Chemistry Chapter 3. Atoms: The Building Blocks of Matter

Chemistry Chapter 3 Atoms: The Building Blocks of Matter

I. From Philosophical Idea to Scientific Theory

History of the Atom The Ancient Greeks were the first to come up with the idea of the atom. Democritus and Leucippus believed that all matter was made up of tiny particles, which they called atomos. Sand On A Seashore They reasoned that if you kept breaking sand into smaller and smaller pieces, eventually you would get a piece that couldn t be broken anymore.

History of the Atom: Democritus and Leucippus s Theory of Matter: a) Point #1 - All matter is made up of undividable particles called atoms. b) Point #2 - There is a void, which is empty space between atoms. c) Point #3 Atoms are completely solid d) Point #4 Atoms are homogeneous, with no internal structure e) Point #5 Atoms vary in size, shape, weight

History of the Atom: Aristotle: Greek Philosopher that believed that all matter was composed of four elements: Earth, Wind, Fire, Water. And that those four elements in different proportions would give you the varying types of matter

Hot Light FIRE wind Wet water Earth Cool and Heavy

Who was right? Even though Democritus and Leucippus were closer to being right, Aristotle won the argument. Why?

1) Greeks did not experiment, they argued--aristotle was more famous so He won! 2) His ideas carried throughout the Middle Ages. but alchemists, chemists, and some science hobbyists made discoveries that changed everything and paved the way for a new theory.

Foundations of Atomic Theory 1) By the late 1700 s, scientists had determined: 1)An element could not be broken down by ordinary chemical means 2)Compounds were composed of elements 3)Elements have different physical and chemical properties. 2) The transformation of a substance or substances into one or more new substances is known as a chemical reaction.

3. Antoine Lavoisier - credited with the discovery of the Law of Conservation of Mass. He focused on the measuring of the weights of reactants and products during combustion reactions. He concluded that during the combustion of phosphorus and sulfur in air, the products weighed more than the original because the air combined with the reactants in the reaction.

Law of Conservation of Mass Mass is neither created nor destroyed during ordinary chemical reactions or physical changes Visual Concept

5) Law of definite proportions: a chemical compound is always composed of the same elements in exactly the same proportions by mass regardless of the size of the sample or source of the compound. (The mass of the compound is the sum of the masses of the elements that make it.) Pure water has the same mass ratio no matter how much you have or how it was made. Salt (NaCl) has the same mass ratio no matter if it is in a mine or on your table.

6) Law of multiple proportions: when different compounds are formed by a combination of the same elements, different masses of one element combine with the same fixed mass of the other element in a ratio of small whole numbers. https://youtu.be/d6hbmg8niru

Dalton s Atomic Theory John Dalton Was an English School Teacher Unlike the Greeks, Dalton performed experiments to test and correct his atomic theory. He studied the ratios in which elements combine in chemical reactions and formulated hypotheses and theories which could be tested

Dalton s Postulates (Parts of his theory): 1) All matter is composed extremely small particles called atoms. 2) Atoms of a given element are identical in size, mass and other properties; atoms from different elements differ in size, mass, and other properties. 3) Atoms cannot be subdivided, created or destroyed. 4) Atoms of different elements combine in simple whole-number ratios to form chemical compounds. 5) In chemical reactions, atoms are combined, separated, or rearranged.

Chemical Reaction

Most of Dalton s Atomic Theory is accepted today Dalton said: Atoms cannot be subdivided, created, or destroyed However!

one major change is the fact that an atom can be divided And A given element can now have atoms with different masses (Isotopes)

II. The Structure of the Atom

History of the Atom: In the 1800 s, scientists proved that the atom could be divided, and that the number and arrangement of these particles determine that atom s chemical and physical properties.

Atomic Structure An atom is the smallest particle of an element that retains the chemical properties of that element. 1. The nucleus is a very small, positively charged, region located at the center of an atom. a. Made up of at least one positively charged particle called a proton and one or more neutral particles called neutrons.

Atomic Structure 2) Surrounding the nucleus are shells occupied by negatively charged particles called electrons. 3) Protons, neutrons, and electrons are often referred to as subatomic particles.

Discovery of the Electron 1) Cathode Rays and Electrons a) Experiments in the late 1800s showed that cathode rays were composed of negatively charged particles. b) These particles were named electrons.

Discovery of the Electron J.J. Thomson an English physicist discovered the existence of electrons in 1897.

By bringing positively charged metal plates near the cathode ray, the path was altered. Since unlike charges attract each other, Thomson determined that the cathode ray was made up of negatively charged particles.

He performed experiments using a cathode ray tube. A sealed glass tube containing different gases that when connected to high voltage electricity a glowing beam called a cathode ray is created.

Since no difference was found by using different gases in the tube, and different metals for the plates. Thomson concluded that all atoms contain electrons.

