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Chapter 4 Atoms and Elements Homework Assigned Problems (odd numbers only) Recommended: Exercises, 1-27 Required: Problems, 29-101 Required: Cumulative Problems, 103-117 Optional: Highlight Problems, 119 Indivisible: The Atomic Theory Matter is anything with a mass and occupies space Matter (in our world) is composed of combinations of about 100 basic substances called elements 109 elements have been discovered and isolated 88 are found in nature 21 are (synthetic) man-made Oxygen most abundant element (by mass) on earth The Atomic Theory An element is a pure substance that cannot be broken down into simpler substances by a chemical means Single atom of that element Sample of the element large enough to weigh on a scale Generally referring to the presence of that element (compound), not necessarily in its free form The Atomic Theory An atom is the smallest particle of an element that can exist and still have properties of that element All atoms of a certain type are similar to one another and different from all other types 109 different types are known and each “type” is a different element The Atomic Theory (1808) Dalton proposed a set of five statements that summarize the modern scientific concept about atoms: 1) All matter is made from small particles called atoms (109 different types) All atoms of a given type are similar to one another and significantly different from all other types 2) ` Dalton’s Atomic Theory The number and arrangement of different types of atoms in a pure substance (a compound) determine its identity 4) A chemical change is a combination, separation, or rearrangement of atoms (forms new substances) 5) Only whole atoms take part or result from any chemical reaction 3) ` Dalton’s Atomic Theory Atoms are indivisible in a chemical process (indestructible) All atoms present at beginning are present at the end Atoms are not created or destroyed, just rearranged Atoms of one element cannot change into atoms of another element Cannot turn Lead into Gold by a chemical reaction Thomson’s Model of the Atom J.J. Thomson (1897) used a gas discharge tube to examine a glowing stream of light called a cathode ray Determined that the ray was made of tiny negatively charged particles we call electrons He determined the electrons were smaller than a hydrogen atom Since electrons are smaller than atoms they must be parts of an atom Atoms must be divisible Atoms of different elements all produced these same electrons Thomsons’s Model of the Atom Defined by Thomson Tiny, negatively charged particle Electrical charge = 1 ¯ Very light compared to mass of atom 1/2000th the mass of a H atom Moves very rapidly within the atom Thomson’s Model of the Atom Thomson’s experiment showed that atoms have a structure and are divisible Thomson reasoned that electrons must be a fraction of the entire size of the atom since their mass is much smaller that the whole atom Thomson also reasoned since atoms are neutral, the electrons were embedded in a sphere of uniform positive charge Thomson (1898) proposed the “Plum Pudding” model or “Raisin Muffin” model of the atom Thomson’s Model of the Atom Thomson Atomic Model (early 1900’s): Proposed a uniform, positive sphere of matter with small negative electrons attached to the surface of the sphere This became known as the plum-pudding model The Nuclear Atom: Rutherford’s Experiment 1911 Rutherford designed an experiment to test the Thomson model (“plumpudding”) of the atom Rutherford directed positively charged particles (alpha particles) towards a thin gold foil sheet Rutherford expected the particles to pass straight through a uniform area of mass and positive charge The Nuclear Atom: Rutherford’s Experiment Some particles are scattered Most particles pass straight through foil Source of particles Beam of particles Fig4_5 Screen to detect scattered particles Thin metal foil The Nuclear Atom: Rutherford’s Experiment Results: Most (alpha) particles mostly went straight through A few particles were unexpectedly deflected from their expected (straight) path A few deflected nearly back towards alpha particle source The Nuclear Atom Rutherford proposed: A very small, dense core at the center of the atom This dense core was called the “nucleus” It contains most of the mass of the atom and it has a positive charge (protons) Most of an atom is empty space filled with electrons The Nuclear Atom (Rutherford Model) The nucleus is the center (core) of the atom Extranuclear The nucleus region has most of the mass of the atom protons neutrons The extranuclear region It contains all the electrons Electrons occupy most of the total volume of the atom nucleus The Nuclear Atom (Rutherford Model) Rutherford’s Nuclear Theory of the Atom A very dense, small center exists in the center of the atom called the nucleus Volume of nucleus is about 1/10 trillionth the volume of the entire atom Nucleus is basically the entire mass of the atom The protons and neutrons are located in the nucleus Most of the atom is empty space with fast-moving electrons The Nuclear Atom (Rutherford Model) The nucleus The core of the atom Positively charged Contains most of the mass of the atom Within a neutral atom, there are equal numbers of protons and electrons, so atom has a net