Chapter 4: Atomic Structure Section 4.1 Defining the Atom

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1 Chapter 4: Atomic Structure Section 4.1 Defining the Atom Early Models of the Atom atom the smallest particle of an element that retains its identity in a chemical reaction Democritus s Atomic Philosophy Democritus: Greek philosopher ( BC) among the first to suggest the existence of atoms believed that atoms were indivisible and indestructible beliefs did not explain chemical behavior lacked experimental support because idea was not based on the scientific method his ideas were later challenged by Plato and Aristotle Figure 4.2 How does a mixture of atoms of different elements differ from a compound? In a mixture of atoms of different elements, each element in the mixture retains its chemical properties. In a compound, different elements have been chemically combined to form a new substance with chemical properties different from its component elements. Dalton s Atomic Theory John Dalton: English chemist and school teacher ( ) by using experimental methods, Dalton transformed Democritus s ideas on atoms into a scientific theory studied the ratios in which elements combine in chemical reactions Dalton s Atomic Theory all elements are composed of tiny particles called atoms atoms of the same element are identical; atoms of one element are different from those of any other element atoms of different elements can physically mix together or can chemically combine in simple whole number ratios to form compounds chemical reactions occur when atoms are separated, joined, or rearranged; atoms of one element are NEVER changed into atoms of another element as a result of a chemical reaction Checkpoint What happens to atoms in a chemical reaction according to Dalton s atomic theory? atoms are separated, joined, or rearranged Sizing Up the Atom atoms are very small despite their small size, individual atoms are observable with instruments such as scanning tunneling microscopes

2 Chapter 4: Atomic Structure Section 4.2 Structure of the Nuclear Atom Subatomic Particles it is now known that atoms are divisible; they can be broken down into subatomic particles three types of subatomic particles electrons protons neutrons Electrons discovered by J.J. Thomson English physicist ( ) negatively charged subatomic particles Thomson s experiment involved passing electric current through gases at low pressure he sealed the gases in glass tubes fitted at both ends with metal disks called electrodes the electrodes were connected to a source of electricity one electrode, the anode, became positively charged the other electrode, the cathode, became negatively charged the result was a glowing beam (cathode ray) that traveled from the cathode to the anode a cathode ray is deflected by a magnet and electrically charged metal plates Thomson knew that opposite charges attract and like charges repel, so he hypothesized that a cathode ray is a stream of tiny negatively charged particles moving at high speed he called them particles corpuscles Thomson s additional experiment set up an experiment to measure the ratio of the charge of an electron to its mass found this ratio to be constant the charge to mass ratio of electrons did not depend on the kind of gas in the cathode ray tube or the type of metal used for the electrodes concluded that electrons must be parts of the atoms of all elements Robert Millikan: U.S. physicist ( ) carried out experiments to find the quantity of charge carried by an electron calculated the mass of the electron electrons carry one unit of negative charge mass is 1/1840 the mass of a hydrogen atom Checkpoint How do negatively charged plates affect the path of cathode rays? a negatively charged plate repels a cathode ray

3 Properties of Subatomic Particles Particle Symbol Relative Charge Relative mass (mass of proton = 1) Actual mass (g) Electron e 1 1/ x Proton p x Neutron n x Protons and Neutrons What happens after a hydrogen atom (lightest atom) loses an electron? atoms have not net electric charge; they are neutral electric charges are carried by particles of matter electric charges always exist in whole number multiples of a single basic unit when a given number of negatively charged particles combines with an equal number of positively charged particles, an electrically neutral particle is formed so, a particle with one unit of positive charge should remain when a typical hydrogen atom loses an electron Eugen Goldstein ( ) 1886 observed a cathode ray tube and found rays traveling in the direction opposite to that of the cathode rays called these rays canal rays concluded that they were composed of positive particles he had discovered PROTONS James Chadwick ( ) English physicist confirmed the existence of another subatomic particle he discovered NEUTRONS subatomic particles with no charge but with a mass nearly equal to that of a proton The Atomic Nucleus Scientists wondered how subatomic particles were put together in an atom. most thought it likely that the electrons were evenly distributed throughout an atom filled uniformly with positively charged material the plum pudding model electrons were stuck into a lump of positive charge, similar to raisins stuck in dough Rutherford s Gold Foil Experiment Ernest Rutherford ( ) former student of Thomson 1911 University of Manchester, England Rutherford and coworkers decided to test what was the current theory of atomic structure

