Chapter 2Atoms and Elements

Example: Two samples of carbon dioxide are decomposed into their constituent elements. Once sample produces 25.6 g of oxygen and 9.60 g of carbon, and the other produces 21.6 g of oxygen and 8.10 g of carbon. Show that these results are consistent with the law of definite proportions.

-Sample 1: 25.6 g O and 9.60 g C -Sample 2: 21.6 g O and 8.10 g C Proportion O:C -Sample 1: mass O/mass C = 25.6g/9.60g = 2.67 -Sample 2: mass O/mass C = 21.6g/8.10g = 2.67

If a 10.0-g sample of calcite contains 4.0 g of calcium, how much calcite contains 0.24 g of calcium?

-Sample 1: 4.0 g Ca and 10.0 g calcite -Sample 2: 0.24 g Ca mass calcite, g [g calcite(1)/ g Ca(1)] * g Ca(2) = g calcite(2) (10g calcite/4g Ca) * .24gCa =.60 g calcite

Hematite contains 2.327 g of Fe for every 1.00 g of oxygen. Litharge contains 3.490 g of Fe per gram of oxygen. Show these results are consistent with the Law of Multiple Proportions.

-hematite: 2.327 g Fe per 1 g O -litharge: 3.490 g Fe per 1 g O Fe in litharge:Fe in hematite mass Fe in litharge per 1gO / mass Fe in hematite per 1gO = 3.490g/2.327g = 1.5 1.5 = 3:2

Show that two oxides of nitrogen are consistent with the Law of Multiple Proportions

-nitrogen dioxide: 2.28 g O per 1 g N -dinitrogen monoxide: 0.570 g O per 1 g N -O in nitrogen dioxide: O in dinitrogen monoxide mass O in Nitrogen dioxide per 1gN / mass O in dinitrogen monoxide per 1gN = 2.28g/.570g = 4.00

How many protons, neutrons, and electrons are in 247Cm?

247 protons, 247 neutrons, 247 electrons 247 protons, 96 neutrons, 96 electrons 96 protons, 247 neutrons, 151 electrons 96 protons, 151 neutrons, 96 electrons<=======

How many neutrons are in an atom of Cl-37?

37 37,000 20<======(37-17=20) 20,000 Cannot be determined without additional information.

How many protons, electrons, and neutrons are in an atom of Cr? Mass#: 52 Atomic#: 24

52-24=28 n=28 p=24 e=24

2.5 Summary of Subatomic Particles

A proton and a neutron have the same mass, so each is defined as weighing 1 amu. MORE ACCURENT MASSES THAN PIC: proton: 1.67262 x 10^-24 g, 1.00727 amu Electron: .00091 x 10^-24 g, .00055 amu neutron: 1.67493 x 10^-24 g, 1.00866 amu

2.5 Atomic Number

All atoms of the same element always have the same number of protons (unique to that element) - An atom's atomic number indicates the number of protons in an atom. -Elements are arranged on periodic table in order of atomic # - The atomic number is located at the top of the element block above the elemental symbol. Each element has a unique name and symbol. -symbol either one or two letters -one capital letter or one capital letter and one lowercase

2.4 Rutherford's Conclusions

An atom is mostly empty space. -because almost all the particles went straight through An atom contains a dense particle that was small in volume compared to the atom but large in mass. - because of the few particles that bounced back This dense particle was positively charged. -because of the large deflections of some of the particles Remember α particles are (+) charged! *LOOK AT INTERACTIVE MODEL PP SLIDE 17*

2.2 Law of Conservation of Mass

Antoine Lavoisier 1743-1794 In a chemical reaction, matter is neither created nor destroyed. Total mass of the materials you have before the reaction must equal the total mass of the materials you have at the end. -total mass of reactants = total mass of products

2.5 Isotopes

Atomic Number -Number of protons -Z Mass Number -Protons + Neutrons -Whole number -A Abundance = relative amount found in a sample

Oxides of Carbon

Carbon combines with oxygen to form two different compounds, carbon monoxide and carbon dioxide. Carbon monoxide contains 1.33 g of oxygen for every 1.00 g of carbon. Carbon dioxide contains 2.67 g of oxygen for every 1.00 g of carbon. Since there are twice as many oxygen atoms per carbon atom in carbon dioxide than in carbon monoxide, the oxygen mass ratio should be 2. mass of O that combines with 1g of C in CO(2)/ mass of O that combines with 1g of carbon in CO = 2.67g/1.33g = 2

Practice—Decide if each statement is correct according to Dalton's Model of the Atom

