Thursday September 27, 2012 (Begin Atomic Structure Research Project)

List six scientists that were involved in the quest to understand the structure of the atom. John Dalton Ernest Rutherford JJ Thompson Neils Bohr Max Planck Albert Einstein Bell Ringer Thursday,

Announcements None today

Assignment Currently Open Summative or Formative? Date IssuedDate Due Date Into GradeSpeed Final Day QUIZ 3S39/14 9/2010/2 Quiz – Science Lab Safety S49/17 WS – Categories, Properties, and Phases of Matter F129/179/21 Quiz – Matter Vocabulary S59/18 QUIZ 4S69/21 10/5 Lab – Physical Properties of Matter F139/24

A Brief History of the Discovery of Atomic Structure

AristotleDemocritus Ancient Philosophers

Aristotle vs. Democritus More than 2000 years ago in ancient Greece: Aristotle believed in four elements: Earth, Air, Fire, and Water. Democritus believed that matter was made of small particles he named “atoms”. Both Aristotle and Democritus used observation and inference to explain the existence of everything.

Important Work in the Study of Atomic Structure Dalton’s five key points Thomson’s cathode ray experiment that lead to the discovery of electrons Rutherford’s gold foil experiment that found the atomic nucleus Bohr’s discovery of fixed electron paths Plank’s discovery that objects emit energy in small, specific amounts called “quanta” Einstein’s discovery that electromagnetic radiation has a dual wave-particle nature

Research Project Philosophers and scientists have been interested in the structure of matter for over 2,000 years. In the early 1800s, English school teacher John Dalton provided the first scientific evidence for the atom, the basic particle of matter. Since that time, many other scientists have conducted research and made important discoveries in the field of atomic structure. The scientists that we will consider the works of in this project are Dalton, J.J. Thomson, Ernest Rutherford, Neils Bohr, Max Planck, and Albert Einstein. History of Atomic Structure Research

Research Project Your assignment is to work with your lab partner to make a poster illustrating and describing the contributions of the six scientists listed above to the understanding of the structure of the atom. This poster is due on Friday, October 5th no later than 5:00 pm. Your team should begin by organizing your thoughts and constructing a draft of your poster on a regular sheet of paper. This draft should be shown to Mr. C before you actually begin work on the poster board. History of Atomic Structure Research

The Particle Theory Revisited English school teacher John Dalton, in 1808, described atoms as tiny particles that could not be divided. Dalton thought each element was made of its own kind of atom. He was building on the ideas of Democritus in ancient Greece.

John Dalton Recall that in 1808, an English schoolteacher named John Dalton reasoned that: elements were composed of atoms. only whole numbers of atoms can combine to form compounds. Expanding on these notions, Dalton published his theory on atom and their structure, characteristics, and properties.

The Five Key Points of Dalton’s Atomic Theory 1.All matter is composed of extremely small particles called atoms. 2.Atoms of a given element are identical in size, mass and other properties: atoms of different elements differ in these ways. 3.Atoms cannot be subdivided, created or destroyed. 4.Atoms of different elements combine in simple whole-number ratios to form compounds 5.In chemical reactions, atoms are combined, separated or rearranged.

Later Alterations to Dalton’s Atomic Theory Atoms are no longer thought of as indivisible. Atoms have in more recent years been divided into smaller particles: protons, neutrons and electrons. Neutrons have been further divided into quarks and gluons. We now know that are atoms of a given element do not necessarily have the same mass. Elements consist of various isotopes – same number of protons but differing numbers of neutrons.

The Modern Atomic Theory Dalton turned Democritus’ idea into a scientific theory. The important concepts of all matter being composed of atoms and atoms of any one element differing from atoms of another element remain unchanged.

What is an atom? An atom is the smallest particle of an element that retains the chemical properties of that element. An atom is the smallest particle of matter – if you break an atom into smaller pieces, those pieces are not considered to be matter. All atoms consist of two “regions” the nucleus the electron cloud

What Does an Atom Look Like?

We can only attempt to illustrate the structure of the atom in a simplified way.

The Discovery of the Electron In England, in 1897, J. J. Thompson discovered that electrons were smaller particles of an atom and were negatively charged. Thompson knew atoms were neutrally charged, but couldn’t find the positive particle.

The Atom’s Electron Cloud The electron cloud contains electrons with a negative (-) electromagnetic charge. The electron cloud is very large compared to the nucleus. If the electron cloud of an atom was the size of this room, the nucleus would be the size of a grain of sand!

The Discovery of the Electron J. J. Thompson In the late 1800s, experiments were conducted in cathode-ray tubes – an electric current was passed through various gases at different temperatures. The surface of the tube opposite the cathode glowed. An object placed between the cathode and the opposite end of the tube cast a shadow on the glass. A paddle wheel placed on rails between the electrodes rolled along the rails from cathode to anode. This supported the existence of cathode rays.

The Discovery of the Electron Thompson hypothesized that the glow was caused by a stream of particles, and he called them cathode rays. With a current, the rays traveled through the tube from cathode to anode.

