Chapter 4 Atoms and Elements

Homework Assigned Problems (odd numbers only) “Questions and Problems” 4.1 to 4.65 (begins on page 96) “Additional Questions and Problems” 4.75 to 4.105 (page 122-123) “Challenge Questions” 4.107 to 4.111, (page 124)

Elements 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

“Element” 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 Chemical means are a reaction, an electric current, or a beam of light

Chemical symbols Each element has a unique 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) Two capital letters indicate two elements such as CO

Names of Symbols and Some Common Elements Required: Know the name and symbol of some of the most common elements Table 4.2 on p. 95 (know the names and symbols listed) A periodic table will be given on each test or quiz Required: Know how to use a periodic table to find needed information

Periodic Table A chart of the elements with similar chemical properties arranged into vertical columns (groups) Horizontal rows are called periods The elements arranged (in rows) in order of increasing atomic mass (atomic number) 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”

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

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

Classification 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 Table 1A 8A 1 2A 3A 4A 5A 6A 7A 2 3 4 5 6 7

Metals/Nonmetals 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

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 Atom 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

Dalton’s Atomic Theory (1808) A set of five statements that summarize the modern scientific concept about atoms 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 A series of five statements that summarizes the modern day theory of the atom `

Dalton’s Atomic Theory The number and arrangement of different types of atoms in a pure substance determine its identity A chemical change is a combination, separation, or rearrangement of atoms (forms new substances) Only whole atoms take part or result from any chemical reaction A series of five statements that summarizes the modern day theory of the atom `

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

Cathode Rays and Electrons J.J. Thomson (1897) used a gas discharge tube to investigate a beam 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

Parts of an Atom: The Electron Defined by Thompson Tiny, negatively charged particle Charge is -1 Very light compared to mass of atom 1/2000th the mass of a H atom Moves very rapidly within the atom

Thompson’s Model of Atoms 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

Thompson’s Model of Atoms 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 In the early 1900s plum pudding model

Rutherford’s Experiment 1911 Rutherford designed an experiment to test the Thompson model (“plum-pudding”) 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

Rutherford’s Experiment

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

Rutherford’s Experiment 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

Parts of an Atom 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)

Nucleus of the Atom (Rutherford Model) 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

Nucleus of the Atom (Rutherford Model) The nucleus is the center (core) of the atom The nucleus has most of the mass of the atom protons neutrons The extranuclear region it contains all the electrons Extranuclear region nucleus Over 99.9 % of the mass is concentrated in the nucleus (called the nuclear region)

Nucleus of the Atom (Rutherford Model) The nucleus is 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

Mass of Subatomic Particles: The Proton Has the same magnitude charge as the electron, but oppositely charged Has a charge of +1 Weighs about 2000 times an electron Is found in the nucleus In a neutral atom: Number of protons = identity of the compound Number of protons = number of electrons

Mass of Subatomic Particles: The Neutron The last of the three subatomic particles to be discovered, also located in the nucleus The mass is about the same as a proton Has no charge (neutral) Variable amounts are possible in atoms of the same element This is the basis for isotopes Dalton reasoned the atoms of an element were identical because they had the same chemical properties But atoms can exist can differ in a way that has minimal effect on its chemical properties

Mass of Subatomic Particles The three subatomic particles have extremely small masses 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 One amu is 1/12 the mass of one carbon atom, so the approximate mass of one proton or neutron is 1.00 amu

Atomic Number = number of protons in an atom Atomic Number (Z) All elements in periodic table arranged according to the atomic number Equal to the number of protons in the nucleus of an atom Determines the identity of the atom Is also equal to the number of electrons in the neutral atom The top number in each square in the periodic table Atomic Number = number of protons in an atom

Mass Number = number of protons + number of neutrons Mass Number (A) The total number of protons and neutrons in an atom Mass number is always a whole number (no decimals) An oxygen atom has a mass number of 16 (8 protons and 8 neutrons) Mass Number = number of protons + number of neutrons

Isotopes and Atomic Mass 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 and Atomic Mass 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

Nuclear (Isotopic) Symbols A notation used when necessary to differentiate between isotopes 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 Reduces the number of masses to deal with to 109 from many hundreds This weighted average takes into account the natural abundances and the atomic masses of the isotopes of an element

Atomic Mass 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 the following table: Symbol Name # Protons #Neutrons #Electrons Hydrogen 1 1 Fluorine 9 10 9 Copper 29 35 29 Hydrogen 1 1 1

end Remaining slides (Section 4.6) will continue with chapter 10

Electron Energy Levels Electrons possess energy; they are in constant motion in the large empty space of the atom The arrangement of electrons in an atom corresponds to an electron’s energy The electron resides outside the nucleus in one of seven fixed energy levels Energy levels are quantized: Only certain energy values are allowed A quantized property is one that can only have certain values If the atom could have all possible energies, then the result would be a continuous spectrum instead of lines

Electron Energy Levels Electrons of similar energy are grouped into energy levels The major energy levels in an atom are called the principal shells symbolized by n, the principal quantum number As n goes from 1 to 2, 3, 4, etc., the electron’s energy and distance from the nucleus increases The maximum number of electrons in an energy level is equal to 2n2

Electron Arrangements The chemical properties of an element are determined by the number and arrangement of electrons in the nucleus The electron arrangement (configuration) is a statement of the number of electrons in each energy level The number of electrons an atom has of various energies The electron arrangements of the first 18 elements can be written by placing electrons in order of increasing energy

Valence Electrons The electrons that reside in the highest energy level (n) They are the furthest electrons from the nucleus Determine the chemical properties of an element Number at the top of each column for elements (1A-8A) equals the number of valence electrons for each element in that group

Electron-Dot Symbols Consists of an element’s symbol and one dot for each valence electron placed about the symbol Used only for main group elements (1A to 8A) Main group elements in the same group have the same number of valence electrons The number of valence electrons is the same as the group number Also called Lewis structures

Electron-Dot Symbols

Electron-Dot Symbols Write the symbol of the element Determine the number of valence electrons by the group number Use dot or X to represent an electron Group Number 1 2 3 4 5 6 7 8 Li• Be• •B• •C• •N• •O: :F: :Ne: • •• Li• Li+1 :F: [:F:]-1 • ••

Electron-dot symbols Examples Determine the number of valence electrons for Ba, As, and Br. Write the electron-dot symbol for each of these elements Ba: 2 valence electrons As: 5 valence electrons Br: 7 valence electrons Ba As Br

Ionization Energy Energy needed to remove one electron from an atom in the gas state It measures how tightly an atom holds its electrons The lower the ionization energy, the easier it is to remove the electron Metals have low ionization energies Nonmetals have high ionization energies Ionization Energy decreases down the group Ionization Energy increases across the period Left to right Valence electron farther from nucleus

Ionization Energy The more tightly an electron is held, the higher the ionization energy The outermost electrons are the easiest to remove; as the energy level increases the farther the electron is from the nucleus As n gets larger, removal of an electron requires less energy Helium requires the most energy of any element due to its full (stable) energy level

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