Models of Atomic Structure 1 John Dalton 1766

Models of Atomic Structure 1

John Dalton (1766 -1844) • Dalton was the first to define an element • Element = a pure substance made up of one type of particle or atom. • Each element has its own distinct properties and cannot be broken down into simpler substances by means of chemical change • Dalton’s vision of the atom is also called the “Billiard Ball” model because he thought that the atom was a solid sphere of matter 2

Dalton’s Atomic Theory: • • all matter is made up of small particles called ATOMS atoms cannot be created, destroyed or divided into smaller particles all atoms of the same element are identical in mass and size, but they are different in mass and size from the atoms of other elements compounds are created when atoms of different elements link together in definite proportions 3

Dalton’s Theory continued…. • Dalton thought of atoms as tiny spherical objects which had hooks on them • These “hooks” allowed atoms to combine with other atoms • This model of an atom has no protons, electrons, or neutrons • Today we know that atoms can be destroyed via nuclear reactions • Also, there are different kinds of atoms (differing by their masses, not chemical properties) within an element that are known as "isotopes" 4

William Crookes (1832 -1919): • Using a gas discharge tube, he noticed that a positively charged pinwheel (“cathode”) would rotate when an electric charge was passed through the tube, • Crookes believed that this electric charge (called cathode rays) must have mass as well as motion • These cathode rays were thought to be made of fast moving objects which he called ‘corpuscles’ (tiny bits of matter) 5

Cathode Rays • A cathode ray tube (CRT) is a hollow vessel with an electrode at either end. • The tube is evacuated and then partially filled with a gas. • When high voltage is applied to the electrodes of a cathode ray tube, a cathode ray flows from the negative electrode (cathode) to the positive electrode (anode). This causes the gas to glow. • Since glowing gas originates from the cathode, it is referred to as a “Cathode Ray”. • The cathode ray was assumed to be a beam of negatively charged particles that we now know as electrons. 6

The Cathode Ray 7

J. J. Thomson (1856 -1940) • Thomson took Crookes’ work and created a new experiment • He designed an experiment to investigate how these charged corpuscles would move in an electric field • He found that the corpuscles were attracted towards positively charged plates therefore the corpuscles must be negatively charged • Called them ELECTRONS Video Demo 8

Protons • Thomson conducted similar experiments, this time with anode rays and negatively charged plates • Anode rays were attracted to the negatively charged plates - therefore they must contain positively charged particles • Thomson called these particles PROTONS 9

Thomson’s Model • Thomson pictured the atom as a combination of protons and electrons • He thought that the electrons (raisins) were embedded in a sphere of positively charged protons (bun) • The electrons were thought to be mobile particles that leave the atom when energized by an outside source such as heat, light, or electricity • Called the “Raisin Bun” model 10

Thomson’s inferences. . . • All atoms contain both protons and electrons. • All protons are identical, all electrons are identical. However, electrons differ from protons. • Electrons have a negative charge, protons have a positive charge. • An electron has the same amount of charge as a proton, even though the charges are opposite in kind. • A proton has much more mass than an electron. 11

The Electron Microscope How do Electron Microscopes Work? • Electron Microscopes (EMs) use a focused beam of electrons instead of light to "image" the specimen and gain information as to its structure & composition • A stream of electrons is formed and accelerated toward the specimen using a positive electrical potential • This stream is confined and focused into a thin, monochromatic (single colour/wavelength) beam. • This beam is focused onto the sample using a magnetic lens • Interactions occur inside the irradiated sample, affecting the electron beam From www. sciencemuseum. org. uk/on-line/electron/section 4/sem. asp 12

Rutherford’s early work. . . • • • 1898 - heard about the discovery of radioactivity by Marie and Pierre Curie Did his own experiments with radiation and discovered that there were three different types Alpha particles - made of matter (bare helium nuclei) with 4 times the mass of a proton and two times the charge of a proton Beta particles - made of matter, the mass of an electron and the same negative charge as an electron Gamma rays - made of energy with no mass or charge 13

Rutherford’s gold foil experiment • • • 1909 - Rutherford did further studies on the atom Used alpha particles shot from a chunk of polonium Alpha particles were shot at a piece of gold foil Expected that most of the alpha particles would pass though the gold foil and strike the fluorescent screen set up behind the foil (why? He assumed that there were fairly large spaces between the atoms) Found that some of the alpha particles rebounded from the foil much like a ball would bounce off of a wall 14

