Chapter 4: Elements and the Periodic Table Development of atomic theory

Transcription

1 Chapter 4: Elements and the Periodic Table Development of atomic theory Name Date Theory Experiment Democritus 400 BC atomos smallest particle none, philosophical argument Aristotle 350 BC hyle continuous matter none, philosophical argument John Dalton 1803 AD billiard ball indivisible, laws of: conservation of matter smallest particle definite proportion multiple proportion JJ Thomson 1897 plum pudding electrons Cathode ray tube / e - beam in positive pudding (a Crookes tube) ( mass / charge ) ratio e Ernest Rutherford 1911 nuclear model small, α particle / gold foil dense nucleus with e - surrounding empty space Niels Bohr 1913 energy levels (planetary bright line spectra of the elements model of the atom) Erwin Schrödinger 1926 wrote an equation to find used a mathematical equation electron probability clouds James Chadwick 1932 discovered the neutron bombarded Be foil with α particles Dalton s Atomic Theory 1. All elements are composed of extremely small particles called atoms that cannot be divided 2. Atoms of the same element are exactly alike and have the same mass while atoms of different elements are different and have different masses 3. An atom of one element cannot be changed into an atom of a different element nor can atoms be created or destroyed in any chemical change, only rearranged 4. Every compound is composed of atoms of different elements combined in a specific ratio Modern atomic model Atoms are made of three particles Particle Symbol Charge Relative Mass (amu) protons (p+) 1+ 1 } nucleus neutrons (n 0 ) 0 1 electrons (e 1 ) Particle charges Note in the chart above that the charge of protons and electrons is equal size but opposite in sign Neutrons have no charge Since atoms have a neutral charge they must contain the same number of p + and n 0 Particle mass comparison Note in the chart above that the mass of protons and neutrons is about equal (about 1 amu) Electrons have very little mass (p + and n 0 have about 1836 times more mass than an e )

2 Size and scale of atoms Atoms range in size from about 75 pm (75 x m) for hydrogen to about 265 pm for cesium Nuclei range from about 1.75 fm (1.75 x m) for hydrogen to about 15 fm for uranium This makes a hydrogen atom about times larger than its nucleus and a cesium atom is about times larger than its nucleus and uranium atoms about times their nuclei As a scale model If the nucleus of a hydrogen atom was the size of a baseball, the electrons would be one mile away Isotopes: atoms of the same element (the same atomic number or number of protons in their nucleus) that have differing numbers of neutrons (or different mass numbers) Atomic number: the number of protons in the nucleus of an atom Atomic number defines what element an atom is Mass number: the number of protons and neutrons in the nucleus of an atom Mass number defines what isotope an atom is Hyphen notation: a means of writing a single nuclide in an isotope Example: carbon 13 has 6 p + (it is carbon) and 7 n 0 (it is one of the nuclides of carbon) Calculating p +, n 0, and e in a nuclide Because atoms are neutral, they have the same number of p + and e Subtracting the atomic number (p + ) from the mass number (p + + n 0 ) gives the number of n 0 Nuclear symbol: the hyphen notation carbon 13 can be written as a nuclear symbol, C Example: Find the number of p +, n 0, and e in a nuclide of boron 11 B, therefore 5 p +, 5 e, and 6 n 0 (because 11 5 = 6) Counting atoms in chemical formulas Chemical compounds have definite composition and so their formulas have small, whole number ratios of atoms Water = H 2 O Sodium tetraborate = Na 2 B 4 O 7 Magnesium chlorate = Mg(ClO 3 ) 2 Example: count the atoms of each element in magnesium chlorate, Mg(ClO 3 ) 2 Mg magnesium: 1 Cl chlorine: 2 Mg(ClO 3 ) 2 is shown here O oxygen: 6 so, there are two Cl and six O The Periodic Table organizing the elements 1869 scientists looked for ways to organize the 63 known elements in order to find patterns Dmitri Mendeleev Made cards showing known elements and listed chemical and physical properties Chemical properties showed the oxygen atom ratio in compounds Physical properties listed melting point, density, color, etc. When the cards were placed in order by atomic mass, Mendeleev noticed that certain patterns repeated (Li, Na, and K for example) so he started a new column to match properties He noticed two important things: A few element properties did not match exactly in order of atomic mass he moved these He noticed three missing elements and predicted their properties All three of these elements were easily found by other scientists (Sc, Ga, and Ge)

