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Chapter 12 Introduction to Atoms. Development of the Atomic Theory F Atom - the smallest particle into which an element can be divided and still be the.

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Presentation on theme: "Chapter 12 Introduction to Atoms. Development of the Atomic Theory F Atom - the smallest particle into which an element can be divided and still be the."— Presentation transcript:

1 Chapter 12 Introduction to Atoms

2 Development of the Atomic Theory F Atom - the smallest particle into which an element can be divided and still be the same substance F It still retains its characteristic properties F Atoms make up elements F Elements combine to form compounds F Because all matter is made up of elements or compounds, atoms are considered the building blocks of matter. F Atom - the smallest particle into which an element can be divided and still be the same substance F It still retains its characteristic properties F Atoms make up elements F Elements combine to form compounds F Because all matter is made up of elements or compounds, atoms are considered the building blocks of matter.

3 Democritus Proposes the Atom F In 440 B.C. Democritus believed that there was a point where a particle could not be divided any further. F This was against popular belief held by Aristotle F He called this particle an atomos (Greek for atom), which means indivisible F He believed these particles were constantly moving and they form different materials by joining together F In 440 B.C. Democritus believed that there was a point where a particle could not be divided any further. F This was against popular belief held by Aristotle F He called this particle an atomos (Greek for atom), which means indivisible F He believed these particles were constantly moving and they form different materials by joining together

4 Dalton Creates an Atomic Theory Based on Experiments F John Dalton, a British chemist, wanted to know why elements combine in specific proportions to form compounds F i.e. water (H 2 O) always has two hydrogen and one oxygen atoms bound together F He performed experiments on compounds and concluded that they combined in specific proportions because of atoms F John Dalton, a British chemist, wanted to know why elements combine in specific proportions to form compounds F i.e. water (H 2 O) always has two hydrogen and one oxygen atoms bound together F He performed experiments on compounds and concluded that they combined in specific proportions because of atoms

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6 F After many experiments and observations Dalton published his atomic theory in F He stated the following: F 1) All substances are made of atoms. ATOMS Are small particles that cannot be created, divided, or destroyed F 2) Atoms of the same element are exactly alike, and atoms of different elements are different F 3) Atoms join with other atoms to make new substances F This was the first step in our understanding of the atom and our current atomic theory F After many experiments and observations Dalton published his atomic theory in F He stated the following: F 1) All substances are made of atoms. ATOMS Are small particles that cannot be created, divided, or destroyed F 2) Atoms of the same element are exactly alike, and atoms of different elements are different F 3) Atoms join with other atoms to make new substances F This was the first step in our understanding of the atom and our current atomic theory

7 Thomson Finds Electrons in the Atom F In 1897, a British scientist named J. J. Thomson discovered that atoms are made of small particles. F This was contrary to Daltons atomic theory F Thomson experimented with a cathod ray F He noticed that the ray was affected (bent) by a positive charge F He concluded that the ray contained negatively charged particles F In 1897, a British scientist named J. J. Thomson discovered that atoms are made of small particles. F This was contrary to Daltons atomic theory F Thomson experimented with a cathod ray F He noticed that the ray was affected (bent) by a positive charge F He concluded that the ray contained negatively charged particles

8 F These negatively charged particles are now called electrons F Thomson revised Dalton's atomic theory to include the presence of electrons even though he did not know how they were arranged in the atom F He came up with the plum pudding model F Model - is a representation of an object or system F Thomson’s model illustrated positively charged material with negatively charged particles located throughout. F These negatively charged particles are now called electrons F Thomson revised Dalton's atomic theory to include the presence of electrons even though he did not know how they were arranged in the atom F He came up with the plum pudding model F Model - is a representation of an object or system F Thomson’s model illustrated positively charged material with negatively charged particles located throughout.

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11 Rutherford Opens an Atomic “Shooting Gallery” F In 1909, a former student of Thomson’s named Ernest Rutherford tested Thomson’s theory F He aimed positively charged particles, larger than protons, at gold foil F Most particles passed straight through F But to Rutherford’s amazement; F Some were deflected at various angles F And a few were bounced straight back F It became obvious to Rutherford that Thomson’s model was wrong F In 1909, a former student of Thomson’s named Ernest Rutherford tested Thomson’s theory F He aimed positively charged particles, larger than protons, at gold foil F Most particles passed straight through F But to Rutherford’s amazement; F Some were deflected at various angles F And a few were bounced straight back F It became obvious to Rutherford that Thomson’s model was wrong

