# Ch. 3: Atoms: The Building Blocks of Matter

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Ch. 3: Atoms: The Building Blocks of Matter
3.1 The Atom: From Philosophical Idea to Scientific Theory 3.2 The structure of the atom 3.3 Counting Atoms

4.1 An atom is the smallest particle of an element that retains its identity in a chemical reaction. Philosophers and scientists have proposed many ideas on the structure of atoms. The lab technician shown here is using a magnifying lens to examine a bacterial culture in a petri dish. When scientists cannot see the details of what they study, they try to obtain experimental data that help fill in the picture.

3 laws Law of conservation of mass: Mass can neither be created nor destroyed during ordinary chemical and physical reactions. Problem 1: When ammonium nitrate (NH4NO3) explodes, the products are nitrogen, oxygen, and water. When 40. g of ammonium nitrate explode, 14 g of nitrogen and 8 grams of oxygen form. How many grams of water form? Law of definite proportions: A pure compound, whatever its source, always contains definite or constant proportions of the elements by mass. Ex g of NaCl always contains g of sodium and g of chlorine chemically combined. Sodium chloride has definite proportions of sodium and chlorine, irrespective of the source. Problem 1: If you have 5.00 g of NaCl, how much of it is sodium? Problem 2: What percent of NaCl is sodium?

3 laws Law of multiple proportions: It states when two elements form more than one compound, the masses of one element in these compounds for a fixed mass of the other element are in ratios of small whole numbers. Ex. Carbon and oxygen form two compounds: carbon monoxide (CO) and carbon dioxide (CO2). Carbon monoxide contains g of oxygen for each g of carbon, whereas carbon dioxide contains g of oxygen for each g of carbon. In other words, the ratio of oxygen that combines with g of carbon is 1:2 (small whole number) Problem 4: Hydrogen and oxygen can combine to form water (H2O) and hydrogen peroxide (H2O2). The mass of oxygen in 1.00 mole of water is 16.0 g and the mass of oxygen in 1.00 mol of hydrogen peroxide is 32.0 g. What is the ratio of oxygen in the two compounds?

Early Models of the Atom
4.1 Early Models of the Atom Democritus’s Atomic Philosophy 400 BC (430?) “Everything is made up of a few simple parts called atomos.” Atomos means “uncuttable” in Greek. He envisioned atomos as small, solid particles of many different sizes and shapes. Democritus’s ideas were limited because they didn’t explain chemical behavior and they lacked experimental support. His ideas were rejected because Aristotle supported *the “earth, air, water, and fire” concept of matter. Democritus believed that atoms were solid particles that are indivisible and indestructible.

John Dalton, 1803 (1808?) By using experimental methods, Dalton transformed Democritus’s ideas on atoms into a scientific theory. Dalton’s atomic theory is described in four parts (following slides): “Each element consists of a particular kind of atom. All atoms of a particular element are identical.” His atoms had a specific mass, size, and chemical behavior.

Early Models of the Atom
4.1 Early Models of the Atom 2. Atoms of the same element are identical. The atoms of any one element are different from those of any other element. 1. All elements are composed of tiny indivisible particles called atoms. According to Dalton’s atomic theory, an element is composed of only one kind of atom, and a compound is composed of particles that are chemical combinations of different kinds of atoms. a) Atoms of element A are identical. b) Atoms of element B are identical, but differ from those of element A. c) Atoms of elements A and B can physically mix together. d) Atoms of elements A and B can chemically combine to form a compound. Interpreting Diagrams How does a mixture of atoms of different elements differ from a compound?

Early Models of the Atom
4.1 Early Models of the Atom 3. Atoms of different elements can physically mix together or can chemically combine in simple whole-number ratios to form compounds. 4. Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of one element are never changed into atoms of another element in a chemical reaction. According to Dalton’s atomic theory, an element is composed of only one kind of atom, and a compound is composed of particles that are chemical combinations of different kinds of atoms. a) Atoms of element A are identical. b) Atoms of element B are identical, but differ from those of element A. c) Atoms of elements A and B can physically mix together. d) Atoms of elements A and B can chemically combine to form a compound. Interpreting Diagrams How does a mixture of atoms of different elements differ from a compound?