3 Organization of the modern periodic table Periods horizontal rows on the periodic table (sometimes called series) There are 7 periods on the modern periodic table Each period starts at the left with a very active metal, then come less active metals, then the metalloids, then the nonmetals with the noble gases finishing each period Groups vertical columns on the periodic table (sometimes called families) There are 18 groups on the modern periodic table Sometimes, the lanthanides and actinides are placed differently Short form: the lanthanides and actinides are placed below the usual 18 groups Long form: lanthanides are between Ba and Lu and actinides are between Ra and Lr Blocks remember, blocks help you remember electron configurations from Groups (or families) -- vertical columns on the table are numbered in different ways Modern group labels (as seen above) are the integers 1 18 but many chemists still use the older American scheme (refer to IA IIA IIIB IVB VB VIB VIIB VIIIB IB IIB IIIA IVA VA VIA VIIA VIIIA There are also trivial names for main group elements (s- and p-blocks) IA Alkali metals IIA Alkaline earth metals IIIA Icosagens IVA Crystallogens VA Pnictogens VIA Chalcogens VIIA Halogens VIIIA Noble gases Transition metals (d-block elements) Inner transition metals or rare earth metals (f-block elements) Metals, metalloids, and nonmetals (see Periods (or series) horizontal rows are numbered to show the highest energy level

4 Chemical symbols one or two letter symbols for each element The first letter is always capitalized If there is a second letter in the symbol, it is always small case Two capital letters in a row denote two separate elements in a compound Example: Co = cobalt while CO is carbon monoxide Average atomic mass most elements have an isotopic mixture B is about 80% 11 B and 20% 10 B so that the average is 10.8 amu Cl is about 75% 35 Cl and 25% 37 Cl for an average of about 35.5 amu Typical information on modern Periodic Tables Atomic number Chemical symbol Element name Atomic mass Formation of elements in stars In the extreme temperatures found in stars, matter exists in an ionized state called a plasma There are actually four states of matter: solid, liquid, gas, and plasma Big bang: the only elements that formed during the big bang were H, He, and a very small trace of Li How a sun the size of ours generates He by nuclear fusion from: MSFC Solar Physics ( Nuclear fusion in stars the size of our sun can create He, C, N, and O More massive stars can create elements as heavy as Mg and Si The most massive stars have cores with elements all the way up to Fe To produce elements more massive than Fe requires a supernova with enough energy to reach billions of degrees K enough energy to make all the elements heavier than Fe in the very massive star s final hours of life The elements: metals, metalloids, and nonmetals from: Wikipedia (

5 The properties of metals Physical properties of metals Malleable can be hammered or rolled into thin sheets or other shapes Ductile can be pulled or drawn into long wires Conductivity most metals are good conductors of heat and electricity Luster most metals are very shiny or have high metallic luster Magnetic many metals (but not all) are attracted to magnets Chemical properties of metals Reactivity metals react by losing electrons to form positive ions Some metals are useful because they are extremely reactive (Li in batteries) Some metals are useful because they exhibit very low reactivity (Au and Pt) Corrosion destruction of a metal because of its reactivity with oxygen from the air Metals in the Periodic Table Alkali metals metals found in group 1 (or IA, s block) Highly reactive losing one electron Have low density and react violently with water Sodium and potassium ions are needed by the body and are a requirement for life Alkaline earth metals metals found in group 2 (or IIA, s block) Also very reactive losing two electrons Like group 1 metals, group 2 metals are never found as uncombined elements in nature Magnesium (when mixed with a small amount of aluminum) makes lightweight wheels and ladders Calcium ions are needed by the body for bone and muscle growth Ca and Mg are found in dairy products and in leafy green vegetables Transisition metals metals found in groups 3 through 12 (or IIIB through IIB, d block) These metals are usually much less reactive and often useful as conductors or for strong building materials Iron is needed to make hemoglobin in the blood, a requirement for carrying oxygen Inner Transisition metals metals found in the two rows at the bottom of the short form of the Periodic Table and do not have group numbers Lanthanides the top row, these metals tend to be soft, malleable, shiny, and have high conductivity Lanthanides are often found mixed in nature and are hard to separate because they have very similar properties They are often used to make alloys Nd and Sm are used to make very powerful magnets used for modern speakers Actinides the bottom row of which only Ac, Th, Pa, and U occur naturally on earth Most of the actinides are synthetic elements formed in particle accelerators Mixed group metals metals found in the bottom left corner of the p block The most familiar of these metals are Al, Sn, and Pb Pb was used in the past to make lead pipes for water, but since it was discovered that lead was very toxic it is no longer used for this purpose The symbol for lead comes from its old name, plumbum, which also explains why people who used to work with lead pipes for water lines are called plumbers Sn is used to line iron cans to prevent the iron from rusting Al is used for many purposes because it is very lightweight and very common in the Earth s crust