Copper atoms can combine with zinc atoms to make gold atoms -Incorrect -According to Dalton, atoms of one element cannot turn into atoms of another element by a chemical reaction. He knew this because if atoms could change it would change the total mass and violate the Law of Conservation of Mass. Water is composed of many identical molecules that have one oxygen atom and two hydrogen atoms -Correct -According to Dalton, atoms combine together in compounds in small whole-number ratios, so that you could describe a compound by describing the number of atoms of each element in a molecule. He used this idea to explain why compounds obey the Law of Definite Proportions. Some carbon atoms weigh more than other carbon atoms - Incorrect -According to Dalton, all atoms of an element are identical Since the mass ratio of Fe:O in hematite is 1.5 times larger than the Fe:O ratio in litharge, there must be 1.5 Fe atoms in hematite and 1 Fe atom in litharge - Incorrect -According to Dalton, atoms must combine in small whole-number ratios. If you could combine fractions of atoms, that would mean the atom is breakable and Dalton's first premise would be incorrect. You can get the mass Fe:Fe ratio to be 1.5 if the formula for hematite is Fe2O3 and litharge is FeO.

2.2 Dalton's Atomic Theory

Dalton proposed a theory of matter based on it having ultimate, indivisible particles to explain these laws. *Each element is composed of tiny, indestructible particles called atoms. *All atoms of a given element have the same mass and other properties that distinguish them from atoms of other elements. *Atoms combine in simple, whole-number ratios to form molecules of compounds. *In a chemical reaction, atoms of one element cannot change into atoms of another element. - They simply rearrange the way they are attached.

2.5 Isotopes and Atomic Mass

Determine the number of protons, neutrons, and electrons present in the following isotopes of nitrogen: a. N-15 b. N-14 c. N-16 Nitrogen's atomic number is 7 -so all have 7 protons. A nitrogen atom has no overall charge -they all also have 7 electrons. The number of neutrons is different for each of the isotopes. -Number of neutrons = mass number - number of protons a. 15-7= 8 neutrons b. 14-7= 7 neutrons c. 16-7= 9 neutrons

2.3 Electrons

Electrons are particles found in all atoms. Cathode rays are streams of electrons. The electron has a charge of −1.60 × 1019 C. The electron has a mass of 9.1 × 10−28 g.

2.3 Discovery of the Electron

Further study revealed the atom is composed of even smaller particles Cathode rays are produced when high electrical voltage is applied between two electrodes within a partially evacuated glass tube When connected to a high-voltage power supply, a glowing area is seen emanating from the cathode. *J. J. Thomson* Believed that the cathode ray was composed of tiny particles with an electrical charge Designed an experiment to demonstrate that there were particles by measuring the amount of force it takes to deflect their path a given amount

2.1: Scanning Tunneling Microscope (1981)

Gerd Binnig and Heinrich Rohrer found that as you pass a sharp metal tip over a flat metal surface, the amount of current that flowed varied with distance between the tip and the surface. Measuring this "tunneling" current allowed them to scan the surface on an atomic scale─ essentially taking pictures of atoms on the surface.

2.4 Rutherford's Experiment

How can you prove something is empty? Fire something through it! - Use large target atoms. - Use very thin sheets of target so it will not absorb "bullet". - Use very small particle as bullet with very high energy. --- but not so small that electrons will affect it bullet = alpha particles, target atoms = gold foil -a particles have a mass of 4 amu and charge of +2 cu. -Gold has a mass of 197 amu and is very malleable (easily formed into very thin sheets).

2.3 Thomson's Conclusions

If the particle has the same amount of charge as a hydrogen ion, then it must have a mass almost 2000× smaller than hydrogen atoms! The only way for this to be true is if these particles were pieces of atoms. = Apparently, the atom is not unbreakable. Thomson believed that these particles were the ultimate building blocks of matter. These cathode ray particles became known as electrons.

2.4 Radioactivity

In the late 1800s, Henri Becquerel and Marie Curie discovered that certain elements would constantly emit small, energetic particles and rays. These energetic particles could penetrate matter. Ernest Rutherford discovered that there were three different kinds of emissions. (Ch. 19) -alpha, a, particles with a mass 4× H atom and 2+ charge - beta, b, particles with a mass ~1/2000 H atom and - charge gamma, g, rays that are energy rays, not particles with no measurable charge This discovery allowed scientists to experimentally probe the structure of the atom

Thomson's Experiment

Investigated the effect on a cathode ray of placing an electric field around tube (1) Charged matter is attracted to an electric field. (2) Light's path is not deflected by an electric field.

isotopes

Isotopes are atoms of the same element - Same number of protons - Different number of neutrons - They have different mass numbers Isotopes can be written in symbolic notation Isotopes can also be written by stating the element name followed by the mass number. -For example, carbon-12, carbon-13, and carbon-14.

