Chapter 7Atoms, Elements, Compounds and

Mixtures

7.1 Structure of an Atom• OBJECTIVES:• Summarize how models of the atom

have changed.• Name and describe parts of an atom• Calculate the number of protons,

neutrons and electrons in an atom given its mass number.

Structure of an Atom• YOU have learned that all matter in the

universe is made up of tiny particles.

• A glass of water has many water particles, each too small to see.

• If you could see them, what would they look like?

• What makes them different from the particles of other kinds of matter?

Atoms and Elements• Water particles can actually be

divided into even smaller pieces of matter.

• These particles are examples of the most basic units of matter called atoms.

• Atoms can't be broken down into smaller pieces by any common methods of separating matter.

Atoms and Elements• Atoms are the building blocks of the

universe.• Scientists have identified nearly 100

different kinds of naturally occurring atoms.

• Each kind of atom has unique properties and is called an element.

• An atom of an element can't be broken down and retain its properties.

Atoms and Elements• The particles of matter can

be made of single atoms, two or more atoms of the same element, or two or more atoms of different elements.

• This water particle is made up of three atoms, two of hydrogen and one of oxygen.

Models of the Atom

• All atoms share the same basic structure.

• During the past 200 years, scientists have proposed different models for this structure.

• Each model was the best one for its time.

• With new observations or experiments, however, the model had to be changed.

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Dalton's Model

• * In the early 1800s, John Dalton performed experiments with gases.

• His results convinced him that matter was made up of tiny particles.

• …and that each element must be made of its own unique kind of particle and that these particles combine in simple ways.

Dalton's Model

• Dalton called these basic particles atoms and pictured them as tiny, solid spheres.

Dalton's ModelBased on his experiments, Dalton developed a theory of the structure of matter. His theory contained four main concepts:

1. All matter is composed of tiny, indivisible particles called atoms.

2. Atoms of each element are exactly alike.3. Atoms of different elements have different

masses.4. Atoms of different elements can join to

form compounds.

Thomson's Model

• At the end of the 1800s, J. J. Thomson discovered that atoms were not just simple, solid spheres and contained even smaller, subatomic particles.

• The subatomic particles Thomson discovered were very small and negatively charged.

• Thompson called them electrons.

Thomson's Model• Thomson knew that atoms are electrically

neutral. Therefore, he reasoned, an atom must contain enough positive charge to balance the negative charge of the electrons.

• Thomson developed an atomic model in which electrons were stuck into a positively charged sphere.

• A positive charge in the substance of the sphere balanced the electrons' charge .

Rutherford's Model• By the early 1900s, scientists knew that

the positive charge of an atom comes from subatomic particles called protons.

• A proton is a positive particle with a mass much greater than that of an electron.

• At that time, scientists hypothesized that electrons and protons were evenly scattered throughout an atom.

Rutherford's Model

• In 1911 , Ernest Rutherford set out to test this theory.

• After experimenting with a beam of positively charged light, he concluded that the protons are concentrated in a small area at the center of the atom.

• He called this region the nucleus.

Rutherford's Model

• In his model, an atom is mostly empty space. The nucleus is tiny compared to the whole atom, but it contains nearly all the atom's mass.

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Rutherford's Model

Bohr's Model

• Niels Bohr modified Rutherford's model in 1913. He proposed that each electron in an atom has a fixed amount of energy.

• This energy keeps an electron moving around the nucleus within a specific region called an energy level.

• In Bohr's model, energy levels surround the nucleus in rings or shells, like the layers of an onion.

Bohr's Model

• In the Bohr Model, an electron can move from one energy level to another just as you can climb up or down a ladder, by absorbing or releasing a specific amount of energy.

• Just as you can't be between rungs on a ladder, an electron can't be between energy levels.

• Bohr's model has been called the ‘planetary model’ as it compares electrons to planets and the nucleus to the sun.

Bohr's Model

•

Electron Cloud Model

• Today scientists know that electrons do not actually orbit the nucleus as in Bohr's planetary model.

• The electron cloud model is now used to describe atoms.

• In this model, electrons dart about within an energy level in an ever-changing path.

• Most of this path falls into a region called an electron cloud.

• At any given time, there is a high probability that the electron exists in the electron cloud.

Electron Cloud Modelhttp://www.youtube.com/watch?v=zYeRSgiypbc• The idea of an electron cloud is not so

strange. You have probably seen the blur of a fan when it spins at high speed.

