during chemical reactions or physical changes. Law of Definite Proportions = compounds contain the same elements in the same proportions, regardless of sample size

Dalton’s Atomic Theory 1808 – John Dalton proposed an explanation based on experimentation All matter is made of atoms Atoms of an element are identical Each element has different atoms Atoms of different elements combine in constant ratios to form compounds Atoms are rearranged in chemical reactions

Thomson Model Scientists observed that rays were deflected away from a negatively charged object. This is the Cathode Ray Experiment Thomson noted that these negative particles were a part of all atoms.

Adding Electrons to the Atomic Model Dalton’s “Billiard ball” model (1800-1900) Atoms are solid and indivisible. Thomson “Plum pudding” model (1900) Negative electrons in a positive framework. The Rutherford model (around 1910) Atoms are mostly empty space. Negative electrons orbit a positive nucleus.

Radioactive substance Ernest Rutherford (1911) Shot alpha (α) particles at a sheet of gold foil Zinc sulfide screen Thin gold foil Lead block Radioactive substance

Rutherford Most particles passed through. So, atoms are mostly empty. Some positive -particles deflected or bounced back! Thus, a “nucleus” is positive & holds most of an atom’s mass.

Early 1900-1913 What was happening in the world? Speed limits invented President McKinley is shot. Queen Victoria dies. First vacuum cleaner created. Nobel Prize is created. Post office begins working. Serbia and Greece declare war on Bulgaria. Titanic sinks 1912.

Neils Bohr (1913) Electrons orbit the nucleus in “shells”. Neils Bohr proposed a model in which electrons surround the nucleus like the planets do the sun. Bohr Model

Cloud Model 1920’s Electrons don’t orbit, they surround nucleus in a cloud. This is where electrons are likely to be found—the more energy an electron has, the farther from the nucleus it is.

To Summarize 1. Democritus—Ball Model 2. Dalton—Ball Model, 5 key points 3. Thomson—Plum Pudding Model, Found Electrons 4. Rutherford—Gold Foil Experiment, Found Nucleus 5. Bohr—Orbitals, Found that electrons travel around nucleus 6. Current Theory—Cloud Model

Draw the Model Progression…

Structure of the Atomic Nucleus All atomic nuclei are composed of protons (+) and neutrons (neutral) and electrons (-) Subatomic particle masses: Electron = 9.109 x 10-31 kg Proton = 1.673 x 10-27 kg Neutron = 1.675 x 10-27 kg

Atomic Nucleus Nuclei are held together by nuclear forces Short-range forces that hold the nucleus together

The Sizes of Atoms Atomic radius = the distance from the center of the nucleus to the outer portion of its electron cloud Expressed as picometers (pm) 1pm = 10-12 m Atomic radii range from 40 to 270 pm Nuclear radius is about 0.001 pm

Observing & Collecting Data Observing = using the senses to get information Data collection may be: Qualitative  descriptive data Color, texture, odor, etc Quantitative  numerical Measurements of mass, distance, volume, temperature, etc