Main assumptions or postulates of Dalton
Summary of John Dalton’s atomic theory as listed by tutorvista website: John Dalton, a British school teacher, published his theory about atoms in the year His findings were based on experiments and also from laws of chemical combination. Main assumptions or postulates of Dalton All matter consists of indivisible particles called atoms. Atoms of the same element are similar in shape and mass, but differ from the atoms of other elements. Atoms cannot be created or destroyed. Atoms of different elements may combine with each other in a fixed, simple, whole number ratio to form compound atoms. Atoms of same element can combine in more than one ratio to form two or more compounds. Atoms are the smallest unit of matter that can take part in a chemical reaction.

Continued…Drawbacks of Dalton's atomic theory of matter
The indivisibility of an atom was proved wrong, for, an atom can be further subdivided into protons, neutrons and electrons. However an atom is the smallest particle, which takes part in chemical reactions. According to Dalton, the atoms of same element are similar in all respects. This is wrong because atoms of some elements vary in their mass and density. Such atoms of the same element having different masses are called isotopes. For example, chlorine has two isotopes having mass numbers 35 a.m.u and 37 a.m.u. Dalton also said atoms of different elements are different in all respects. This has been proved wrong in certain cases like argon and calcium atoms, which have the same atomic mass of 40. Such atoms of different elements that have the same atomic mass are called isobar. According to Dalton atoms of different elements combine in simple whole number ratio to form compounds. This is not seen in complex organic compounds like sugar C12H22O11. The theory completely fails to explain the existence of allotropes. The difference in properties of charcoal, graphite, diamond went unexplained in spite of being made up of same kind of atoms.

Merits of Dalton's atomic theory
Continued…tutorvista comments Merits of Dalton's atomic theory It has enabled us to explain the laws of chemical combination. Dalton was the first person to recognize a workable distinction between the ultimate particle of an element (atom) and that of a compound (molecule).

3.2 Structure of the Nuclear Atom
4.2 3.2 Structure of the Nuclear Atom Three kinds of subatomic particles are electrons, protons, and neutrons. Cathode-ray tubes are found in TVs, computer monitors, and many other devices with electronic displays.

Subatomic Particles Electrons 4.2
In 1897, the English physicist J. J. Thomson (1856–1940) discovered the electron. Electrons are negatively charged subatomic particles. Thomson performed experiments that involved passing electric current through gases at low pressure. The result was a glowing beam, or cathode ray, that traveled from the cathode to the anode.

4.2 Subatomic Particles Cathode Ray Tube A cathode ray is deflected by a magnet. A cathode ray is deflected by electrically charged plates. In a cathode-ray tube, electrons travel as a ray from the cathode (-) to the anode (+). A television tube is a specialized type of cathode-ray tube. Thomson concluded that a cathode ray is a stream of electrons. Electrons are parts of the atoms of all elements.

Charge and mass of the electron
Thomson’s experiment revealed that the electron has a very large charge for its tiny mass. In 1909, Robert A. Millikan led to the final determination that the mass of the electron is 1/1837 the mass of the simplest type of hydrogen atom, or x kg. He also confirmed that the electron carries a negative electric charge. Based on this information, it was inferred that because atoms are electrically neutral, they must contain a positive charge to balance the negative electrons, and because electrons have such a small mass, atoms must contain other particles to account for most of the mass of an atom.

Review: Properties of Electric Charge
Electric charge may be only (+) or (-). Opposite charges attract each other Like charges repel.

*History of Electric Charge
Discovered by Ben Franklin in the 1750’s in his famous “kite” experiment. Franklin coined the following terms to help describe his findings: positive (to indicate an object that donates electric charge) negative (an object that accepts electric charge) Plus, minus, battery, conductor, and charge Unfortunately, Franklin decided to describe electricity as moving from (+) to (-). Later, it was discovered (by Thomson) that electricity is the motion of negative charges (electrons), which move from the (-) terminal on a battery to the (+) one. Today, physicists and physics students still use the convention that electricity flows from (+) to (-), even though they know that electrons flow the opposite way!

4.2 Subatomic Particles In 1886, Eugen Goldstein (1850–1930) observed a cathode-ray tube and found rays traveling in the direction opposite to that of the cathode rays. He concluded that they were composed of positive particles. Such positively charged subatomic particles are called protons. In 1932, the English physicist James Chadwick (1891–1974) confirmed the existence of yet another subatomic particle: the neutron. Neutrons are subatomic particles with no charge but with a mass nearly equal to that of a proton.

