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Chapter 4. The Structure of the Atom. Section 4.1. Early Theories of Matter. Greek Philosophies of Matter. A few thousand years ago, people did not know what a controlled experiment was Scientific ideas came not from experimentation, but from intellectual thought
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Chapter 4 The Structure of the Atom
Section 4.1 Early Theories of Matter
Greek Philosophies of Matter • A few thousand years ago, people did not know what a controlled experiment was • Scientific ideas came not from experimentation, but from intellectual thought • Early people thought that matter was compose of earth, water, air, and fire • They also thought that matter could be infinitesimally to smaller and smaller pieces
Democritus and His Theory • Democritus was the first person to propose that matter could not be divided into infinitesimally small pieces • Proposed that • Matter is composed of empty space through which atoms move • Atoms are solid, homogeneous, indestructible, and indivisible • Different kinds of atoms have different shapes and sizes • The different properties of matter are due to the different size, shape, and movement of atoms • Apparent changes in matter result from changes in the way atoms are group together, and not from changes in the atoms themselves
Democritus v. Aristotle • “What holds atoms together?” was a common criticism for Democritus’ theory which he could not explain • Aristotle did not agree with Democritus’ view because the theory clashed with his idea of nature • Aristotle asked how “nothingness,” or empty space could exist and how atoms could move through it, which Democritus could not explain • Because of these challenges, Democritus was eventually rejected by the intellectual community
John Dalton • Used experimentation to back up his atomic theory • All matter is composed of extremely small particles called atoms • All atoms of a given element are identical, having the same size, mass, and chemical properties. Atoms of a specific element are different from those of any other element • Atoms cannot be created, divided into smaller particles, or destroyed • Different atoms combine in simple whole-number ratios to form compounds • In a chemical reaction, atoms are separated, combined, or rearranged
Daltons Theory Cont’d. • By using experimentation to back his theory, Daltons theory became a major step towards our current understanding of the atom • Was Daltons theory completely correct?
Defining Atomic Size • What is the size of an atom? • The world’s population is nearly 7,000,000,000 and a typical penny has nearly 29,000,000,000,000,000,000,000 copper atoms • The diameter of a copper atom is 1.28x10-10m • 7billion copper atoms side by side would result in a line only about one meter long
Section 4.2 Subatomic Particles and the Nuclear Atom
Discovering the Electron • Scientists began to study the relationship between electric charge and matter in the 1800’s • Scientists were studying this relationship with a newly invented apparatus called the cathode ray tube
Electrons, cont’d. • By observing the deflection of the stream in the CRT, Thomson was able to conclude two things • The cathode rays were actually charged particles, not just a wave of energy • The charge of the particles was negative (but the exact value of the charge was not yet known) • These particles were called electrons
Thomson’s Atomic Model • Thomson’s concluded that electrons were common to all types of matter • His model of the atom placed electrons evenly throughout a positive uniform charge in the atom • It was called the plum pudding model • Now it is better known as the chocolate chip cookie model
Rutherford and the Nuclear Atom • Rutherford was a Kiwi, working at Cambridge University • He was studying how positively charged alpha particles interacted with solid matter • Rutherford made hypotheses based on Thomson’s plum pudding model of the atom • He expected most of the alpha particles to pass straight through the matter, will very little deflection
Results • Rutherford was astounded to find his results • As he predicted, most alpha particles passed straight through the material with only slight deflection • However, some particles were deflected at very extreme angles, and some even bounced straight back • He compared this to firing an artillery cannon at a sheet of paper, and having it bounce back toward you
Explanation for the Results • Rutherford concluded that the plum pudding model was incorrect based on his findings • He developed a new model of the atom based on his findings • He concluded that most of the atom is empty space, thus allowing most of the alpha particles to pass straight through • However, there is a tiny, dense, positively charged region, called the nucleus at the center of every atom • The nucleus contains nearly all of an atoms mass • The nucleus is the cause of some alpha particles being strongly deflected
Completing the Atom • In 1920, Rutherford searched for the reason the nucleus was positively charged. • He discovered the proton, a subatomic particle carrying a charge equal to and opposite of that of an electron (proton has a +1 charge) • In 1932, the nucleus was also shown to contain a neutron, a particle that has nearly identical mass to a proton, but is neutral in charge
Section 4.3 How Atoms Differ
Atomic Number • In the early 1900’s, shortly after Rutherford’s gold foil experiment, Henry Moseley discovered that atoms of each element contain a unique positive charge in the nuclei • After Rutherford discovered the proton, Moseley concluded that the number of protons is unique to each element • The number of protons in the nucleus of an element is known as the atomic number • This can be found on any periodic table
Because all atoms are neutral (how do we know this?), the number of protons is also equal to the number of electrons, which is equal to the atomic number • Atomic number = number protons = number electrons
Isotopes and Mass Number • Dalton’s atomic theory told us that atoms of the same element were identical • However, we later learned that this was incorrect • An example being potassium. There are three types of potassium atoms. All three types contain 19 protons, but one type of potassium contains 20 neutrons, another 21 neutrons, and the third has 22 neutrons • Atoms that have the same number of protons, but different numbers of neutrons are called isotopes
Isotopes • Do you think the three different isotopes of potassium have the same mass? • In order to identify the different isotopes from one another, chemists use the atoms mass number • Mass number is equal to the number of protons plus the number of neutrons in the nucleus • The isotope of potassium that has 20 neutrons has a mass number of 39 (19p+ + 20n0) • Number of neutrons = mass number – atomic number
There are two ways in which we indicate the isotope of an element • Potassium-39 the 39 indicates the mass number. How many protons does potassium have? How many neutrons? • The second way is to use the elemental symbol where the top number is the mass number and the bottom number is the atomic number
Atomic Mass • Atomic mass of an element is the weighted average of the isotopes of that element • For example, chlorine exists naturally as a mixture of about 75% chlorine-35 and 25% chlorine-37 • Will the weighted atomic mass be closer to 35 or 37? Why? • What is the atomic mass of chlorine?