charge of zero The Properties of Protons, Neutrons, and Electrons Experimentation in the early 20th century (Thomson and Rutherford) proved atoms were not indivisible spheres Atoms are comprised of smaller particles: Subatomic particles More experiments led to the discovery of two more fundamental subatomic particles: Protons and neutrons Electron: Negatively charged (1897) Proton: Positively charged (1919) Neutron: No electrical charge (1932) The Properties of Protons, Neutrons, and Electrons: Mass of Subatomic Particles The usual standards of mass such as the gram or pound are not practical for use with atoms Chemists base the mass of atoms on the atomic mass scale A relative scale based on the mass of one carbon atom: 12.00 amu (atomic mass unit) One amu is 1/12 the mass of one carbon atom, so the approximate mass of one proton or neutron is 1.00 amu The Properties of Protons, Neutrons and Electrons: The Electron The electron is a subatomic particle that has one unit of negative charge (-1) Its relative mass is about 1/2000 times a proton It is found in the region outside of the nucleus The Properties of Protons, Neutrons and Electrons: The Proton The proton has the same magnitude charge as the electron, but oppositely charged The proton has one unit of positive charge (+1) Its relative mass is about 2000 times an electron It is located in the nucleus In a neutral atom: Number of protons = number of electrons An element is a pure substance in which all atoms have the same atomic number No. of protons is a characteristic of an element The Properties of Protons, Neutrons, and Electrons: The Neutron The neutron is the last of the three subatomic particles to be discovered The neutron is neutral: It has no unit charge Its relative mass is also about 2000 times an electron It is located in the nucleus Variable amounts are possible in atoms of the same element: This is the basis for isotopes Elements: Defined by Their Numbers of Protons The atomic number is equal to the number of protons in the nucleus of an atom Atomic Number = number of protons in an atom Determines the identity of the atom It is also equal to the number of electrons in the neutral atom The top number in each square in the periodic table All elements in periodic table arranged according to the atomic number Elements: Chemical Symbol Each element has a unique chemical symbol One or two letter abbreviations If two letters, the second is lower case The letter symbol often corresponds to the name of the element F = Fluorine P = Phosphorous Some symbols derived from the Latin or Greek names Lead – Pb (plumbum) Gold – Au (aurum) Sodium – Na (natrium) Elements: Names of Symbols and Some Common Elements Required: Know the name and symbol of some of the most common elements (from list given) See handout given in class A periodic table will be given on each test or quiz Required: Know how to use a periodic table to find needed information The Periodic Law and the Periodic Table In early part of 19th century many chemical facts were being obtained for the elements known (at that time) As the number of known elements increased, scientists began to attempt useful classifications Scientists had discovered that certain groups of elements had similar chemical properties Many attempts were made to arrange or classify the elements in order to explain the similarities The Periodic Law and the Periodic Table In 1869, Mendeleev attempted to arrange the elements by their relative masses Mendeleev (Meyer) noted that similar chemical properties recur periodically when the elements are arranged by increasing atomic mass Mendeleev then arranged the elements with similar chemical properties in the same vertical columns The periodic law (Mendeleev): When elements are arranged in order of increasing atomic mass (number), elements with similar properties occur at periodic intervals The Periodic Law and the Periodic Table The periodic table is a chart of the elements with similar chemical properties arranged into vertical columns (groups) Horizontal rows are called periods The periodic table has two categories of elements: Main group elements are those in the columns labeled with numbers (1A-8A) Transition elements are those in the columns labeled with the letter “B” (1B-8B) Group number The Periodic Law and the Periodic Table Each box in the periodic table designates an element, with that element’s symbol in the center of the box The number above the symbol is the element’s atomic number The elements are arranged in the order of increasing atomic number (across the period) The Periodic Table 1A 1 2 3 4 5 6 7 2A 8A 3A 4A 5A 6A7A The Periodic Law and the Periodic Table nonmetalss metals Metals Everything to the left of the metal/nonmetal barrier Shiny solid, good conductor of electricity, ductile and malleable Nonmetals Everything to the right of the metal/nonmetal barrier Dull appearance, not ductile or malleable not good conductors of electricity The Periodic Law and the Periodic Table Metalloids Elements with properties intermediate between those metals and nonmetals On the metal/nonmetal barrier Have some physical properties of metals but some chemical properties of nonmetals Semiconductors Si, Ge, As, Sb, Te The Periodic Law and the Periodic Table Families of Elements Certain groups