4.2 Subatomic Particles Table 4.1 summarizes the properties of electrons, protons, and neutrons.

4.2 The Atomic Nucleus How can you describe the structure of the nuclear atom? Plum pudding model-- J.J. Thomson and others supposed the atom was filled with positively charged material and the electrons were evenly distributed throughout. Rutherford atomic model--This model of the atom turned out to be short-lived, however, due to the work of Ernest Rutherford (1871–1937).

The Atomic Nucleus 4.2 Rutherford’s Gold-Foil Experiment
In 1911, Rutherford and his coworkers at the University of Manchester, England, directed a narrow beam of alpha particles at a very thin sheet of gold foil. Alpha particles scatter from the gold foil.

The Atomic Nucleus 4.2 The Rutherford Atomic Model
Ernest Rutherford concluded that the atom is mostly empty space. All the positive charge and almost all of the mass are concentrated in a small region called the nucleus. The nucleus is 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.

Four Forces in the Nucleus
Repulsive: 1. Electrostatic: between p+ (like charges repel) Attractive: 2. “Strong force”: between p+ + + 3. “Weak force”: between p+ and n 4. Gravity: between all particles Strong > electrostatic > weak > gravity Forces in the nucleus: Same charges repel each other but when two protons are extremely close to each other, there are strong attractive forces between them. These forces also exist between p-n and n-n. These short-range forces between p-p, p-n, and n-n are called as nuclear forces and they keep the nuclear particles together.

4.3 3.3 Counting Atoms The atomic number of an element is the number of protons in the nucleus of an atom of that element. Elements are different because they contain different numbers of protons.

Practice #1

4.3 Mass Number The total number of protons and neutrons in an atom is called the mass number. The number of neutrons in an atom is the difference between the mass number and atomic number.

4.3 Isotopes Isotopes are atoms that have the same number of protons but different numbers of neutrons. Because isotopes of an element have different numbers of neutrons, they also have different mass numbers. Despite these differences, isotopes are chemically alike because they have identical numbers of protons and electrons.

E Isotopic notation Atomic number = # p A Z
Mass number = p + n (in the nucleus) Element symbol A E Z Atomic number = # p Elements are put in order of atomic number on the periodic table. Ex: An atom of carbon with 7 neutrons: An atom of lead with 125 neutrons: 13C 6 207Pb 82

Mass Number Au is the chemical symbol for gold. 4.3 How many protons,
electrons, and neutrons does a gold atom have? The atomic number is 79. Therefore, there are 79 protons and 79 electrons. The mass number is 197, which is the total number of protons and neutrons. Therefore, = 118 neutrons. Au is the chemical symbol for gold. Applying Concepts How many electrons does a gold atom have?

Practice #2

4.3 Atomic Mass Some elements and their isotopes An atomic mass unit (amu) is defined as one twelfth of the mass of a carbon-12 atom. It is useful to to compare the relative masses of atoms to a standard reference isotope. Carbon-12 is the standard reference isotope. Carbon-12 has a mass of exactly 12 atomic mass units.

Atomic Mass 4.3 The atomic mass of an element is a weighted average mass of the atoms in a naturally occurring sample of the element. A weighted average mass reflects both the mass and the relative abundance of the isotopes as they occur in nature.

Atomic Mass 4.3 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. For example, carbon has two stable isotopes: Carbon-12, which has a natural abundance of 98.89% and a mass of amu Carbon-13, which has a natural abundance of 1.11% and a mass of amu. Silver is found in two isotopes with atomic masses and amu, respectively. The first isotope represents 51.82% and the second 48.18%. Determine the average atomic mass of silver. ( )(.5182)= ( )(.4818)= = amu

Relating Mass to Numbers of Atoms
The mole: One mole of something contains x 1023 units of that substance. Ex. One mole of Hydrogen atoms contains exactly x 1023 atoms of Hydrogen. Avogadro’s number: It is defined as the number of particles in one mole of any pure substance. This number is found to be x 1023. Molar mass: Mass of one mole of a substance. Molar mass can be calculated by expressing the mass in amu or g. The molar mass of C = 12 g

10.1 What is a Mole?

Converting numbers of atoms to moles
Sample problem 10.2 On your calculator be sure to put the denominator in parentheses or divide by both 6.02 and 1023

Converting moles to number of atoms
Sample problem 10.3

The Mass of a Mole of an Element
10.1 The Mass of a Mole of an Element The atomic mass of an element expressed in grams is the mass of a mole of the element. The mass of a mole of an element is its molar mass. Ex. On the periodic table, carbon has a mass of amu. We say 12 grams per mole or just 12 grams.