6 Nonmetals and metalloids Physical properties of nonmetals Brittle if hammered, most nonmetals will shatter and they do not bend well Cleave if the right pressure is applied, many nonmetals will break along a flat plane Conductivity most nonmetals are poor conductors of heat and electricity Luster most nonmetals are dull but may have a sheen or luster Of the 16 nonmetals 10 are gases, 5 are solids (I, Se, S, P, and C), and 1 is a liquid (Br) Chemical properties of metals Reactivity nonmetals react by gaining electrons to form positive ions or sharing electrons to form molecules Nonmetals in the halogen family react with metals to form salts (NaCl) Oxygen reacts with metals to form rust or calx (Fe rusts and Mg forms a white calx) Families of nonmetals in the Periodic Table, the p block Carbon family the crystallogens found in group 14 (or IVA) Have four electrons to gain, lose or share Carbon is the only nonmetal in this family Especially important because it can make chains and so it plays a big role in the chemistry of living things Nitrogen family the pnictogens found in group 15 (or VA) Nitrogen forms a diatomic molecule (N 2 ) Most living things need nitrogen but are unable to get it from the air Some plants form a symbiotic relationship with bacteria that can fix N 2 Nitrogen is a major component of most fertilizers Phosphorus is the only other nonmetal in group 15 Phosphorus compounds are used to make matches Phosphorus is needed for the backbone of DNA but this is the element that is the limiting reactant for life Oxygen family the chalcogens found in group 16 (or VIA) Oxygen also forms a diatomic molecule (O 2 ) Most living things need oxygen We breathe oxygen from the air Ozone (O 3 ) in the upper atmosphere protects us from UV radiation Sulfur is a common element Often forms very smelly compounds like H 2 S, the rotten egg smell Is used to make rubber for tires (vulcanization process) Sulfur is used to make sulfuric acid (H 2 SO 4 ), an important chemical in industry Halogen family group 17 (or VIIA), F 2, Cl 2, Br 2, I 2, and At 2 These nonmetals form salts (halogen means salt forming ) All the halogens are very reactive Fluorine is the most reactive element and will burn in water, ground glass, and metals Fluorides are added to water and toothpaste to strengthen teeth Chlorine is poisonous but is used to kill bacteria (pools and water) and to bleach clothes Chlorides (like calcium chloride) are used to melt ice on walkways and roads Silver bromide is used in photography Noble gases group 18 (or VIIIA) Noble gases are found in the air but are very unreactive He was first discovered in the sun (by spectroscopy) Ne is used (as well as other noble gases) for neon signs Ar is used in light bulbs

7 Hydrogen usually listed above group 1 (or IA) which are metals, but H is a nonmetalγ Hydrogen is unique in that it is the only reactive element with only one electron shell The first energy level can only hold two electrons, so H can either gain or lose electrons Hydrogen atoms make up about 90% of the atoms in the universe Only 1% of the mass of Earth s crust, the oceans, and atmosphere are H Hydrogen is rarely found in the elemental state on Earth, most if found in water (H 2 O) Metalloids Metalloids form a stair-step border between metals and nonmetals on the Periodic Table The properties of metalloids are between those of metals and notmetals They are brittle, hard, and somewhat reactive All the metalloids form solids at room temperature The most common metalloid is silicon (Si), most commonly found in SiO 2 in sand Semiconductors materials that conduct electricity under some conditions but not under other conditions The most useful property of the metalloids is their varying ability to conduct electricity The ability of many metalloids to conduct electricity can depend on temperature, exposure to light, or the presence of impurities Computer chips, lasers, transistors, and solid state light emitting diodes (LED lights) Radioactive elements Henri Becquerel discovered radioactivity in 1896 while investigating a mineral that contained uranium ( 92 U) Becquerel assumed that sunlight was the source of energy that could expose photographic film or plates One cloudy day, he placed the mineral next to a plate wrapped in protective paper but later discovered that even in the dark drawer the mineral gave off energy that exposed the plates This led to the question, what was the energy source that produced the penetrating radiation? Becquerel presented his findings to the Curies (Marie and Pierre) After study, Marie and Pierre concluded that the energy source was a reaction taking place in the nuclei of the uranium atoms Radioactivity is the name Marie gave to this spontaneous emission of energy by unstable atomic nuclei Marie found that some minerals containing uranium were more radioactive than pure uranium causing her to conclude that these minerals contained small amounts of other radioactive elements The Curies eventually isolated two new elements, polonium and radium Types of natural radioactive decay Nuclear Type of Radiation Symbol Symbol Penetrating Power Alpha decay α particles He Very low blocked by a sheet of paper Beta decay β particles e Medium blocked by a thin Al sheet Gamma radiation γ radiation γ High thick Pb or concrete required A nuclear equation can represent nuclear reactions Th Ra He

8 Using radioactive isotopes Tracers radioisotopes used to track chemical reactions or industrial processes Phosphorus 32 is used to trace where and how plants use phosphorus for growth Radioactive gases can help detect leaks in pipes Gamma radiation can be used to take photographs of metals to detect weak spots Diagnosis radioisotopes used to track chemical reactions or industrial processes Technetium 99 is used to diagnose problems in bones, liver, kidneys, and the digestive system Iodine 131 is used to detect problems with thyroid function Treatment radioisotopes used to track chemical reactions or industrial processes High-energy gamma rays can be used like a surgical knife to kill cancer cells