Thomson s Experiment - Voltage source + Vacuum tube Metal Disks

Thomson s Experiment - Voltage source +

Thomson s Experiment - Voltage source + Passing an electric current makes a beam appear to move from the negative to the positive end

Thomson s Experiment Voltage source + By adding an electric field: -

Thomson s Experiment Voltage source + By adding an electric field -

Thomson s Experiment Voltage source + By adding an electric field he found that the moving pieces were negative -

Thomson s Atomic Model Thomson could not find a positively charged particle, so he believed that the electrons were like plums embedded in a positively charged pudding, thus it became known as the plum pudding model.

Thomson s Cathode-Ray Tube Experiment Click below to watch the Visual Concept. Visual Concept

1909 Robert Millikan determines the mass of the electron.

1909 Robert Millikan determines the mass of the electron. Robert Millikan sprayed very fine drops of oil into the drum, where they dropped through a very small hole. The drum had two electric plates on the inside.

Millikan watched through a scope and measured the speed at which the drops fell. When adding an electrical charge the drops fell slower and he could actually add enough charge to cause the drops to stop completely in mid-air.

The oil drop apparatus Through his experiments, he determined both the mass and the amount of charge for the electron

Millikan s Oil Drop Experiment Click below to watch the Visual Concept. Visual Concept

Conclusions from the Study of the Electron 1. Cathode rays have identical properties regardless of the element used to produce them. All elements must contain identically charged electrons. 2. Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electron 3. Electrons have so little mass, that atoms must contain other particles that account for most of the mass.

Ernest Rutherford and the Nucleus Discovery of the Atomic Nucleus 1) Ernest Rutherford s gold foil experiment led to the discovery of a very densely packed bundle of matter with a positive electric charge. a) Rutherford called this positive bundle of matter the nucleus.

Ernest Rutherford and the Nucleus (1910) Rutherford believed in the plum pudding model of the atom. He wanted to see how big atoms were, so he used radioactivity and shot alpha particles, (positively charged pieces given off by uranium) through gold foil which can be made a few atoms thick.

Lead block Uranium Florescent Screen Gold Foil

What he expected-- The alpha particles would pass through without changing direction very much

Because, he thought the mass was evenly distributed in the atom (Plum Pudding)

What he got:

A new model must be devised! However what he found was completely unexpected. Many of the particles did go straight through the gold foil, but several were deflected, some were turned at 90 degrees or more!

Rutherford Concluded : a) The atom is mostly empty space b) It has a small dense, positively charge center c) Alpha particles are deflected when they get near this center or nucleus + Chapter menu Resources Copyright by Holt, Rinehart and Winston. All rights reserved.

Rutherford s Gold Foil Experiment Click below to watch the Visual Concept. Visual Concept

2) In 1886 twelve years before Thomson, a man named Eugen Goldstein observed in a cathode ray tube rays flowing in the opposite direction. He called these rays canal rays and concluded that they were composed of positive charges. This lead eventually to the discovery of the proton 3) In 1932 English physicist James Chadwick confirmed the existence of another subatomic particle, the neutron Canal Ray Tube

Composition of the Atomic Nucleus Except for the nucleus of the simplest type of hydrogen atom, all atomic nuclei are made of protons and neutrons. a) A proton has a positive charge equal in magnitude to the negative charge of an electron. b) Atoms are electrically neutral because they contain equal numbers of protons and electrons.

Composition of the Atomic Nucleus A neutron is electrically neutral they have no charge, or are neutral (they are NOT neutrally charged!) Atoms of different elements have a different number of protons, thus the number of protons determines that atom s identity.

Electron Cloud 1) Electrons are negatively (-) charged subatomic particles. They are found orbiting around the nucleus in shells.

2) In a neutral atom of any element, the number of protons in the nucleus and the number of electrons orbiting around the nucleus is always equal this number is known as the Atomic Number. NEUTRAL ATOM : PROTONS = ELECTRONS

Parts of the Atom Click below to watch the Visual Concept. Visual Concept

III. Counting Atoms

A. Atomic Number 1) Atoms of different elements have different numbers of protons. Atoms of the same element all have the same number of protons. The atomic number of an element is the number of protons of each atom of that element.

Atomic Number

B. Isotopes 1) Isotopes are atoms of the same element that have different masses. The isotopes of a particular element all have the same number of protons and electrons but different numbers of neutrons. Most of the elements consist of mixtures of isotopes.

C. Mass Number 1) The mass number is the total number of protons and neutrons that make up the nucleus of an isotope. 2) MASS NUMBER = PROTONS + NEUTRONS

Mass Number

D. Designating Isotopes 1) Hyphen notation: The mass number is written with a hyphen after the name of the element. Uranium-235 2) Nuclear symbol: The superscript indicates the mass number and the subscript indicates the atomic number. 235 92 U

Designating Isotopes, continued 3) The number of neutrons is found by subtracting the atomic number from the mass number. mass number atomic number = number of neutrons 235 (protons + neutrons) 92 protons = 143 neutrons 4) Nuclide is a general term for a specific isotope of an element.