Rutherford’s a-particle experiment: Video 15

Radiation Beta Particles (-) • Fast moving electrons Gamma Rays • Energy wave Alpha Particles (++) • 4 x the mass of a proton Rutherford used radiation and his gold foil experiment to postulate that atoms must contain: - - + ++ - - A Nucleus - a very positive, dense and massive core An Electron Cloud - a low density, negative envelope around the nucleus 16

Rutherford’s Inferences • There must be a small dense, positively charged particle within the gold foil (to account for the rebounding of a small percentage of the particles) • the atoms must be mostly empty space (to account for the high percentage of particles that simply pass straight through the foil) 17

Rutherford’s Conclusions. . . • Atoms are mostly EMPTY SPACE with a small, dense, positively charged core which he called the NUCLEUS • The nucleus was surrounded by a clouds of electrons that was very large in volume, but very light in mass compared to the nucleus • This cloud of electrons was negatively charged 18

Rutherford’s Conundrum • Rutherford found that gold atoms have 79 protons in their nuclei, but that this made up less than half of the mass he calculated for their nuclei • Thought that the protons could not be alone in the nucleus and that there had to be another type of particle that was uncharged (“neutral”) in the nucleus • These neutral particles would have roughly the same mass as a proton • Rutherford’s inference was confirmed in the 1930’s by James Chadwick’s discovery of the NEUTRON • Neutrons are important to the structure of the atom because they counteract the repulsive forces between the protons within the nucleus • If there were no neutrons, all of the protons in the nucleus would repel each other and the nucleus would fall apart! 19

Niels Bohr • So far we have seen that Rutherford’s experiment gave rise to the nuclear model of the atom • In 1912, Niels Bohr began working with Ernest Rutherford • He believed that Rutherford’s model was incomplete • Why weren’t the negatively charged electrons attracted to the positively charged nucleus? 20

Bohr’s Experiments… • Bohr looked at light emitted from gas discharge tubes, specifically hydrogen • He noticed that the light spectrum for hydrogen showed only four lines, rather than a continuous spectrum 21

Bohr’s Conclusions… • Bohr decided that the only way the spectrum could show four lines would be if the electrons were forced to stay in fixed orbits, much like the planets around the sun • The only way the atom could give off light would be if the electrons jumped from one orbit to another • Bohr decided these regions were three dimensional and sphere like • These regions are known as electron shells 22

The Bohr-Rutherford Model • This model combines the ideas of both Rutherford and Bohr • It suggests that the atom contains a dense positive core (the nucleus) and that the electrons exist in orbits or electron shells 23

Introducing Isotopes • We have seen that atoms of the same element are all similar, each having the same number of electrons, protons, and neutrons • This would also tell us that the atomic mass of each of these elements would be the same • However, scientists studying radioactivity found that some substances had different atomic masses, while maintaining the same chemical properties • They concluded that these must be different atomic forms of the same element • These atoms that differ in atomic mass are called Isotopes 24

Isotopes • Atoms with the same number of electrons in their orbits, and protons in their nuclei, but different numbers of neutrons • Since these atoms have the same number of protons, they have the same atomic number and are atoms of the same chemical element, but because of the different number of neutrons they differ in their mass number 25

Isotopes of Hydrogen and Carbon 6 Protons 6 Neutrons 6 Protons 7 Neutrons 6 Protons 8 Neutrons 26

Half-life • The half life of an isotope is the amount of time it takes for half of the atoms to decay into a more stable form • There are many naturally occurring isotopes that exist around us because their half-lives are longer than the age of the earth! • E. g Uranium – 238 has a half life of 4. 5 billion years • Most isotopes have short half lives and must therefore be made in the lab to study or use 27

Radioactive decay • Alpha decay • • • Many nuclei are radioactive. This means they are unstable, and will eventually try to decay to more stable isotopes They do this by emitting a particle which allows them to enter a lower energy state This process occurs until a stable nucleus is reached There are 3 common types of decay – alpha, beta, and gamma The difference between them is the particle that is emitted by the nucleus – Nucleus emits an alpha particle (helium nucleus) – Alpha particles are very useful because the don’t travel far in air before being absorbed • Beta decay – Most often emits an electron – Neutron is changed into proton – More penetrating than alpha particles, but much less than gamma particles • Gamma decay – Nucleus changes from high to low energy level – Energy is released as light – Very penetrating 28

Carbon Dating "Radiocarbon (C-14) Dating, " Microsoft® Encarta® Encyclopedia 2000. 29