Introductory Chemistry: A Foundation FOURTH EDITION by Steven S. Zumdahl University of Illinois Elements, Atoms & Ions Chapter 4 1 2 Elements Aims: To learn about the relative abundances of the elements, learn the names of elements Table 4.1: Distribution (Mass Percent) of the 18 Most Abundant Elements in the Earth's Crust, Oceans, and Atmosphere Over 114 known, of which 88 are found in nature others are man-made Abundance is the percentage found in nature oxygen most abundant element (by mass) on earth and in the human body the abundance and form of an element varies in different parts of the environment Each element has a unique symbol The symbol of an element may be one letter or two if two letters, the second is lower case 3 4 Table 4.2: Abundance of elements in the human body The Symbols for the Elements Aim: to learn the names/symbols for some of the elements You need to know the elements in Table 4.3 for Quiz #3!! 5 6
Table 4.3: The names and symbols of the most common elements 7 Dalton s Atomic Theory Aims: Learn Dalton s Atomic Theory Learn Law of Constant Composition In the 18 th century, scientists studying the nature of things agreed upon the following things: Most natural materials are mixtures of pure substances Pure substances are either elements or combinations of elements A given compound always contains the same proportions (by mass) of the elements. e.g., water always contains 8 g of oxygen for every 1 g of hydrogen. This principle is known as the Law of Constant Composition. It means a given compound always has the same composition. 8 Figure 4.1: John Dalton (1766-1844) An English scientist and teacher was aware of these observations and formulated an explanation. His explanation became known as Dalton s Atomic Theory Dalton s Atomic Theory 1. Elements are composed of atoms tiny, hard, unbreakable, spheres 2. All atoms of a given element are identical all carbon atoms have the same chemical and physical properties 3. Atoms of a given element are different from those of any other element carbon atoms have different chemical and physical properties than sulfur atoms 9 10 Dalton s Atomic Theory 4. Atoms of one element combine with atoms of other elements to form compounds. Law of Constant Composition all samples of a compound contain the same proportions (by mass) of the elements Chemical Formulas Describe the proportions of elements in a compound Dalton s Atomic Theory 5. Atoms are indivisible in a chemical process. 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 11 12
Formulas Aims: Learn how a formula describes a compounds composition Learn how to write formulas a compound is a distinct substance that is composed of atoms of two or more elements Formulas describe the compound by describing the number and type of each atom in the simplest unit of the compound molecules or ions Formulas Describe Compounds Rules for Writing Formulas each element represented by its letter symbol the number of atoms of each element is written to the right of the element as a subscript if there is only one atom, the 1 subscript is not written polyatomic groups are placed in parentheses if more than one If subscript is one (1), then it is not written 13 14 Figure 4.2: Dalton pictured compounds as collections of atmosphere NO, NO 2, and N 2 O are represented Structure of the Atom Aims: Learn the internal parts of the atom Understand Rutherford s experiment to characterize the atom s structure Dalton s theory explained compounds as a collection of atoms. But what were the atoms themselves like? 15 16 The Nature of the Atom Many scientists pondered the nature of the atom in the 1800s Physicist J.J. Thomson showed that atoms of any kind can emit tiny negative particles. Therefore all atoms must contain these tiny, negative particle known as electrons Since J.J. Thomson knew that atoms are neither positively or negatively charged, he concluded that there must also be positive particles present in the atom to balance the charge to 0 (zero) Are Atoms Really Unbreakable? J.J. Thomson investigated a beam called a cathode ray he determined that the ray was made of tiny negatively charged particles we call electrons his measurements led him to conclude that these electrons were smaller than a hydrogen atom if electrons are smaller than atoms, they must be pieces of atoms if atoms have pieces, they must be breakable Thomson also found that atoms of different elements all produced these same electrons 17 18
