How long have people been interested in understanding matter and its structure? Thousands of years Hundreds of years A few years Never

History of Atomic Structure

Ancient Philosophy Who: Aristotle, Democritus When: More than 2000 years ago Where: Greece What: Aristotle believed in 4 elements: Earth, Air, Fire, and Water. Democritus believed that matter was made of small particles he named “atoms”. Why: Aristotle and Democritus used observation and inferrence to explain the existence of everything.

Democritus Aristotle

Early Greek Theories fire air water earth Democritus 400 B.C. - Democritus thought matter could not be divided indefinitely. This led to the idea of atoms in a void. fire air water earth Aristotle 350 B.C - Aristotle modified an earlier theory that matter was made of four “elements”: earth, fire, water, air. Aristotle was wrong. However, his theory persisted for 2000 years.

Alchemists Who: European Scientists When: 800 – 900 years ago Where: Europe What: Their work developed into what is now modern chemistry. Why: Trying to change ordinary materials into gold.

Alchemic Symbols

Particle Theory Who: John Dalton When: 1808 Where: England What: Described atoms as tiny particles that could not be divided. Thought each element was made of its own kind of atom. Why: Building on the ideas of Democritus in ancient Greece.

John Dalton

John Dalton 1800 -Dalton proposed a modern atomic model based on experimentation not on pure reason. 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 reactions. His ideas account for the Law of Conservation of Mass

The Law of Conservation of Mass Atoms are neither created nor destroyed The Law of Constant Proportions Atoms in compounds are in fixed ratios…CO2 is a different ration than CO.

Discovery of Electrons Who: J. J. Thompson When: 1897 Where: England What: Thompson discovered that electrons were smaller particles of an atom and were negatively charged. Why: Thompson knew atoms were neutrally charged, but couldn’t find the positive particle.

J. J. Thompson

Atomic Structure I Who: Ernest Rutherford When: 1911 Where: England What: Conducted an experiment to isolate the positive particles in an atom. Decided that the atoms were mostly empty space, but had a dense central core. Why: He knew that atoms had positive and negative particles, but could not decide how they were arranged.

Ernest Rutherford

Ernest Rutherford Rutherford shot alpha () particles at gold foil. Zinc sulfide screen Thin gold foil Lead block Radioactive substance path of invisible -particles 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.

Atomic Structure II Who: Niels Bohr When: 1913 Where: England What: Proposed that electrons traveled in fixed paths around the nucleus. Scientists still use the Bohr model to show the number of electrons in each orbit around the nucleus. Why: Bohr was trying to show why the negative electrons were not sucked into the nucleus of the atom.

Niels Bohr

Bohr’s model Electrons orbit the nucleus in “shells” Electrons can be bumped up to a higher shell if hit by an electron or a photon of light. There are 2 types of spectra: continuous spectra & line spectra. It’s when electrons fall back down that they release a photon. These jumps down from “shell” to “shell” account for the line spectra seen in gas discharge tubes (through spectroscopes).

Electron Cloud Model Electrons travel around the nucleus in random orbits. Scientists cannot predict where they will be at any given moment. Electrons travel so fast, they appear to form a “cloud” around the nucleus.

The Atom All matter is composed of tiny particles called Atoms. All atoms are composed of smaller particles: Protons- atomic particles with a positive charge. Neutrons- atomic particles with no charge. Electrons- atomic particles with a negative charge

Atomic Number The atomic number- depends on the number of protons in a type of element. In an electrically neutral atom, the number or electrons is the same at the number of protons. Number of protons = number of electrons Atomic Number Atomic Mass

Atomic Mass Atomic mass is based on the number of protons and neutrons in an atom of an element. A certain element has an atomic mass of 16 and an atomic number of 8. The atomic number equals the number of protons in the element’s atoms. To find the number of protons, subtract the atomic number (8) from the atomic mass (16). 16 8 8 neutrons

Dalton’s Theory All matter is made of atoms. Atoms are indivisible and indestructible. All atoms of a given element are identical in mass and properties Compounds are formed by a combination of two or more different kinds of atoms. A chemical reaction is a rearrangement of atoms.

J.J. Thompson’s Theory Discovered a negatively charged particle in the atom, that he named the electron. Chocolate Chip Cook Model of the Atom. Positive and negative charges are distributed evenly throughout the atom.

Lord Rutherford’s Theory Gold Foil Experiment: Fired alpha particles at a piece of gold foil. Most particles when through the foil, but about 1 in 10,000 got deflected. The new evidence allowed him to formulate a new atomic model with a central nucleus. He suggested that the dense positive charge was in the center and the negatively charged electrons were being held in place by attractive forces. Remember, opposites attract!