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HIGH SCHOOL SCIENCE Physical Science 9: Atomic Structure

WILLMAR PUBLIC SCHOOL 2013-2014 EDITION CHAPTER 9 Atomic Structure In this chapter you will: 1. Compare and contrast , , and bos- ons. 2.Describe the . Identify three subatomic and compare their proper- ties. 3.Distinguish the of an element from the number of an , and use these numbers to describe the structure of at- oms. 4.Describe ancient Greek models of . 5.Describe Dalton’s atomic theory and his evi- dence for the existence of . 6.Explain Thomson’s, Rutherford’s, and Bohr’s atomic models as well as the cloud model. 7.Distinguish the ground state from excited states of an based on electron configurations. SECTION 9.1 For decades scientists have known that atoms consist of and other particles called and . Standard Model They created the explosion in the picture in their search for even smaller particles of atoms. Today, scientists think that electrons truly are fundamental particles that cannot be broken down into smaller, simpler particles. They are a type of fundamental particles called leptons. Protons and neutrons, on the other hand, are no longer thought to be fundamental particles. Instead, they are now thought to consist of smaller, simpler particles of matter called quarks. Quarks and leptons are classified as . A is the true building blocks of matter. Scientists theorize that fermions are held OBJECTIVES: together by yet another type of fundamental particles called . 1. Compare and contrast quarks, leptons, and bosons. 2. Describe the standard model.

Vocabulary: fermion standard model

2 A quark is a tiny fundamental or fermion that make of matter. According to the model, all known matter consists up protons and neutrons. There are six types of quarks. In of quarks and leptons that interact by exchanging bosons, ordinary matter, virtually all quarks are of the types called up which transmit fundamental . The standard model is a and down quarks. All quarks have mass, and they have an good theory because all of its predictions have been verified of either +2/3 or -1/3. For example, up quarks by experimental data. However, the model doesn’t explain have a charge of +2/3, and down quarks have a charge of -1/3. everything, including the of gravity and why matter has Quarks also have a different type of charge, called color mass. Scientists continue to search for evidence that will allow charge, although it has nothing to do with the colors that we them to explain these aspects of force and matter as well. see. Quarks are never found alone but instead always occur in groups of two or three quarks.

A lepton is a weak fundamental particle or fermion. There are also six types of leptons, including electrons. Leptons have an electric charge of either -1 or 0. Electrons, for exam ple, have a charge of -1. Leptons have mass, although the mass of electrons is extremely small.

There are four known types of bosons, which are force- carrying particles. Each of these bosons carries a different fundamental force between interacting particles. In addition, there is a particle called the "", which gives objects the they have. Some types of bosons have mass; others are massless. Bosons have an electric charge of +1, -1, or 0.

Based on their knowledge of subatomic particles, scientists have developed a theory called the standard model to explain all the matter in the and how it is held together. The model includes only the fundamental particles in the table. No other particles are needed to explain all kinds

3 Section Review:

1. Make a table comparing and contrasting the three types of fundamental particles. Include an example of each type in your table.

2. In what ways is the standard model incomplete?

4 SECTION 9.2 Atoms are the smallest particles of an element that still have the element’s properties. Individual atoms are extremely Atomic Structure small. In fact, they are so small that trillions of them would fit inside the period at the end of this sentence. Although atoms are very tiny, they consist of even smaller particles. Three main types of particles that make up all atoms are: protons, neutrons, and electrons.

Protons, elections, and neutrons are subatomic particles which can be distinguished by mass, charge, and location in OBJECTIVE: an atom. 1. Identify three subatomic particles and

compare their properties. Relative Relative Symbol Location Charge Mass 2. Distinguish the atomic number of an element Outside the Electron e- 1- 1/1836 from the of an isotope, and use Nucleus p+ 1+ 1 Nucleus these numbers to describe the structure of n 0 1 Nucleus atoms. A proton is a positive charged subatomic particle that is found in the nucleus of an atom. Protons have two up quarks Vocabulary: and one . Each nucleus must contain at least one particle with a positive charge. atoms proton A neutron is a neutral subatomic particle that is found in the neutron electron nucleus of an atom. It has a mass almost exactly equal to that atomic number mass number of a proton. Neutrons have one and two down quarks. isotope An electron is a negatively subatomic particles that is found in the space outside the nucleus. It would take 1836 electrons

5 to equal to the mass of a proton. Thus people say it does not The mass number of an atom is the sum of the protons and have enough mass to be considered or almost 0. Electrons are neutrons in the nucleus on that atom. If you know the atomic one type of lepton thus have very difference properties than number and the mass number of an atom, you can find the protons and neutrons. number of neutrons by subtracting.