The Mass of a Mole of an Element
10.1 The Mass of a Mole of an Element One molar mass of carbon, sulfur, mercury, and iron are shown. One molar mass of carbon, sulfur, mercury, and iron are shown. Each of the quantities contains one mole of the element. Applying Concepts How many atoms of each element are shown?

The Mass of a Mole of an Element
10.1 The Mass of a Mole of an Element

The Mass of a Mole of a Compound
10.1 To calculate the molar mass of a compound, find the number of grams of each element in one mole of the compound. Then add the masses of the elements in the compound. Substitute the unit grams for atomic mass units. Thus 1 mol of SO3 has a mass of 80.1 g.

The Mass of a Mole of a Compound
10.1 The Mass of a Mole of a Compound Molar Masses of Glucose, Water, and Paradichlorobenzene One molar mass is shown for each of three molecular compounds. Inferring How can you know that each sample contains Avogadro’s number of molecules?

Determining molar mass
phosphorus molar mass = g chlorine molar mass = g 1 mole of phosphorus = g 3 mole of chlorine = 3 x = g molar mass of PCl3 = = 137 g/mol

The Mole–Mass Relationship
10.2 The Mole–Mass Relationship How do you convert the mass of a substance to the number of moles of the substance? Use the molar mass of an element or compound to convert between the mass of a substance and the moles of a substance.

These aluminum satellite dishes at the National Radio Astronomy Observatory near Soccoro, New Mexico are naturally protected from corrosion by the formation of a thin film of aluminum oxide (Al2O3).

Converting moles to mass
mass of carbon= 20 x = g mass of hydrogen = 42 x = g mass of C20H42 = g/mol

Rust weakens an iron chain.

Converting mass to moles

The Mole–Volume Relationship
10.2 The Mole–Volume Relationship Avogadro’s hypothesis states that equal volumes of gases at the same temperature and pressure contain equal numbers of particles. Standard temperature and pressure (STP) means a temperature of 0°C or 273 K and a pressure of kPa (kilopascals) or 1 atmosphere (atm). In each container, the volume occupied by the gas molecules is small compared with the container’s volume, so the molecules are not tightly packed. a) The molecules in this container are small. b) This container can accommodate the same number of larger molecules. The volume of a gas varies with temperature and pressure. Because of these variations, the volume of a gas is usually measured at a standard temperature and pressure.

The Mole–Volume Relationship
10.2 The Mole–Volume Relationship At STP, 1 mol or, 6.02  1023 representative particles, of any gas occupies a volume of 22.4 L. The quantity 22.4 L is called the molar volume of a gas. This box, with a volume of 22.4 L, holds one mole of gas at STP.

Determining volume at STP

The Mole–Volume Relationship
10.2 The Mole–Volume Relationship Calculating Molar Mass from Density

Using density and molar volume

10.2 The Mole Road Map The map shows the conversion factors needed to convert among volume, mass, and number of particles. Interpreting Diagrams How many conversion factors are needed to convert from the mass of a gas to the volume of a gas at STP?

10.2 The Mole Road Map The map shows the conversion factors needed to convert among volume, mass, and number of particles. Interpreting Diagrams How many conversion factors are needed to convert from the mass of a gas to the volume of a gas at STP?

Chapter 3 Objectives Explain the law of conservation of mass, the law of definite proportions and the law of multiple proportions Summarize the five essential points of Dalton’s atomic theory Explain the relationship between Dalton’s theory and the three laws Summarize the experiments involving CRTs and how that led to the discovery of the electron Summarize the experiments done by Rutherford that led to the discovery of the nucleus. Visually draw the structure of the atom according to Dalton, Thompson and Rutherford and label the subatomic particles List the properties of protons, electrons and neutrons Define atom What are isotopes Write nuclear symbols for isotopes and calculate number of protons, electrons and neutrons Define atomic number and mass number and describe how they apply to isotopes Distinguish between relative mass, average atomic mass and molar mass. Calculate average atomic mass Define the mole in terms of Avogadro’s number and define molar mass Solve problems involving mass in grams, amount in moles and number of atoms of an element and a compound

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