• The fast-moving blades appear to fill the space between them, just as fast-moving electrons seem to fill the space around the nucleus.

• The paths of an atom's electrons account for nearly all of its volume.

Inside the Nucleus• As scientists learned more about atomic

structure, they found that the nucleus is more complicated than they had thought.

• In 1932, scientists showed that most atomic nuclei contain a third kind of subatomic particle, called a neutron.

• A neutron has about the same mass as a proton but has no electrical charge.

• An atomic nucleus is a positively charged, tightly-packed cluster of protons and neutrons.

Inside the Nucleus

Atomic Numbers and Isotopes•All atoms of an element contain the same number

of protons. •This number, called the atomic number, identifies an element.•Sodium, for example, has an atomic number of 11.

Atomic Numbers and Isotopes•If all iron atoms contain 26 protons, what is the

atomic number of iron?

•The atomic number also represents the number of electrons in an atom. •Remember that an atom is electrically neutral. Thus the number of negative particles must equal the number of positive particles.

Atomic Numbers and Isotopes•As you may recall, Dalton hypothesized that

all atoms of an element are exactly alike. Today scientists know that he was not completely correct.•Atoms of the same element do have the same number of protons and electrons, but they may differ in the number of neutrons they contain.•Atoms of the same element with different numbers of neutrons are called isotopes.

Atomic Numbers and Isotopes

Mass Number and Atomic Mass•Almost every atom contains one or more

neutrons in it’s nucleus.•The number of neutrons does not affect the charge of an atom, but it does affect its mass.•The total number of protons and neutrons in an atom is called its mass number.•The mass number helps to distinguish one isotope from another.•What is the difference between carbon-12 and carbon-14?

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Check & Explain pg. 163

Answer Question 4

Dalton, Thomson, Rutherford and Bohr

Draw one of each model of a neon

atom.

7.2 ElementsObjectives• Describe what elements are.• Give examples of common

elements.• Make a model that relates the

particle model to a familiar property of matter.

Elements• High in the Rocky Mountains or on

the streets of New York City, the oxygen in the air you breathe is the same.

• The air is different, but the oxygen atoms in both places have the same properties. All oxygen atoms have eight protons and eight electrons.

Elements• Oxygen is an element.• As you have learned, elements

are the basic kinds of matter in the universe.

• An element is a substance made of just one kind of atom.

• It cannot be broken down or changed by chemical means.

Elements and Matter

• Even though all matter is made up of elements, only a few elements exist in nature in their pure form.

• You may remember that the oxygen and nitrogen in air are elements.

• Diamond is a natural form of the element carbon.

• Occasionally pure deposits of silver, gold, or copper are found.

Elements and Matter

• You would probably not recognize most elements in their pure form because most elements in nature are combined with other elements.

Elements and Matter

• Of the more than 100 known chemical elements, only about 30 play an important role in your daily life.

• About 18 elements do not occur in nature. They are created in laboratories and known as synthetic elements.

Properties of Elements• The properties used to describe the elements

in their pure forms include luster, texture, color, density and the ability to conduct electricity.

• Elements differ in how they react with other elements. Most elements are solids but some are gases and others are liquids.

Chemical Symbols• Chemists use symbols to represent the

names of elements (and atoms). • A chemical symbol is one or two letters

taken from the name of the element.• In some cases, the symbol is derived from

the element's name in a language other than English.

• The symbol for gold, Au, comes from aurum, the Latin word for gold.

Chemical Symbols

• Symbols form a kind of universal chemical shorthand. Ca means calcium to chemists in Japan, Mexico, Kenya, India, and everywhere else.

Chemical Symbols

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Answer questions 1 & 2

Check & Explain pg. 167

Page 167

Objectives• Describe the properties of a compound.

• Give examples of common compounds.

• Define operationally the composition of a compound by writing its chemical formula.

7.3 Compounds

• Nearly all the products you use are made of more than one element.

• The clothes you wear, the food you eat for breakfast, and the toothpaste you use to brush your teeth are all combinations of elements.

• Almost everything you can think of is made up of some combination of elements.

Compounds

• There are millions of compounds in, on, and around the earth. Many compounds are found in living things.

• Compounds also make up most of the nonliving world.

• Water is a compound.

Defining Compounds

• How are compounds formed?• Many are created by geologic processes deep in the

earth. • Organisms must manufacture compounds to stay

alive. Plants are always making the compound glucose.

• Many products you buy are made of compounds that aren't found in the natural world.

