Department of Chemistry CHEM1010 General Chemistry *********************************************** Instructor: Dr. Hong Zhang Foster Hall, Room 221 Tel:

CHEM1010/General Chemistry _________________________________________ Chapter 2. (L5)-Introduction to Atoms Today’s Outline..Chapter 1 Review: Chemistry is about matter and its changes..Introduction to Chapter 2..Lavoisier: The law of mass conservation -Chemistry as a quantitative science -Lavoisier: The father of modern chemistry -Mass conservation law

Chapter 2. (L5)-Introduction to Atoms Chapter 1 review In the last four lectures, we talked about what chemistry is about...We learned that chemistry is about matter and its changes...Matter is different, and there are so many different things...We can distinguish between different things by their properties or characteristics, but only those we can measure accurately will be useful in science.

Chapter 2. (L5)-Introduction to Atoms Chapter 1 review..By using the quantifiable properties of matter, we can distinguish between things accurately...The properties of matter fall into two basic categories: Physical properties and chemical properties...Commonly physical properties include: Mass, color, temperature, size (length, area, volume), density, hardness, heat capacity, boiling point, freezing point, etc.

Chapter 2. (L5)-Introduction to Atoms Chapter 1 review..Measurement of a property has two components: Value and unit. The values depend on the units used...The unit system used in science is the International System of Units or the SI Unit System. Commonly used SI units...Scientific expression of large and small numbers: Powers of 10 and prefixes...Conversion between different expressions of numbers.

Chapter 2. (L5)-Introduction to Atoms Chapter 1 review..Measurement of physical properties: Mass, length, area, volume, time, density..Concept of energy, heat, and temperature Any changes of matter require energy or release energy Conservation of energy. Units for energy..Temperature Units for temperature Conversion between different units

Chapter 2. (L5)-Introduction to Atoms Chapter 1 review..Classification of matter Pure substances Mixture (of pure substances)..Three states of matter Solid Liquid Gas

Chapter 2. (L5)-Introduction to Atoms Introduction to Chapter 2..We now have known a lot about matter or things. Now, we are asking this fundamental question: How to explain so many different things and their changes using a unified theory?..Starting from now, we will learn how the chemical theory explaining different things and their changes was created in the context of our human history and the history of science and chemistry.

Chapter 2. (L5)-Introduction to Atoms Introduction to Chapter 2..First, let’s see how we work in science...In science and chemistry, we are working on two ends, one is the scientific observations, and the other is the scientific theory. And then, the next is to connect two, meaning we need to create theories and hypotheses to explain the observations and predict new chemical changes...By the way, we need to be very clear about what is the theory and what are the observations.

Chapter 2. (L5)-Introduction to Atoms Introduction to Chapter 2..In science and chemistry, on both ends we are working, we need to be quantitative, meaning we need numbers for observations, and the theory and hypotheses need to be able to explain the observations quantitatively,..Being qualitative is fine, often the first step (very important), but it’s not enough.

Chapter 2. (L5)-Introduction to Atoms Introduction to Chapter 2 Now, let’s first work on the theory end, and have a review about some general ideas regarding some theory about chemistry in the early history...Two views of matter, taking beach and sand as an example for the argument: A. Matter is non-continuous, separable B. Matter is continuous, inseparable

Beach and Sand

Chapter 2. (L5)-Introduction to Atoms Introduction to Chapter 2..Leucippus (ca. 450 b.c.) and Democritus, the Greek philosophers: Water drops cannot be further separated into smaller drops at certain point; this is the early Greek idea about matter...Democritus: The inseparable particles called atoms; this is the ancient Greek idea of atoms about the composition of matter..Aristotle (ca. 384-ca. 322 b.c.): Matter is continuous, not atomistic..The Aristotle’s idea dominated for about 2000 years until the emergence of modern chemistry.

Chapter 2. (L5)-Introduction to Atoms Introduction to Chapter 2..The ancient Greek philosophers originated the atomistic view about matter, and then modern science picked up the same approach, and also built its view about matter based on similar ideas, but in a scientific way...As we learned from language and music, the similar basic idea is used in chemistry to explain different things, that is, we try to reduce things to some elemental entities or things.