The Electron Tiny, negatively charged particle Very light compared to mass of atom 1/1836 th the mass of a H atom Move very rapidly within the atom Thomson s Plum Pudding Model Thomson concluded from his studies: 1. Atom breakable!! 2. Atom has structure 3. Electrons suspended in a positively charged electric field must have positive charge to balance negative charge of electrons and make the atom neutral 4. mass of atom due to electrons 5. atom mostly empty space compared size of electron to size of atom 19 20 Figure 4.3: The plum pudding model Figure 4.4: Ernest Rutherford (1871-1937) A physicist, who found that something was deflecting the α-particles he was studying He set up an experiment to find out what it was 21 22 Figure 4.5: Rutherford s experiment on α- particle bombardment of metal foil α-particles have a mass of 4 amu & charge of +2 c.u. gold has a mass of 197 amu & is very malleable 23 Rutherford s Gold Foil Expt How can you prove something is empty? The Plum Pudding model postulated that the atom was an empty cloud of positive charge with electrons scattered through it If the Plum Pudding Model was correct, the α-particles would fly right through his gold foil (like a bullet through a piece of paper) Rutherford expected the α-particles to fly through the foil with at most a minor deflection But some particles experienced large deflections!! Therefore the Plum Pudding Model is not correct! The large deflections were due to positive particles hitting a positively charged nucleus 24
Rutherford s Results Figure 4.6: (a) The results that the metal foil experiment would have yielded if the plum pudding model had been correct; (b) Actual results Over 98% of the α particles went straight through About 2% of the α particles went through but were deflected by large angles About 0.01% of the α particles bounced off the gold foil 25 26 Rutherford s Nuclear Model 1. The atom contains a tiny dense center called the nucleus the volume is about 1/10 trillionth the volume of the atom 2. The nucleus is essentially the entire mass of the atom 3. The nucleus is positively charged the amount of positive charge of the nucleus balances the negative charge of the electrons 4. The electrons move around in the empty space of the atom surrounding the nucleus Structure of the Nucleus The nucleus was found to be composed of two kinds of particles Some of these particles are called protons charge = +1 mass is about the same as a hydrogen atom Since protons and electrons have the same amount of charge, for the atom to be neutral there must be equal numbers of protons and electrons The other particle is called a neutron has no charge has a mass slightly more than a proton 27 28 The Modern Concept of the Atom Aim: To understand the main features of subatomic particles The Modern Atom We know atoms are composed of three main pieces - protons, neutrons and electrons The nucleus contains protons and neutrons Particle Electron Proton Neutron Relative Mass 1 1836 1839 Relative Charge 1-1+ 0 (none) The nucleus is only about 10-13 cm in diameter The electrons move outside the nucleus with an average distance of about 10-8 cm therefore the radius of the atom is about 10 5 times larger than the radius of the nucleus 29 30
Figure 4.9: A nuclear atom viewed in cross section Components of Atoms If all atoms are composed of the same components (electrons, protons, and neutrons) Why do different atoms have different chemical properties?? The answer is the number and arrangement of the electrons Electrons account for most of the volume of an atom Electrons are the part of the atom that intermingle with other atoms, so the number and arrangement of electrons affect chemical behavior 31 32 Bohr s Model We ll Talk about this more in Chapter 10 Planetary Model Based upon the orbits of our solar system. Not a 100% correct model, but good enough to explain some concepts. 33 The Orbits or Shells (Bohr) Also talk more about this in Chapter 10 So there are these orbits around the nucleus of the atom where the electrons are. The first orbit can only hold 2 electrons. All other orbits can hold 8 electrons. Octet Rule Atoms will arrange themselves in order to obtain 8 electrons. Except: hydrogen and helium Valence Orbit: the outer most orbit This is the orbit that is used to create ion and is used in bonding. The electrons in this orbit are called valence shell electrons. So it is kind of important Bohr s model can only be used for the first 3 periods in the periodic table. 34 Isotopes Aims: Learn the following terms Isotope Atomic number Mass number Understand the symbols used to describe atoms Isotopes All atoms of an element have the same number of protons The number of protons in an atom of a given element is the same as the atomic number found on the Periodic Table Atoms of an element with different numbers of neutrons are called isotopes All isotopes of an element are chemically identical undergo the exact same chemical reactions Isotopes of an element have different masses Isotopes are identified by their mass numbers mass number = protons + neutrons 35 36