• For example, if you look at the list of ingredients on food packages you will probably see some compounds you don't recognize. People create these compounds in factories and chemical plants.

Defining Compounds

• The properties of a compound are different from those of the elements that make it up.

• Hydrogen and oxygen are both gases. but they combine to form water.

• The elements that make it up always combine in a specific proportion.

• Carbon dioxide is two parts oxygen and one part carbon (CO2).

Properties of Compounds

Properties of CompoundsStudy the text and photographs on pg.

169Answer on a separate piece of paper:-What is the difference between the compounds

carbon dioxide and carbon monoxide?-How does the compound sodium chloride differ from

the elements sodium and chloride?-What compound is common in rocks, and what

elements does the compound contain?-What is the natural form of calcium carbonate, and

what three elements make it up?

Properties of CompoundsStudy the text and photographs on pg.

169Answer on a separate piece of paper:-CO2 has 2 atoms of oxygen and 1 atom of carbon and

CO has one C & one O.-NaCl2 is table salt, an edible crystal. Sodium is a

silvery metal and chlorine is a poisonous gas.-Silicon Dioxide; silicon and oxygen.-Limestone; Calcium, carbon and oxygen.

• Compounds differ in the kinds of atoms that make them up. Compounds also differ in the way the atoms are joined.

• Compounds can be classified into two groups based on how their atoms are bonded, or joined to each other.

• 1. Molecular Compounds• 2. Ionic Compounds

Types of Compounds

• A molecule is a particle of matter made up of two or more atoms held together by the sharing of electrons.

• A compound made up of molecules is a molecular compound.

• Although many compounds are molecules, some molecules are not compounds.

• Oxygen gas, for example, exists as molecules formed by two oxygen atoms.

Molecular Compounds

• An ionic compound is a combination of positive and negative ions, not really called atoms.

• The ions are held together by electrical attraction.

• Most ionic compounds are solids. In ionic compounds that are solids, the ions are arranged in a regular three-dimensional crystal structure.

• Ionic compounds are usually soluble in water. When melted or dissolved in water, they conduct electricity.

Ionic Compounds

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Ionic Compounds

• A chemical formula is a combination of symbols and numbers that represent the composition of a compound.

• The symbols show the kinds of atoms in the compound. The numbers, called subscripts, show the number of each kind of atom.

• When more than one atom of an element is present in a compound, a subscript is written to the right and below the element's symbol.

• If there is only one atom of the element, no subscript is used.

Formulas of Compounds

• How do you write a formula for a compound?

• First you need to know what it made of. A molecule of carbon dioxide, for example, has one atom of carbon and two atoms of oxygen.

• The formula is CO2.

Formulas of Compounds

Formulas of Compounds

Objectives• Compare and contrast mixtures and

compounds.• Distinguish between homogeneous and

heterogeneous mixtures.• Define operationally a method for

separating a mixture.

7.4 Mixtures

• How is a mixture different from a compound?

• The different parts of a compound -either atoms or ions- are combined chemically.

• The different parts of a mixture, in contrast, are simply mixed

together.

Comparing Mixtures and Compounds

Also, mixtures and compounds differ in other ways:

1. The makeup, or composition, of a mixture can vary. The composition of a compound, on the other hand, is constant.

2. The components of a mixture keep their original properties. You can still taste the sodium chloride dissolved in sea water. A compound, however, has properties different from the elements that make it up.

Comparing Mixtures and Compounds

3. Because the components of a mixture are not combined chemically, they can usually be separated by physical means. Distillation and filtering are examples of physical means used to separate mixtures.

4. In contrast, the elements in a compound must usually be separated by chemical means, such as the addition of heat energy.

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Comparing Mixtures and Compounds

There are several types of mixtures:

1.Homogenous2.Heterogeneous3.Alloys

Types of Mixtures

• All parts of a homogeneous mixture contain the same amount of each component.

• Most mixtures formed by dissolving a compound in a liquid are homogeneous mixtures.

• Perfume is a homogeneous mixture of dozens of fragrance compounds dissolved in alcohol.

Homogenous Mixtures

• Not every part of a heterogeneous mixture has the same composition.

• One part of the mixture has more of one component than another.

• Cooking spices are often a heterogeneous mixture of many herbs mixed together.

Heterogeneous Mixtures

• Different metals can be combined to form a special kind of mixture, called an alloy.

• An alloy is made by heating two or more metals until they melt together.

• All coins are made of alloys.• Is an alloy a homogeneous or a heterogeneous mixture?

Alloys

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