Chapter 2. (L5)-Introduction to Atoms Introduction to Chapter 2..Now, philosophically, we have the approach or some general ideas about explaining different things...But the devil is in the details...How was this general idea developed into the quantitative theory of modern chemistry?.. How would the modern theory of chemistry explain the many observations in chemistry, including some very fundamental ones.

Chapter 2. (L5)-Introduction to Atoms Introduction to Chapter 2..Now, to facilitate our discussion, let’s invent a theory, say, matter is composed of four elemental entities: Quala, bala, wala, zala. And then let’s see if this theory would work in explaining the observations in a scientific way...And then, let’s learn the real theory of modern chemistry, and then see how that real theory works...The comparison between the two should be instructive.

Chapter 2. (L5)-Introduction to Atoms Introduction to Chapter 2..OK, sure, this can be a theory, but then what? What is the size of quala, bala, wala, zala? How do we know quala, bala, wala, zala are the smallest entities that cannot be divided again to smaller ones? How can this theory explain our observations about matter and its changes? How do we test this theory to see if it’s right or wrong?

Chapter 2. (L5)-Introduction to Atoms Introduction to Chapter 2..By the same token, we can ask similar questions about the atomic theory started from the ancient Greek history long time ago...So, we are asking these questions: How many atoms are there exactly? How small are the atoms exactly? Can atoms be further divided into subatomic components? What is the composition of atoms? How do we use this theory to explain our observations about matter and its changes?

Chapter 2. (L5)-Introduction to Atoms Chemistry as a quantitative science..Now, let’s turn to the other end...Before we move on to answer the previous questions, we need to know first how modern chemistry became a quantitative science and what quantitative observations about matter and chemical changes this quantitative science has led to...Once we have the quantitative observations in chemistry, then in science, we need to come up with some theory to explain the observations.

Chapter 2. (L5)-Introduction to Atoms Chemistry as a quantitative science..Quantitative science means we can measure chemical changes accurately, and repeatedly with the same results, meaning the observations are reproducible...Now, let’s first learn some fundamental observations in chemistry, which all chemical theories have to be able to explain and which have become the very basis for the whole chemistry.

Chapter 2. (L5)-Introduction to Atoms Lavoisier: Father of modern chemistry..In many senses, modern chemistry started with Antoine Lavoisier ( ), a French scientist/chemist...He did many quantitative chemical experiments and measurements...An important step was he weighed the total mass of the substances involved in a reaction before and after the reaction took place (doing chemistry quantitatively). Example: 2HgO (red) = 2Hg (liquid metal) + O 2  g 93.57g 6.43g This is the most important reaction he performed. He named the gas oxygen...He started using systematic names for chemical elements.

Chapter 2. (L5)-Introduction to Atoms The law of mass conservation After many similar discoveries, Lavoisier summarized his findings in a scientific law: The law of mass conservation: Matter is neither created or destroyed during a chemical reaction; in other words, the total mass of the reaction products (ending substances) is always equal to the total mass of the reactants (starting substances) This is the scientific basis for chemical reaction equations (we will learn more later on).

Chapter 2. (L5)-Introduction to Atoms The law of mass conservation.. What is a scientific law: It is a generalization of certain natural phenomenon based on many observations of similar individual cases...The approach of induction: From many individual cases to come up with the generalization. Example: All horses have four legs. Of course, we cannot observe all the horses and then come to this generalization. We get the generalization based on many many repeatable similar observations.

Chapter 2. (L5)-Introduction to Atoms Significance of the law of mass conservation..We cannot create things out of nothing. We need resources...We cannot destroy things we don’t want completely. We have to deal with wastes...Things are limited in our Earth, at least. If we create some things, then we cannot avoid loosing something else.

Chapter 2. (L5)-Introduction to Atoms Quiz Time 1. A scientific law is based on (a) a few times of similar observations; (b) many repeatable similar observations; (c) theory; (d) guess 2. A scientific law is correct (a) for ever; (b) a few years; (c) until a majority of people are against it; (d) before new observations contradicting to it occur 3. The scientific basis of chemical reaction equations is: (a) a vote by scientists; (b) an agreement between scientists and people; (c) mass conservation law; (d) mysterious inspiration