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13 Rutherford Presents a New Atomic Model F In 1911 Rutherford revised the atomic theory F He concluded that atoms are mostly empty space with lightweight negative electrons moving around F And in the center of the atom is a tiny, extremely dense, positively charged region - Nucleus F He calculated that the diameter of the nucleus was 100,000 times smaller than the atom itself F In 1911 Rutherford revised the atomic theory F He concluded that atoms are mostly empty space with lightweight negative electrons moving around F And in the center of the atom is a tiny, extremely dense, positively charged region - Nucleus F He calculated that the diameter of the nucleus was 100,000 times smaller than the atom itself

14 Bohr States That Electrons Can Jump Between Levels F In 1913 a Danish scientist who worked with Rutherford, Niels Bohr, suggested that electrons travel around the nucleus in definite paths F These paths are located at levels at certain distances from the nucleus F He also believe that electrons could jump from one level to the next F Bohr’s model was valuable in predicting some atomic behavior, but was too simple to explain all atomic behavior F In 1913 a Danish scientist who worked with Rutherford, Niels Bohr, suggested that electrons travel around the nucleus in definite paths F These paths are located at levels at certain distances from the nucleus F He also believe that electrons could jump from one level to the next F Bohr’s model was valuable in predicting some atomic behavior, but was too simple to explain all atomic behavior

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16 The Modern Theory: Electron Clouds Surround the Nucleus F Many scientists have contributed to our current understanding of the atom F This led to the following change to the atomic theory; F Electrons do not travel in definite paths around the nucleus F The exact paths cannot be predicted F The regions around the nucleus where electrons are likely to be found are called - Electron Clouds F Many scientists have contributed to our current understanding of the atom F This led to the following change to the atomic theory; F Electrons do not travel in definite paths around the nucleus F The exact paths cannot be predicted F The regions around the nucleus where electrons are likely to be found are called - Electron Clouds

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18 Chapter 12 Quiz 1 F 1) __________is the smallest particle into which an element can be divided and still be that element F 2) __________ is a unifying explanation for a broad range of hypothesis and observations that have been supported by testing F 3) __________ are the negatively charged particles found in all atoms F 4) ___________ discovered the nucleus F 5) ___________ are regions where electrons are likely to be found F Bonus) __________ are atoms that have the same number of protons, but different numbers of neutrons F 1) __________is the smallest particle into which an element can be divided and still be that element F 2) __________ is a unifying explanation for a broad range of hypothesis and observations that have been supported by testing F 3) __________ are the negatively charged particles found in all atoms F 4) ___________ discovered the nucleus F 5) ___________ are regions where electrons are likely to be found F Bonus) __________ are atoms that have the same number of protons, but different numbers of neutrons

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20 Section 2: The Atom

21 How Small Is an Atom? F An average sized atom (i.e. aluminum) has a diameter of cm F That’s three hundred millionths of a centimeter F It would take a stack of 50,000 aluminum atoms to equal the thickness of a sheet of aluminum foil F There are 2 x atoms in one penny! F 20,000,000,000,000,000,000,000 atoms F That’s twenty thousand billion billion atoms F That is 4,000,000,000,000 times more atoms than people on Earth! F An average sized atom (i.e. aluminum) has a diameter of cm F That’s three hundred millionths of a centimeter F It would take a stack of 50,000 aluminum atoms to equal the thickness of a sheet of aluminum foil F There are 2 x atoms in one penny! F 20,000,000,000,000,000,000,000 atoms F That’s twenty thousand billion billion atoms F That is 4,000,000,000,000 times more atoms than people on Earth!

22 What’s Inside an Atom? F As tiny as an atom is, it consists of even smaller particles F Protons, Neutrons, and Electrons F Protons and Neutrons make up the nucleus, which is at the center of the atom F As tiny as an atom is, it consists of even smaller particles F Protons, Neutrons, and Electrons F Protons and Neutrons make up the nucleus, which is at the center of the atom

23 F Protons - the positively charged particles of the nucleus F All protons are identical F Each has an approximate mass of 1.7 x grams or 1 amu F Neutrons - particles of the nucleus that have no charge F Neutrons have a slightly greater mass than protons F The nucleus of the atom is small, but very dense F Protons - the positively charged particles of the nucleus F All protons are identical F Each has an approximate mass of 1.7 x grams or 1 amu F Neutrons - particles of the nucleus that have no charge F Neutrons have a slightly greater mass than protons F The nucleus of the atom is small, but very dense

24 Outside the Nucleus F Electrons - negatively charged particles in atoms F Electrons are found moving around the nucleus within electron clouds F Electrons are very small in mass compared to protons and neutrons ( amu) F It takes more than 1,800 electrons to equal the mass of one proton F Electrons - negatively charged particles in atoms F Electrons are found moving around the nucleus within electron clouds F Electrons are very small in mass compared to protons and neutrons ( amu) F It takes more than 1,800 electrons to equal the mass of one proton