2.4 Relative Mass and Charge

It is sometimes easier to compare things to each other rather than to an outside standard. When you do this, the scale of comparison is called a relative scale. We generally talk about the size of charge on atoms by comparing it to the amount of charge on an electron, which we call −1 charge units. -A proton has a charge of +1 cu. -Protons and electrons have equal amounts of charge, but opposite signs. We generally talk about the mass of atoms by comparing it to 1/12th the mass of a carbon atom with 6 protons and 6 neutrons, which we call 1 atomic mass unit. -Protons have a mass of 1 amu. - Electrons have a mass of 0.00055 amu, which is generally too small to be relevant.

2.5 Structure of the Nucleus

It was discovered that the same element could have atoms with different masses, which he called isotopes. There are two isotopes of chlorine found in nature, - a mass of about 35 amu -A mass of about 37 amu. The observed mass is a weighted average of the weights of all the naturally occurring atoms. -The percentage of an element that is one isotope is called the isotope's natural abundance. - The atomic mass of chlorine is 35.45 amu. All isotopes have the same number of protons (to still be considered that atom) The different mass numbers of these atoms are due to a difference in the number of neutrons.

2.2 Law of Multiple Proportions

John Dalton 1766-1844 When two elements (call them A and B) form two different compounds, the masses of B that combine with 1 g of A can be expressed as a ratio of small whole numbers.

2.2 Law of Definite Proportions

Joseph Proust 1754-1826 -All samples of a given compound, regardless of their source or how they were prepared, have the same proportions of their constituent elements. Examples: A 100.0 g sample of sodium chloride contains 39.3 g of sodium and 60.7 g of chloride. Mass of Cl/mass of Na= 60.7g/39.3g = 1.54 A 200g sample of sodium chloride contains 78.6g of sodium and 121.4g of chloride. mass of Cl/mass of Na = 121.4g/78.6g = 1.54 A 58.44g sample ofsodium chloride contains 22.99g of sodium and 35.44g of chloride. mass of Cl/mass of Na = 35.44g/22.99g = 1.541

2.1 Scanning Tunneling Microscope

Later scientists found that not only can you see the atoms on the surface, but the instrument allows you to move individual atoms across the surface.

2.4 Some Problems

Lithium has 3 protons stuck together in the nucleus -Shouldn't they repel each other? 3 protons weigh 3 amu. Li actually weighs 6.941 amu! Where does the extra mass come from? -Each proton weighs 1 amu. -Electron's mass is only about 0.00055 amu and Li has only 3 electrons - No way for the extra 4.9 amu of mass are due to e-.

Which of the following statements is false?

Most of the atom's mass and all of its negative charge are contained in a small core called the nucleus. <======== Most of the volume of the atom is empty space, throughout which tiny, negatively charged electrons are dispersed. There are as many negatively charged electrons outside the nucleus as there are positively charged particles (named protons) within the nucleus, so that the atom is electrically neutral.

2.4 Rutherford's Results

Over 98% of the a particles went straight through. About 2% of the a particles went through but were deflected by large angles. About 0.005% of the a particles bounced off the gold foil. "... as if you fired a 15" cannon shell at a piece of tissue paper and it bounced back" Remember α particle are (+)!

2.5 Structure of the Atom

Rutherford proposed that the nucleus had a particle that had the same amount of charge as an electron but opposite sign. (+) - based on measurements of the nuclear charge of the elements These particles are called protons. -charge = +1.60 × 1019 C -mass = 1.67262 × 10−24 g 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. - All atoms of an element contain the same number of protons. -All atoms are neutral and therefore, the number of protons equals the number of electrons.

2.4 A New Theory of the Atom

Since the atom is no longer indivisible, Thomson must propose a new model of the atom to replace the first statement in Dalton's Atomic Theory. -rest of Dalton's theory still valid at this point Thomson proposes that instead of being a hard, marble-like unbreakable sphere, the way Dalton described it, that it actually has an inner structure. The discovery of negatively charged particles, and the fact that atoms are charge-neutral -There must be a positive charge that neutralized the negative charge

2.2 Early Philosophy of Matter

Some early philosophers believed that matter had an ultimate, tiny, indivisible particle. - Leucippus and Democritus Other philosophers believed that matter was infinitely divisible. - Plato and Aristotle Since there was no experimental way of proving who was correct, the best debater was the person assumed correct, i.e., -Aristotle. In the late 17th century, the scientific approach to understanding nature became established. For the next 150+ years, observations about nature were made that could not easily be explained by the infinitely divisible matter concept.

2.5 The Periodic Table of the Elements

Some symbols are one capital letter, like C, S, and I. Others are two letters, and the second is lower case, like Br and Sr Some symbols come from the element 's name, like C for carbon. Others come from the Latin name of the element, like Au for gold (aurum) and Cu for copper (cuprum)

2.5 Symbolic Notation

Symbolic notation is a method used to represent an atom's atomic symbol, mass number, and atomic number.