Sample Problem How many protons, electrons, and neutrons are there in an atom of chlorine-37?

Sample Problem Solution Given: name and mass number of chlorine-37 Unknown: numbers of protons, electrons, and neutrons Solution: atomic number = number of protons = number of electrons mass number = number of neutrons + number of protons

Solution continued: mass number of chlorine-37 atomic number of chlorine = number of neutrons in chlorine-37 mass number atomic number = 37 (protons plus neutrons) 17 protons = 20 neutrons An atom of chlorine-37 is made up of 17 electrons, 17 protons, and 20 neutrons.

E. Relative Atomic Masses 1) Because the masses of atoms are extremely small numbers, chemists instead compare the masses of atoms using simple whole numbers called Atomic Mass Units

F. Average Atomic Masses of Elements 1) Average atomic mass is the weighted average of the atomic masses of the naturally occurring isotopes of an element. 2) Calculating Average Atomic Mass a) The average atomic mass of an element depends on both the mass and the relative abundance of each of the element s isotopes.

1) Copper consists of 69.15% copper-63, which has an atomic mass of 62.929 601 amu, and 30.85% copper-65, which has an atomic mass of 64.927 794 amu. Average Atomic Mass = (Mass Number x Relative Abundance) + (Mass Number x Relative Abundance)

(0.6915 62.929 601 amu) + (0.3085 64.927 794 amu) = 63.55 amu b) The calculated average atomic mass of naturally occurring copper is 63.55 amu.

Average Atomic Mass Click below to watch the Visual Concept. Visual Concept

G. Relating Mass to Numbers of Atoms 1) The Mole a) The mole (abbreviated mol) is the SI unit for amount of substance.

Chapter 3 Section 3 Counting Atoms 2) Avogadro s Number a) Avogadro s number 6.022 1415 10 23 is the number of particles in exactly one mole of a pure substance. Amadeo Avogadro

3) Molar Mass a) The mass of one mole of a pure substance is called the molar mass of that substance. b) Molar mass is usually written in units of g/mol. c) The molar mass of an element is numerically equal to the atomic mass of the element in atomic mass units.

4) Gram/Mole Conversions a) Chemists use molar mass as a conversion factor in chemical calculations. b) For example, the molar mass of helium is 4.00 g He/mol He. 1) To find how many grams of helium there are in two moles of helium, multiply by the molar mass. 4.00 g He 2.00 mol He = 8.00 g He 1 mol He

5) Conversions with Avogadro s Number a) Avogadro s number can be used to find the number of atoms of an element from the amount in moles or to find the amount of an element in moles from the number of atoms. b) In these calculations, Avogadro s number is expressed in units of atoms per mole.

Avogadro's Number and The Mole

Sample Problem What is the mass in grams of 3.50 mol of the element copper, Cu?

Sample Problem Solution Given: 3.50 mol Cu Unknown: mass of Cu in grams Solution: the mass of an element in grams can be calculated by multiplying the amount of the element in moles by the element s molar mass. moles Cu grams Cu = grams Cu moles Cu

Sample Problem Solution, continued The molar mass of copper from the periodic table is rounded to 63.55 g/mol. 3.50 mol Cu 63.55 g Cu 1 mol Cu = 222 g Cu

Chapter 3 Section 3 Counting Atoms Sample Problem A chemist produced 11.9 g of aluminum, Al. How many moles of aluminum were produced? Chapter menu Resources Copyright by Holt, Rinehart and Winston. All rights reserved.

Chapter 3 Section 3 Counting Atoms Sample Problem Solution Given: 11.9 g Al Unknown: amount of Al in moles Solution: moles Al grams Al grams Al = moles Al The molar mass of aluminum from the periodic table is rounded to 26.98 g/mol. 1 mol Al 11.9 g Al = 0.441 mol Al 26. 98 g Al Chapter menu Resources Copyright by Holt, Rinehart and Winston. All rights reserved.

Chapter 3 Section 3 Counting Atoms Sample Problem Solution Given: 3.01 10 23 atoms of Ag Unknown: amount of Ag in moles Solution: moles Ag Ag atoms = moles Ag Avogadro's number of Ag atoms 23 3.01 10 Ag atoms 1 mol Ag 23 6.022 10 Ag at oms = 0.500 m ol Ag Chapter menu Resources Copyright by Holt, Rinehart and Winston. All rights reserved.

Chapter 3 Section 3 Counting Atoms Sample Problem Solution Given: 1.20 10 8 atoms of Cu Unknown: mass of Cu in grams Solution: Cu atoms moles Cu Avogadro's number of Cu atoms The molar mass of copper from the periodic table is rounded to 63.55 g/mol. 14 1.27 10 g Cu grams Cu moles Cu 1 mol Cu 63.55 g Cu 6.022 10 Cu atoms 1 mol Cu = grams Cu 8 1.20 10 Cu atoms = 23 Chapter menu Resources Copyright by Holt, Rinehart and Winston. All rights reserved.