Figure 4.10: Two isotopes of sodium Mass Number = # protons + # Neutrons Atomic number = # protons Symbols and Calculations You should be able to do the following: Interpret a symbol for an isotope (Ex 4.2) Write a symbol for an isotope (Ex 4.3) Calculate a mass number (Ex 4.4) 37 38 Elements Arranged in a pattern called the Periodic Table Position on the table allows us to predict properties of the element Metals about 75% of all the elements lustrous, malleable, ductile, conduct heat and electricity Nonmetals dull, brittle, insulators Metalloids also know as semi-metals some properties of both metals & nonmetals Figure 4.11: The periodic table 39 40 The Modern Periodic Table Elements with similar chemical and physical properties are in the same column Columns are called Groups or Families Rows are called Periods Each period shows the pattern of properties repeated in the next period Group Figure 4.11: The periodic table Period 41 42
The Modern Periodic Table Main Group = Representative Elements A columns Transition Elements all metals Bottom rows = Inner Transition Elements = Rare Earth Elements metals really belong in Period 6 & 7 Figure 4.12: The elements classified as metals and nonmetals Metalloids 43 44 Group 8 = Noble Gases He, Ne, Ar, Kr, Xe, Rn all colorless gases at room temperature very non-reactive, practically inert found in nature as a collection of separate atoms uncombined with other atoms Important Groups Noble Metals Ag, Au, Pt all solids at room temperature least reactive metals found in nature uncombined with other atoms Figure 4.13: Argon gas consists of a collection of separate argon atoms 45 46 Figure 4.14: Gaseous nitrogen and oxygen contain diatomic (two-atom) molecules Figure 4.15: The decomposition of two water molecules (H 2 O) to form two hydrogen molecules (H 2 ) and an oxygen molecule (O 2 ) 47 48
Important Groups - Halogens Figure 4.16: (a) Sodium chloride (table salt) can be decomposed to the elements sodium metal and chlorine gas (b) Group 7A = Halogens very reactive nonmetals react with metals to form ionic compounds HX all acids Fluorine = F 2 pale yellow gas Chlorine = Cl 2 pale green gas Bromine = Br 2 brown liquid that has lots of brown vapor over it Only other liquid element at room conditions is the metal Hg Iodine = I 2 lustrous, purple solid 49 50 Allotropes Many solid nonmetallic elements can exist in different forms with different physical properties, these are called allotropes the different physical properties arise from the different arrangements of the atoms in the solid Allotropes of Carbon include diamond graphite buckminsterfullerene Figure 4.17: In solid metals, the spherical atoms are packed closely together 51 52 Figure 4.18a: The three solid elemental (allotropes) forms of carbon Figure 4.18b: The three solid elemental (allotropes) forms of carbon 53 54
Figure 4.18c: The three solid elemental (allotropes) forms of carbon (c) Buckminsterfullerene Electrical Nature of Matter Most common pure substances are very poor conductors of electricity with the exception of metals and graphite Water is a very poor electrical conductor Some substances dissolve in water to form a solution that conducts well - these are called electrolytes When dissolved in water, electrolyte compounds break up into component ions ions are atoms or groups of atoms that have an electrical charge 55 56 Figure 4.20: (a) Pure water does not conduct a current; (b) Water containing a dissolved salt conducts electricity Ions ions that have a positive charge are called cations form when an atom loses electrons ions that have a negative charge are called anions form when an atom gains electrons ions with opposite charges attract therefore cations and anions attract each other moving ions conduct electricity compound must have no total charge, therefore we must balance the numbers of cations and anions in a compound to get 0 total charge 57 58 Figure 4.19: The ions formed by selected members of Groups 1, 2, 3, 6, and 7 59 Atomic Structures of Ions Metals form cations For each positive charge the ion has 1 less electron than the neutral atom Na = 11 e -, Na + = 10 e - Ca = 20 e -, Ca +2 = 18 e - Cations are named the same as the metal sodium Na Na + + 1e - sodium ion calcium Ca Ca +2 + 2e - calcium ion The charge on a cation can be determined from the Group number on the Periodic Table for Groups IA, IIA, IIIA Group 1A +1, Group 2A +2, (Al, Ga, In) +3 60
Figure 4.21a: The arrangement of sodium ions (Na + ) and chloride ions (Cl - ) in the ionic compound sodium chloride. Figure 4.21b: Solid sodium chloride highly magnified. 61 62 Atomic Structures of Ions Nonmetals form anions For each negative charge the ion has 1 more electron than the neutral atom F = 9 e -, F - = 10 e - P = 15 e -, P 3- = 18 e - Anions are named by changing the ending of the name to -ide fluorine F + 1e - F - fluoride ion oxygen O + 2e - O 2- oxide ion The charge on an anion can be determined from the Group number on the Periodic Table Group 7A -1, Group 6A -2 Figure 4.7: Schematic of a cathode ray tube 63 64 Figure 4.8: A CRT being used to display computer graphics 65