The Discovery of the Electron

The Atom’s Nucleus The nucleus of an atom is very small compared to the atom as a whole. The nucleus contains two different types of particles. protons having a positive (+) electromagnetic charge neutrons having no (0) electromagnetic charge

Atomic Structure I Again in England, in 1911, Ernest Rutherford conducted an experiment to isolate the positive particles in an atom. He decided that atoms were mostly empty space, but had a dense central core. He knew that atoms had positive and negative particles, but could not decide how they were arranged.

Ernest Rutherford discovered the nucleus of the atom with his famous gold foil experiment

The 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.

Atomic Structure II Niels Bohr, 1913 in England, proposed that electrons travelled in fixed paths around the nucleus. Scientists still use the Bohr model today to show the number of electrons in each orbit around the nucleus. Bohr was trying to show why the negative electrons were not sucked into the nucleus of the atom.

The Bohr Model of the Atom Electrons circle the atom only in allowed paths, or orbits. Orbits have a definite, fixed energy level. The energy level is lowest in the orbit closest to the nucleus. Energy of electrons is higher in orbits successively farther from the nucleus. An electron cannot exist between levels! NO

The Bohr Model of the Atom Electrons in a given orbit have a fixed energy level, but if they gain energy they can move to a higher energy level. When the electron drops back down to its original, lower energy level, it emits a photon of light (of a characteristic color for that particular element.)

Neils Bohr and the Bohr Model of the Atom Niels Bohr, 1913 in England, proposed that electrons travelled in fixed paths around the nucleus. In 1922, he was awarded the Nobel Prize in physics "for his services in the investigation of the structure of atoms and of the radiation emanating from them.” While at Manchester University, Bohr had adapted Rutherford's nuclear structure to Max Planck's quantum theory, and so obtained a model of atomic structure which, with later improvements (mainly as a result of Heisenberg's concepts) remains valid to this day.

The Bohr Model of the Atom Bohr introduced the theory of electrons traveling in orbits around the atom's nucleus, the chemical properties of each element being largely determined by the number of electrons in the outer orbits of its atoms. Electrons circle the atom only in allowed paths, or orbits. Orbits have a definite, fixed energy level. The lowest energy level is in the orbit closest to the nucleus. Energy of electrons is higher in orbits successively farther from the nucleus. An electron cannot exist between levels! NO

The Bohr Model of the Atom Bohr also introduced the idea that an electron could drop from a higher-energy orbit to a lower one, in the process emitting a photon (a package of light, or quantum) of discrete energy. Electrons in a given orbit have a fixed energy level, but if they gain energy they can move to a higher energy level. When the electron drops back down to its original, lower energy level, it emits a photon of light (of a characteristic color for that particular element.)

The Electromagnetic Spectrum

What are examples of electromagnetic radiation? radio waves, microwaves, infrared rays, visible light, ultraviolet light, x-rays, gamma rays What information is represented on the electromagnetic spectrum chart? wavelength, frequency, energy Which form of EM radiation has the longest wavelength? radio waves Which form of EM radiation has the shortest wavelength? gamma rays The Electromagnetic Spectrum

Which form of EM radiation has the highest frequency? gamma rays Which form of EM radiation has the lowest frequency? radio waves Which form of EM radiation has the highest energy? gamma rays Which form of EM radiation has the lowest energy? radio waves Where is visible light located on the EM radiation chart? between infrared rays and ultraviolet light The Electromagnetic Spectrum

A German physicist named Max Planck was studying the emission of light by hot objects. He proposed that a hot object does not emit electromagnetic energy continuously, as would be expected if the energy emitted were in the form of waves. Instead, Planck suggested that the object emits energy in small, specific amounts called “quanta.” A quantum is the minimum quantity of energy that can be lost or gained by an atom. The Particle Description of Light

Planck proposed the following relationship between a quantum of energy and the frequency of radiation. E = hν In the equation, E is the energy, in joules, of a quantum of radiation, v is the frequency of the radiation emitted, and h is a fundamental physical constant now known as Planck’s constant h = × 10 −34 J¢s. The Particle Description of Light

In 1905, Albert Einstein expanded on Planck’s theory by introducing the radical idea that electromagnetic radiation has a dual wave-particle nature. While light exhibits many wavelike properties, it can also be thought of as a stream of particles. Each particle of light carries a quantum of energy. Einstein called these particles “photons.” The Particle Description of Light

A photon is a particle of electromagnetic radiation having zero mass and carrying a quantum of energy. The energy of a particular photon depends on the frequency of the radiation. E photon = hv Einstein explained the photoelectric effect by proposing that electromagnetic radiation is absorbed by matter only in whole numbers of photons. The Particle Description of Light

In order for an electron to be ejected from a metal surface, the electron must be struck by a single photon possessing at least the minimum energy required to knock the electron loose. According to the equation E photon = hv, this minimum energy corresponds to a minimum frequency. If a photon’s frequency is below the minimum, then the electron remains bound to the metal surface. Electrons in different metals are bound more or less tightly, so different metals require different minimum frequencies to exhibit the photoelectric effect. The Particle Description of Light