25 F The charges of protons and electrons are opposite, but equal in strength F Therefore if there are equal amounts of protons and electrons, the overall charge of the atom is zero - neutral F i.e. 10 protons - 10 electrons = 0 F If the number of protons and electrons differ, the atom becomes a charged particle - Ion F Positively charged atoms are Cations F Negatively charged atoms are Anions F The charges of protons and electrons are opposite, but equal in strength F Therefore if there are equal amounts of protons and electrons, the overall charge of the atom is zero - neutral F i.e. 10 protons - 10 electrons = 0 F If the number of protons and electrons differ, the atom becomes a charged particle - Ion F Positively charged atoms are Cations F Negatively charged atoms are Anions

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27 How Do Atoms of Different Elements Differ? F There are 117 different elements, each made of different atoms F The number of Protons determines the element F This is called the Atomic number F Each element is composed of atoms with the same atomic number F There are 117 different elements, each made of different atoms F The number of Protons determines the element F This is called the Atomic number F Each element is composed of atoms with the same atomic number

28 Are All Atoms of an Element the Same? F Isotopes - are atoms that have the same number of protons, but have different number of neutrons F Atoms that are isotopes of each other are the same element because they have the same number of protons F Some isotopes have unstable nuclei and become radioactive F Isotopes - are atoms that have the same number of protons, but have different number of neutrons F Atoms that are isotopes of each other are the same element because they have the same number of protons F Some isotopes have unstable nuclei and become radioactive

29 How Can You Tell One Isotope from Another? F You can identify one isotope from another by its mass number F Mass number - is the sum of the protons and neutrons in an atom F Electrons are not included in this calculation because their mass too small to affect it F To identify a specific isotope write the name of the element followed by a hyphen and the mass number F i.e. carbon - 12 (C-12) has contains 6 protons and 6 neutron, while carbon - 13 has 6 protons and 7 neutrons F You can identify one isotope from another by its mass number F Mass number - is the sum of the protons and neutrons in an atom F Electrons are not included in this calculation because their mass too small to affect it F To identify a specific isotope write the name of the element followed by a hyphen and the mass number F i.e. carbon - 12 (C-12) has contains 6 protons and 6 neutron, while carbon - 13 has 6 protons and 7 neutrons

30 How Do You Calculate the Mass of an Element F Most elements found in nature contain a mixture of different isotopes F i.e. All copper is composed of copper-63 and copper-65 atoms F Atomic Mass Unit (amu) - is the weighted average of the masses of of all naturally occurring isotopes F To calculate the amu of an element, multiple the mass of each element by its percentage of abundance (in decimal form) F Most elements found in nature contain a mixture of different isotopes F i.e. All copper is composed of copper-63 and copper-65 atoms F Atomic Mass Unit (amu) - is the weighted average of the masses of of all naturally occurring isotopes F To calculate the amu of an element, multiple the mass of each element by its percentage of abundance (in decimal form)

31 F Copper consists of copper-63 at 69% abundance, and copper-65 at 31% abundance. What is the amu of copper? F (63 x.69) =43.47 F (65 x.31) = amu * So the amu of copper is amu F Copper consists of copper-63 at 69% abundance, and copper-65 at 31% abundance. What is the amu of copper? F (63 x.69) =43.47 F (65 x.31) = amu * So the amu of copper is amu

32 Now You Try One F Chlorine consists of chlorine-35 at 76%, and chlorine-37 at 24%. What is the amu of chlorine F (35 x.76)=26.6 F (37 x.24)= amu F Chlorine consists of chlorine-35 at 76%, and chlorine-37 at 24%. What is the amu of chlorine F (35 x.76)=26.6 F (37 x.24)= amu

33 What Forces Are at Work in Atoms? F There are four basic forces at work everywhere, including in atoms. F They include: F 1) Gravity - the attraction between two objects based on their masses F Because the masses of the particles in atoms is so small, the gravity between them is small F 2) Electromagnetic Force - particles of the same charge repel, while opposite charges attract. F The electromagnetic force holds electrons around the nucleus F There are four basic forces at work everywhere, including in atoms. F They include: F 1) Gravity - the attraction between two objects based on their masses F Because the masses of the particles in atoms is so small, the gravity between them is small F 2) Electromagnetic Force - particles of the same charge repel, while opposite charges attract. F The electromagnetic force holds electrons around the nucleus

34 F 3) Strong Force - At the close distances between protons in the nucleus, the strong force is greater than the electromagnetic force between the protons F This keeps the nucleus together F 4) Weak Force - In certain unstable, a neutron can change into a proton and a electron. F It is an important force in radioactive atoms F 3) Strong Force - At the close distances between protons in the nucleus, the strong force is greater than the electromagnetic force between the protons F This keeps the nucleus together F 4) Weak Force - In certain unstable, a neutron can change into a proton and a electron. F It is an important force in radioactive atoms


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