of elements have their own special names due to the chemical similarity of the elements in them Alkali Metals Group 1A Alkaline Earth Metals Group 2A Halogens Group 7A Noble Gases Group 8A Become familiar with these group names The Periodic Law and the Periodic Table: Main Group Elements Also called representative elements The elements in the A-groups First two columns (1A and 2A) The last 6 columns (3A to 8A) Easy to predict ionic structure Some groups have names The Periodic Law and the Periodic Table: Transition Elements The elements in the B-groups Middle block of elements (3B through 2B) Includes the two groups at the bottom Lanthanides and Actinides Difficult to predict ionic structure Ions: Losing and Gaining Electrons Electron transfer may occur between two or more atoms It produces charged particles called ions An ion is an atom that is electrically charged from the loss or gain of electrons Atoms are neutral only when the number of protons equals the number of electrons When atoms are lost or gained, this protonelectron balance (neutrality) falls apart This leaves a net charge on the atom Ion charge = number of protons - number of electrons Ions: Losing and Gaining Electrons Cations Positive ions form when an electron or electrons are lost from a metal If the number of protons is the same and the number of electrons decreases, excess positive charge results The number of protons never changes when ions form Sodium Magnesium Aluminum Calcium Sodium Ion Na Na e 2 Mg Mg 2e Magnesium Ion Aluminum Ion Al 3e 2 Calcium Ion 3 Al Ca Ca 2e Ions: Losing and Gaining Electrons Anions Negative ions form when an electron or electrons are gained from a nonmetal If the number of protons is the same and the number of electrons increases, excess negative charge results The number of protons never changes when ions form Fluorine Bromine Oxygen Sulfur Fe F Br e Br 2 O 2e O 2 S 2e S Fluoride Bromide Oxide Sulfide Ions and the Periodic Table Valence electrons are important in determining the bonding characteristic of an atom The outermost electrons in any atom (furthest from the nucleus) The maximum number of valence electrons for any element is eight which is the most stable valence electron configuration Only noble gases have the maximum number 5 of the 6 noble gases have eight valence electrons Helium (the exception) has only two valence electrons Ions and the Periodic Table The number above each main-group gives the number of valence electrons for element in that group The charge of an ion is directly correlated with the number of electrons lost or gained when compounds are formed Atoms of elements tend to lose, gain, or share electrons as to produce a noble gas electron configuration for each of atoms involved The charge on the ion is directly related to the number of electrons lost or gained Ions and the Periodic Table Positive Ions Group IA, IIA, and IIIA metal atoms contain one, two, or three more electrons (respectively) than the preceding noble gas Metal atoms (IA, IIA, IIIA) easily lose electrons to acquire the noble gas electron configuration Produces a (+) charged atom: cation The charge on the cation is directly correlated with the number of electrons lost Ions and the Periodic Table Negative Ions Group VA, VIA, and VIIA nonmetal atoms contain three, four, or five less electrons (respectively) than the nearest noble gas Nonmetal atoms (VA, VIA, VIIA) will gain the necessary number of electrons to acquire the noble gas electron configuration produces a (-) charged atom: anion The charge on the anion is directly related to the number of electrons gained Isotopes: When the Number of Neutrons Varies All atoms of the same element have the same atomic number (Z) The same element can differ in the mass number (A) due to a different number of neutrons All Mg atoms have 12 protons, but may have 12, 13, or 14 neutrons Isotopes: When the Number of Neutrons Varies Atoms that have the same number of protons and electrons but different numbers of neutrons are called isotopes Since isotopes are atoms of the same element, They have the same atomic number They display the same chemical properties All elements have their own exclusive percent natural abundances of the isotopes Isotopes: Mass Number (A) The total number of protons and neutrons in an atom Mass Number = number of protons + number of neutrons Mass number is always a whole number (no decimals) An oxygen atom has a mass number of 16 (8 protons and 8 neutrons) Nuclear (Isotopic) Symbols A notation used when necessary to differentiate between isotopes A X Z A is the mass number Z is the atomic number X is the chemical symbol Atomic Mass A specific element can have several mass values if it exists in isotopic forms For example, oxygen atoms can have any one of three masses but often treated as if it has one mass The atomic mass of an element is the mass of the “average atom” of that element Atomic Mass 1) 2) 3) Atomic mass is a “weighted average mass” based on: The number of isotopes that exist for the element The relative mass of each isotope The percent abundance of each isotope Example: Isotopes and Atomic Mass Complete Symbol Name 1 Hydrogen H 1 19 9 F Fluorine 64 Copper Cu 29 2 Hydrogen H 1 the following table: # Protons #Neutrons #Electrons 1 0 1 9 10 9 29 35 29 1 1 1 end