4 test used relatively massive alpha particles (helium atoms that have lost their two electrons and have a double positive charge because of the two remaining protons) a narrow beam of alpha particles was directed at a very thin sheet of gold foil according to the theory of the time, the alpha particles should have passed easily through the gold, with only a slight deflection due to the positive charge thought to be spread out in the gold atoms the majority of alpha particles passed straight through the gold atoms, without deflection a small fraction of the alpha particles bounced off the gold foil at very large angles some bounced straight back toward the source The Rutherford Atomic Model the Nuclear Atom Rutherford suggested a new theory of the atom proposed that the atom is mostly empty space explained the lack of deflection of most of the alpha particles concluded that all the positive charge and almost all of the mass are concentrated in a small region that has enough positive charge to account for the great deflection of some of the alpha particles Rutherford discovered the NUCLEUS nucleus the tiny central core of an atom and is composed of protons and neutrons In the nuclear atom, the protons and neutrons are located in the nucleus. The electrons are distributed around the nucleus and occupy almost all the volume of the atom. the nucleus is tiny compared to the atom as a whole

5 Chapter 4: Atomic Structure Section 4.3 Distinguishing Among Atoms Atomic Number atoms are composed of protons, neutrons, and electrons protons and neutrons make up the nucleus electrons surround the nucleus elements are different because they contain different numbers of protons atomic number: the number of protons in the nucleus of an atom of an element the number of protons equals the number of electrons the overall charge of an atom is zero (neutral) Atoms of the First Ten Elements Name Symbol Atomic Number Protons Neutrons* Mass Number Electrons Hydrogen H Helium He Lithium Li Beryllium Be Boron B Carbon C Nitrogen N Oxygen O Fluorine F Neon Ne *Number of neutrons in the most abundant isotopes. Mass Number the majority of the mass of an atom is located in the nucleus depends on the number of protons and neutrons (which have almost equal masses) mass number the total number of protons and neutrons in an atom if you know the atomic number and mass number of an atom of any element, you can determine its composition atomic number = # of protons # of protons = # of electrons

6 protons + neutrons = mass number neutrons = mass number atomic number atomic shorthand the atomic number is the subscript and the mass number is the superscript Au may also be written as gold 197 Isotopes all atoms of an element have the same number of protons not all atoms of an element have the same number of neutrons isotopes atoms that have the same number of protons but different numbers of neutrons they also have different mass numbers Hydrogen there are 3 known isotopes of hydrogen each isotope has 1 proton in its nucleus the most common isotope has no neutrons it has a mass number of 1, and is called hydrogen 1 or simply hydrogen the second isotope has one neutron has a mass number of 2 called either hydrogen 2 or deuterium the third isotope has two neutrons has a mass number of 3 called hydrogen 3 or tritium Atomic Mass the mass of an atom can be determined using a mass spectrometer the numbers are very small and impractical for working with it is more useful to compare the relative masses of atoms using a reference isotope as a standard the chosen isotope is carbon 12 (assigned a mass of exactly 12 atomic mass units) atomic mass unit (amu) one twelfth of the mass of a carbon 12 atom the mass of a single proton or a single neutron is about 1 amu in nature most elements occur as a mixture of two or more isotopes each isotope has a fixed mass and a natural percent abundance the atomic mass of an element is a weighted average mass of the atoms in a naturally occurring sample of the element reflects both the mass and the relative abundance of the isotopes as they occur in nature

7 Determining Atomic Mass Based on Relative Abundance to calculate the atomic mass of an element, multiply the mass of each isotope by its natural abundance, expressed as a decimal, and then add the products The Periodic Table A Preview periodic table an arrangement of elements in which the elements are separated into groups based on repeating properties allows you to easily compare the properties of one element (or a group of elements) to another element (or group of elements) period each horizontal row of the periodic table there are 7 on the modern periodic table within a given period, the properties of the elements vary as you move across it from element to element this pattern repeats as you move to the next period group each vertical column of the periodic table also called a family elements in each group have similar chemical and physical properties each group is identified by a number and the letter A or B