Chapter 3 Atoms The Building Blocks of Matter

Chapter 3 Atoms: The Building Blocks of Matter

3. 2 The Structure of the Atom Objectives • Summarize the observed properties of cathode rays that led to the discovery of the electron. • Summarize the experiment carried out by Rutherford and his co-workers that led to the discovery of the nucleus. • List the properties of protons, neutrons, and electrons. • Define atom.

3. 2 The Structure of the Atom • An atom is the smallest particle of an element that retains the chemical properties of that element. • The nucleus is a very small region located at the center of an atom. • The nucleus is made up of at least one positively charged particle called a proton and usually one or more neutral particles called neutrons.

3. 2 The Structure of the Atom, continued • Surrounding the nucleus is a region occupied by negatively charged particles called electrons. • Protons, neutrons, and electrons are often referred to as subatomic particles.

3. 2 The Structure of the Atom Properties of Subatomic Particles

3. 2 The Structure of the Atom Democritus “The universe is composed of two elements: the atoms and the void in which they exist and move. ” Everything is composed of "atoms", which are physically, but not geometrically, indivisible; that between atoms, there lies empty space; that atoms are indestructible, and have always been and always will be in motion; that there is an infinite number of atoms and of kinds of atoms, which differ in shape and size. Of the mass of atoms, Democritus said, "The more any indivisible exceeds, the heavier it is".

3. 2 The Structure of the Atom John Dalton’s Atomic Theory 1. All elements are composed of tiny indivisible particles called ATOMS. (Agreed with Democritus theory from 2000 years earlier) 2. Atoms of the same element are identical. Atoms of any one element are different from atoms of other elements. 3. Atoms of different elements may mix physically or combine chemically in simple whole number ratios to form compounds. 4. Chemical reactions occur when atoms are separated, joined or rearranged. Atoms of one element are never changed into elements of another element as result of a chemical reaction.

3. 2 The Structure of the Atom Discovery of the Electron • Cathode Rays and Electrons • Experiments in the late 1800 s showed that cathode rays were composed of negatively charged particles. • These particles were named electrons.

3. 2 The Structure of the Atom Discovery of the Electron, continued • Charge and Mass of the Electron • Joseph John Thomson’s cathode-ray tube experiments measured the charge-to-mass ratio of an electron. • Robert A. Millikan’s oil drop experiment measured the charge of an electron. • With this information, scientists were able to determine the mass of an electron.

3. 2 The Structure of the Atom JJ Thomson discovered the electron by using the cathode ray tube. Cathode tube. Ray travels from – to +.

3. 2 The Structure of the Atom Discovery of the Electron, continued

3. 2 The Structure of the Atom JJ Thomson cont • Plum pudding model of the atom. Electrons were set in + charged matter like pudding.

3. 2 The Structure of the Atom Robert Millikan Oil Drop Experiment involved measuring the force on oil droplets in a glass chamber sandwiched between two electrodes, one above and one below. With the electrical field calculated, he could measure the droplet's charge, the charge on a single electron being (1. 592× 10− 19 C).

3. 2 The Structure of the Atom Oil Drop Experiment

3. 2 The Structure of the Atom Discovery of the Atomic Nucleus • More detail of the atom’s structure was provided in 1911 by Ernest Rutherford and his associates Hans Geiger and Ernest Marsden. • The results of their gold foil experiment led to the discovery of a very densely packed bundle of matter with a positive electric charge. • Rutherford called this positive bundle of matter the nucleus.

3. 2 The Structure of the Atom Ernest Rutherford determines the nucleus is positively charged. Proves “Plum Pudding Theory” wrong. He came up with a model suggesting that the atom was mostly empty space except in the center where a positively charged mass was concentrated (the cause for the great deflection angles).

3. 2 The Structure of the Atom Gold Foil Experiment

3. 2 The Structure of the Atom Gold Foil Experiment on the Atomic Level

3. 2 The Structure of the Atom Composition of the Atomic Nucleus • Except for the nucleus of the simplest type of hydrogen atom, all atomic nuclei are made of protons and neutrons. • A proton has a positive charge equal in magnitude to the negative charge of an electron. • Atoms are electrically neutral because they contain equal numbers of protons and electrons. • A neutron is electrically neutral.

3. 2 The Structure of the Atom Composition of the Atomic Nucleus, continued • The nuclei of atoms of different elements differ in their number of protons and therefore in the amount of positive charge they possess. • Thus, the number of protons determines that atom’s identity.

3. 2 The Structure of the Atom Forces in the Nucleus • When two protons are extremely close to each other, there is a strong attraction between them. • A similar attraction exists when neutrons are very close to each other or when protons and neutrons are very close together. • The short-range proton-neutron, proton-proton, and neutron-neutron forces that hold the nuclear particles together are referred to as nuclear forces.

3. 2 The Structure of the Atom The Sizes of Atoms • The radius of an atom is the distance from the center of the nucleus to the outer portion of its electron cloud. • Because atomic radii are so small, they are expressed using a unit that is more convenient for the sizes of atoms. • This unit is the picometer, pm.