Download Chapter 4:ааAtomic Structure Section 4.1анаDefining the Atom

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 (460­370 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 (1766­1844) ● 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 whole­number 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 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 (1856­1940) ● 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 charge­to­mass ratio of electrons did not depend on the kind of gas in the cathode­ray 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 (1868­1953) ○ 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 Properties of Subatomic Particles Particle Symbol Relative Charge Relative mass (mass of proton = 1) Actual mass (g) Electron ­ e​
1­ 1/1840 ­28 9.11 x 10​
Proton + p​
1+ 1 ­24 1.67 x 10​
Neutron 0 n​
0 1 ­24
1.67 x 10​
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 whole­number 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 (1850­1930) ○ 1886 ­ observed a cathode­ray 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 (1891­1974) ­ 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 Gold­Foil Experiment ● Ernest Rutherford (1871­1937) ­ former student of Thomson ○ 1911 ­ University of Manchester, England ○ Rutherford and coworkers decided to test what was the current theory of atomic structure ■ 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 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 1 1 0 1 1 Helium He 2 2 2 4 2 Lithium Li 3 3 4 7 3 Beryllium Be 4 4 5 9 4 Boron B 5 5 6 11 5 Carbon C 6 6 6 12 6 Nitrogen N 7 7 7 14 7 Oxygen O 8 8 8 16 8 Fluorine F 9 9 10 19 9 Neon Ne 10 10 10 20 10 *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 ○ protons + neutrons = mass number ■ neutrons = mass number ­ atomic number ● atomic shorthand ○ the atomic number is the subscript and the mass number is the superscript 197 Au 79 ​
● may also be written as gold­197 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 hydrogen­1 or simply hydrogen ■ the second isotope has one neutron ● has a mass number of 2 ● called either hydrogen­2 or deuterium ■ the third isotope has two neutrons ● has a mass number of 3 ● called hydrogen­3 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 carbon­12 (assigned a mass of exactly 12 atomic mass units) ○ atomic mass unit (amu)​
­ one twelfth of the mass of a carbon­12 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 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