Model vs. Theory A Model is a human construct to help us better understand real world systems. A Theory is an explanation of the natural world that can incorporate laws, hypotheses and facts. – It is testable, and can be refined or rejected. A model is used to explain a theory

Empedocles: Greece 450 B.C. Everything is made up of these four elements Different types of matter are combinations of these basic elements Based on philosophy, not experiment Model accepted for 2000 years.

Atoms and their composition The particle theory of matter was supported as early as 400 B.B. by certain Greek thinkers, such a Democritus. He proposed that a substance could be subdivided into smaller and smaller portions, eventually to a stage where particles could not be divided any further. He called nature ’ s basic particle an atom, based on the Greek word meaning indivisible.

The Alchemists The alchemists was a group, which existed from the time of Aristotle to the 1800 ’ s. Their prime aim was to change base metals to gold and even though it couldn ’ t be done, the alchemists are credited with the development of science. They gave science the principles of observation and experimentation.

John Dalton In 1808, John Dalton proposed an explanation for the law of conservation of mass, the law of definite proportions, and the law of multiple proportions. He reasoned that elements were composed of atoms and that only whole numbers of atoms can combine to form compounds.

Dalton ’ s Atomic Theory All mater is made up of tiny particles called atoms. An atom cannot be created, destroyed, or divided into smaller particles. The atoms of one element cannot be converted into atoms of any other element. All the atoms of one element have the same properties, such as mass and size. These properties are different from the properties of the atoms of any other element. Atoms of different elements combine in specific proportions to form compounds.

Small Dense Sphere

The Structure of the Atom The atom is defined as the smallest particle of an element that retains all the chemical properties of that element. In the late 1800 ’ s, scientists believed that atoms were electrical in nature. Sir Crookes set out to prove the electrical nature of atoms by using a cathode-ray tube.

In a cathode-ray tube, electrons travel as a ray through a gas at low pressure from the cathode (negative electrode) to the anode (positive electrode). Crookes attached the cathode and the anode to a power source. When he turned the power on, a glowing beam appeared between the electrodes.

He tested the glowing beam for magnetic effect by using a magnet. When the magnet was placed near the tube, the cathode ray was deflected the same way a wire carrying electric current would. Electric current was known to have a negative charge, so Crookes determined that the cathode ray was also negative.

Crookes also noticed a shadow behind the anode, which meant that the source of the ray was the cathode (the ray was coming from the negative electrode). He determined with certainty, that the ray must have a negative charge.

Joseph John Thomson In 1897, J. J. Thomson performed a similar experiment. He used a cathode-ray tube with an anode and a cathode, but at one end he put a florescent screen. When the cathode ray hit the screen, the ray left a mark. When he used a heavier gas in the tube, the ray left an even bigger mark. He concluded the larger the mark on the screen, the larger the negative charge.

Thomson later discovered that beside the cathode ray was another ray. The second ray was positive. Thomson discovered positively charged particles. Thomson ’ s improved model of the atom had negative and positive particle. This atom was commonly referred to as the plum pudding model.

Robert Millikan In 1909, Robert Millikan determined the mass of an electron to be x kg. His experiment also confirmed that the electron carried a negative charge. His experiment was called the oil drop experiment.

Becquerel In 1896, Becquerel was experimenting with black photography paper and uranium. He would spread the uranium on the paper and expose the paper/uranium to sunlight. The paper would be overexposed as expected, but under the uranium was not exposed. More importantly, around the spots where the uranium was, there were rays. Becquerel discovered radioactivity.

Ernest Rutherford In 1911, Rutherford took the research on radioactivity even further. He found alpha particles (high speed positively charged particles). He also found beta particles (high speed negatively charged particles). Rutherford ’ s experiment is called the gold foil experiment.

In the gold foil experiment, Rutherford shot alpha particles at a thin piece of gold foil. Surrounding the gold foil was a zinc sulfide screen. Each time an alpha particle hit the gold foil, a flash of light was produced. Some of the particles passed through the metal and hit the screen as expected. Some of the particles deflected a bit, which was also expected. What caught Rutherford ’ s eye was the particles that came back and hit the screen in front of the foil.

Rutherford rethought the model of the atom. He put a cluster of positive particles in the center of the atom (nucleus) and surrounded it with negative particles. When the emission hit the negative particles, it went straight through to the screen. When the emission came near the nucleus, it was deflected a bit. It was when the emission hit the nucleus that it bounced right back.

In 1932, Irene Joliet Curie and her husband, Frederic, bombarded Beryllium with alpha particles. A high powered beam was formed. Later in 1932, Sir James Chadwick tested Mme. Curie ’ s high powered penetrating beam. He put a magnet around the beam and found that it did not react. Chadwick discovered a neutral particle in the atom. The neutral particle is the neutron and it has relatively the same mass as the proton, but does not have a charge.

Summary of the Composition of the Atom ParticleSymbolChargeMass # Relative Mass Actual Mass (kg) Protonp+p x Electrone-e x Neutronn0n x

Need for a New Model The Rutherford model of the atom was an improvement over previous models, but it was incomplete. It did not explain where the atom ’ s negatively charged electrons are located in the space surrounding the positively charged nucleus.

The Bohr Model of the Atom Scientists of the 19 th C lacked the concepts necessary to explain line spectra. Even in the first decade of the 20 th C, a suitable explanation proved elusive. This changed in 1913 when Niels Bohr proposed a new model for the hydrogen atom.

Bohr ’ s Model of the Atom

According to Bohr ’ s model, the electrons can circle the nucleus only in allowed paths called orbits. When the electron is in one of these orbits, the atom has a definite, fixed energy. The electron, and therefore the atom, is in its lowest energy state when the electron is in the orbit that is closest to the nucleus The energy of the electron is higher when it is in orbits that are farther from the nucleus. A state in which an atom has a higher energy than it has in its ground state is an excited state.

The electron orbits are called energy levels, and can be compared to step on a ladder. When you are standing on the ladder, your feet are on one step or another, they cannot stand somewhere in between. In the same way, an electron must be in one of the energy levels or another, but cannot be somewhere in between.

When an atom is in an exited state, its electron jumps to a higher energy level. When it falls back down from the excited state, the electron drops down to the lower energy level. In this process, a photon is emitted. This photon has an equal amount of energy to the amount of energy difference between the higher energy level and the lower energy level.

Max Planck Max Planck, a German scientists, was experimenting with the emission of light by hot objects. He proposed that the hot object did not emit a constant energy, which would be expected if the energy was in waves. Instead he suggested that the energy emitted from a hot objects was in small and specific amounts called quantum.