2.4 Rutherford's Interpretation—the Nuclear Model

The atom contains a tiny dense center called the nucleus. -The amount of space taken by the nucleus is only about 1/10 trillionth the volume of the atom. -Analogy: If an atom were the size of an enclosed football arena, the nucleus would be about the size of a pea on the 50-yard line with the electrons occupying the space within the enclosed arena. The nucleus has essentially the entire mass of the atom. -The electrons weigh so little they give practically no mass to the atom. The nucleus is positively charged. -The amount of positive charge balances the negative charge of the electrons. The electrons are dispersed in the empty space of the atom surrounding the nucleus. - Electron cloud

2.3 Thomson's Results

The cathode rays are made of tiny particles. These particles have a negative charge. -because the beam always deflected toward the + plate The amount of deflection was related to two factors, the charge and mass of the particles. Every material tested contained these same particles. The charge:mass ratio of these particles was −1.76 × 108 C/g. -The charge/mass of the hydrogen ion is +9.58 × 104 C/g.

2.2 Reaction of Sodium with Chlorine to Make Sodium Chloride

The mass of sodium and chlorine used is determined by the number of atoms that combine. Since only whole atoms combine and atoms are not changed or destroyed in the process, *the mass of sodium chloride made must equal the total mass of sodium and chlorine atoms* that combine together. 7.7 g Na + 11.9 g Cl2 19.6 g NaCl

.4 Predictions of the Plum Pudding Atom

The mass of the atom is due to the mass of the electrons within it. (incorrect) -Electrons are the only particles in Plum Pudding atoms. (incorrect) The atom is mostly empty space. √ -cannot have a bunch of negatively charged particles near each other as they would repel each other√

2.4 Thomson's Plum Pudding Atom

The structure of the atom contains many negatively charged electrons. These electrons are held in the atom by their attraction for a positively charged electric field within the atom. - There had to be a source of positive charge because the atom is neutral. -Thomson assumed there were no positively charged pieces because none showed up in the cathode ray experiment.

2.4 There Must Be Something Else!

To answer these questions, Rutherford and Chadwick proposed that there was another particle in the nucleus—it is called a neutron. Neutrons have no charge and a mass of 1 amu. - mass = 1.67493 × 10−24 g -----slightly heavier than a proton -no charge

2.5 Mass Number

To determine the number of neutrons in an atom, we must look at the mass number The mass number -located below the elemental symbol in the element block of the periodic table - Equals the number of protons plus the number of neutrons. Mass number = number of protons + number of neutrons

2.3 Millikan's Oil Drop Experiment

Used to deduce the charge of a single electron Oil spayed in fine droplets using an atomizer Droplets allowed to fall through a small hole in the lower part of apparatus Can be viewed by light source and microscope During fall -Drops acquired electrons produced when air was bombarded with a type of energy (ionizing radiation) Negatively charged drops interacted with negatively charged plate at bottom Since (-) oil repels each other - By varying the amount of charge on the plate, the fall of drops could be slowed stopped, or reversed Voltage to halt free falling drops were measured He calculated mass of drops (from radii and density) Then he was able to calculate the charge of each drop Found that measured charge on drop was always a whole-number multiple of -1.60 x 10 -19 *Found that mass of electron was 2000 times lighter than H (9.1 × 10−28 g)

2.5 Charged Atoms

When atoms gain or lose electrons, they acquire a charge. Charged particles are called ions. When atoms gain electrons, they become negatively charged ions, called anions. When atoms lose electrons, they become positively charged ions, called cations.

2.5 Reacting Atoms

When elements undergo chemical reactions, the reacting elements do not turn into other elements. =Statement 4 of Dalton's Atomic Theory This requires that all the atoms present when you start the reaction will still be there after the reaction. Since the number of protons determines the kind of element, the number of protons in the atom does not change in a chemical reaction. However, many reactions involve transferring electrons from one atom to another.

Which subatomic particle determines the identity of the element?

electron proton<====== neutron

2.5

number of protons defines the element

practice for neon

pp slide 38

complete table pratice

pp slide 41

Two samples of the same compound are compared. What does the data represent?Sample 1: 24.22 g carbon and 32.00 g oxygen Sample 2: 36.22 g carbon and 48.00 g oxygen

the law of definite proportions <--------- the law of multiple proportions the law of conservation of mass

2.3 Some Notes on Charge

two kinds of charge + and - opposite charges attract + attracted to - like charges repel + repels + - repels - -to be neutral, something must have no charge or equal amounts of opposite charges.