Development of Atomic Theory

The Greek Philosopher Democritus Philosophers were great thinkers, but not true scientists. Democritus lived from 460-370 BC.

The Greek Philosopher Democritus Developed the “discontinuous” idea of matter with his teacher, Leucippus. If we keep splitting matter in half, we eventually reach a particle that can not be broken down further. Atomos is Greek for “uncuttable” or “indivisible”. The idea of an atom implies that matter contains a lot of empty space!

The Greek Philosopher Aristotle Aristotle was uncomfortable with the idea that matter contains so much empty space. Believed that matter could be continuously divided without end (the “continuous” idea of matter). There is no need for empty space. There are no atoms. All matter is made of the natural elements (earth, water, air, fire, and aether, the heavenly substance)

Real Science: Evidence based ideas. John Dalton (1766-1844) proposed his Atomic Theory in 1803: 1) All matter consists of indivisible atoms. 2) All atoms of a given element are identical. 3) Different elements have different types of atoms.

Real Science: Evidence based ideas. John Dalton’s Atomic Theory also includes: 4) Law of Multiple Proportions: Atoms always combine in small, whole number ratios to produce compounds. (ex. H2O and H2O2) Why does this support the existence of atoms? H2O

Real Science: Evidence based ideas. John Dalton’s Atomic Theory also includes: 5) Law of Definite Composition: A given compound ALWAYS contains the same elements in the same proportions, by mass (for example - H2O). Why does this support the existence of atoms? 89% oxygen by mass 11% hydrogen by mass ALWAYS!

Real Science: Evidence based ideas. 6) Law of Conservation of Mass: In a chemical reaction the mass of the reactants equals the mass of the products so mass is conserved. (from Antoine Lavosier - 1789) How would Dalton explain why this law is always true for chemical reactions?

William Crookes (1848-1923) Invented a tube (cathode ray tube) that emitted strange charged particles. These particles were later shown to be electrons. The atom must contain subatomic particles.

Discovery of Subatomic Structure William Crookes (1848-1923) The existence of electricity implies that atoms must contain negative charges. Crooke developed the cathode ray tube, which produces electrons.

Discovery of Subatomic Structure J.J. Thomson (1856-1940) Recognized that atoms are electrically neutral. For every electron, there must be one proton of equal (but opposite) charge. Plum-Pudding Model

Robert Millikan (1868-1953) Measured the charge of an electron Used the charge/mass ratio from Thomson to then calculate the mass of an electron! Mass of an electron is = 1/2000 AMU Charge of an electron = -1 Unit

Discovery of Subatomic Structure E. Rutherford (1871-1937) J.J. Thomson’s student. Performed the “gold foil” experiment.

Discovery of Subatomic Structure Rutherford’s Gold Foil Experiment

Most of the positively charged a particles went straight through the gold  atoms are mostly empty space. Many a particles were deflected  - charges on outside. Some a particles came straight back!  small + charge at center.

Rutherford’s Model The electrons orbit around the tiny, dense, positively charged nucleus. Rutherford placed all electrons at approximately the same distance from the nucleus so all have the same energy.

Discovery of the Neutron James Chadwick (1891-1974) Rutherford’s co-worker. Knew that atoms are actually “heavier” than they should be if you just add the mass of the protons and electrons together There must be a missing particle! Their neutral nature made neutrons difficult to detect but he did detect them by an ingenious experiment. Neutrons are like glue that holds protons together.

The discovery of the 3 subatomic particles was complete by 1930.

Electrons and Light Observations of light led to the development of the next atomic model – The Bohr Model – the first “quantum” model. Light is produced by moving electrons.

Beyond Rutherford Studies of light (which is produced by moving electrons) soon revealed that not all electrons in an atom have the same energy.

The Atomic Emission Spectrum of Hydrogen (Case in Point!)

Beyond Rutherford Studies of light (which is produced by moving electrons in atoms) suggested that that not all electrons in an atom have the same energy. Scientists reasoned that electrons close to the nucleus might have lower energies, whereas electrons far from the nucleus have higher energies.

Beyond Rutherford The studies of light revealed that electrons can have only specific energies and only certain allowable orbits to represent those energies. In other words, energy is “quantized” in matter (atoms). It can only be absorbed and released in specific amounts, not any amount! Before the quantum model of the atom. The quantum model of the atom rungs of a ladder.

Neils Bohr Neils Bohr (1885-1962) was a Danish physicist that studied electron behavior in atoms and how atoms absorb and release energy in specific amounts called quanta. He added the energy levels to show that electrons in atoms have only certain allowable energy state (orbits), like planets around the sun.

Neils Bohr Each permitted orbits, called an energy level, can hold only a maximum number of electrons: 1st energy level: Max of 2 electrons. 2nd energy level: Max of 8 electrons. 3rd energy level: Max of 18 electrons. For each shell, maximum # electrons = 2n2. Electrons fill the lowest available energy levels before filling higher energy levels!

Neils Bohr The Bohr model explains that electrons moving between energy levels is what produces the atomic emission spectrum. When matter absorbs energy, the electrons jump out to higher energy levels. When they release the energy (as light) they fall back down to lower energy levels!

The Modern Quantum Mechanical Model of the Atom Based on the fact that electrons exhibit significant wave properties Video Clip – Dr. Quantum and the Double Slit Experiment Electrons cannot be located with certainty outside of the nucleus Electrons do not move in predictable paths

The Modern Quantum Mechanical Model of the Atom Electrons are modeled as waves and their positions are determined by calculating high probability regions (orbitals) outside the nucleus where they occur depending upon their energy.