Chapter 4 The Structure of an Atom

Early Theories of Matter - Democritus Thought that matter is composed of the empty space through which atoms move Atoms are solid, homogenous, indestructible and indivisible Different kinds of atoms have different shapes The properties of matter are due to the different sizes, shapes and movements of atoms Changes in matter result from the different groupings of atoms – NOT from changing atomic properties

Early Theories of Matter - Aristotle One of the influential philosophers Denied the existence of atoms

Early Theories of Matter – John Dalton All matter is composed of extremely small particles called atoms All atoms of a given element are identical (same size, mass and chemical properties) Atoms cannot be created, divided and/or destroyed Different atoms combine in whole-number ratios to form compounds In a chemical reaction, atoms are separated, combined and/or rearranged.

What is an atom? An atom is the smallest particle of an element that retains its physical properties

Discovering the Electron Sir William Crookes (English Physicist) discovered the electron while working in a cathode ray lab Cathode Rays were streams of charged particles that carried a negative charge He changed both the electrode and type of gas in the cathode ray, but neither affected the cathode ray produced. Therefore, conclusion was negatively charged particles were found in all forms of matter These are called electrons

Discovering the Electron, Cont. Robert Millikan, an American Physicist, determined the charge of an electron (-1) Mass of an electrons: 9.1 x 10-28 = 1/1840 Mass of an hydrogen atom

The Nuclear Atom Rutherford conducted an experiment with gold foil that created the Rutherford Model of an Atom. He concluded that there was a tiny centrally located dense region of an atom called a nucleus, which contains most of the atoms mass.

Atoms The Structure of an Atom

Space surrounding the nucleus Completing the Atom The Atom is made up of three things: Proton: Found in the nucleus and is positively charged Electron: Found in the space surrounding the nucleus and negatively charged Neutron: Found in the nucleus and is neutrally charged Particle Symbol Location Relative Electrical Charge Relative Mass Actual Mass Electron e- Space surrounding the nucleus 1- 1/1840 9.11 x 10-28 Proton p+ Nucleus 1+ 1 1.673 x 10-24 Neutron No 1.675 x 10-24

How Atoms Differ Atomic Number = Number of Protons = Number of Electrons Chemical Name Chemical Symbol Atomic Number Average Atomic Mass

Isotopes and Mass Number Atoms with the same number of protons but differing amounts of neutrons are isotopes Different isotopes have different mass numbers, which are added after the element, to denote the different amount of neutrons For example, Carbon-15 has 6 Protons and 9 Neutrons

How Do You Find the Number of Neutrons? Number of Neutrons = Mass Number – Atomic Number

Mass of Individual Atoms Atomic Mass Unit is defined as 1/12 of a carbon atom Atomic Mass of an element is the weighted average mass of the isotopes Particle Mass (AMU) Electron 0.000549 Proton 1.007276 Neutron 1.008665

Atoms and Their Interactions Elements A substance that can’t be broken down into simpler chemical substances Everything is made up of elements

Half-Life Half-Life The amount of time it takes for half the amount of a substance to dissipate or transform For example, Carbon-15 has a half-life of 2.449 seconds. That means that every 2.449 seconds, half the amount of substance dissipates and/or transforms.

Half-Life, Continued Answer: 3.75 grams. Carbon-15 has a half-life of 2.449 seconds. That means that every 2.449 seconds, half the amount of substance dissipates and/or transforms. If you start with 30 grams of Carbon-15, how much will you have left after 7.347 seconds? Determine the number of half-lives. (7.347 seconds / 2.449 seconds) = 3 Half-Lives Complete the math for each of the half-lives. The “0” half-life is NOT the first half-life, but instead, the initializing moment – the amount of substance at “0” seconds, before the clock starts. Label the half-lives and determine the amount of substance in each. 0 – 30 grams 1 – 15 grams (30 grams / 2) 2 – 7.5 grams (15 grams / 2) 3 – 3.75 grams (7.5 grams / 2) Answer: 3.75 grams.