The atoms of any given elements always have the same Number of Neutrons = Mass number – Atomic number number of protons. For example, there is one proton in the Every atom of a given element has the same number of nucleus of each and every atom. Atoms of different protons and electrons. Some atoms of the same element may elements have different number of protons, therefore, the have different numbers of neutrons. For example, some number of protons determines the element. carbon atoms have seven or eight neutrons instead of the Atomic number of an element equals the number of usual six. are atoms of the same element that have protons in an atom of that element. Atoms of different different numbers of neutrons and different mass numbers. elements have different atomic numbers. The number of Isotopes of an element have the same atomic number but protons is the atomic number. different mass numbers because they have different numbers of neutrons. Each positive charge in an atom is balanced by a negative charge because atoms are neutral. So the atomic number of Some atoms of hydrogen have no neutrons and a mass an element also equals the number of electrons in an atom. number of one. Some atoms of hydrogen have one neutrons and a mass number of two. Some atoms of hydrogen have two Electrons have almost no mass. Instead, almost all the mass of neutrons and a mass number of three. When it is important an atom is in its protons and neutrons in the nucleus. The to distinguish one hydrogen isotope from another, the nucleus is very small, but it is densely packed with matter. isotopes are referenced to as hydrogen-1, hydrogen-2, and The SI unit for the mass of an atom is the atomic mass unit hydrogen-3. The number after the element is the mass (amu). One atomic mass unit equals the mass of a proton, number. which is about 1.7 × 10−24 g. Each neutron also has a mass of 1 amu. Therefore, the sum of the protons and neutrons in an atom is about equal to the atom’s total mass in atomic mass units.

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Section Review:

1. How are protons, neutrons and electrons distinguished?

2. Which subatomic particles are in the nucleus?

3. Which subatomic particles have charge?

4. Since the up quark has a charge of +2/3 and a down quark has a charge of -1/3, explain how the proton has a 1+ charge?

5. Since the up quark has a charge of +2/3 and a down quark has a charge of -1/3, explain how the neutron has a 0 charge?

6. Why do some say that the electron does not have mass?

7. Why do electrons have very different properties than protons and neutrons?

8. How are atoms of one element different from atoms of other elements?

9. How do you determine the number of protons?

10.How do you determine the number of electrons?

11.How do you determine the number of neutrons?

12.What is the difference between two isotopes of the same element?

13.How do you distinguish between different isotopes of the same element?

7 SECTION 9.3 The history of the atom begins around 450 B.C. with a Greek philosopher named Democritus. Democritus wondered what Atomic Models would happen if you cut a piece of matter, such as an apple, into smaller and smaller pieces. He thought that a point would be reached where matter could not be cut into still smaller pieces, and believed that all matter consists of extremely small particles that could not be divided. Thus he called these "uncuttable" pieces atoms form the Greek word atomos – which means uncut. This is where the modern term OBJECTIVES: atom comes from. Additionally, he thought each type of atom had a specific set of properties. For example, solid atoms 1. Describe ancient Greek models of matter. were rough and prickly while liquid atoms were round and 2. Describe Dalton’s atomic theory and his smooth. evidence for the existence of atoms. Democritus was an important philosopher. However, he was 3. Explain Thomson’s, Rutherford’s, and Bohr’s less influential than the Greek philosopher Aristotle, who atomic models as well as the electron cloud lived about 100 years after Democritus. Aristotle rejected Democritus’s idea of atoms. In fact, Aristotle thought the idea model. of atoms was ridiculous. He did not think there was a limit to 4. Distinguish the ground state from excited the number of matter could be divided. He thought that states of an atom based on electron all substances were built up from only four elements – earth, configurations. air, fire and water. These elements were a combination of four properties – hot, cold, dry and wet. Unfortunately, Aristotle’s ideas were accepted for more than 2000 years. Vocabulary: During that , Democritus’s ideas were more or less forgotten. levels electron cloud orbital electron configuration ground state excited state 8 all matter is made up of individual particles called atoms, which cannot be divided. An atom is the smallest unit of an element. These atoms are represented by tiny, solid sphere with a different mass.

Dalton proposed the theory with four parts:

1. All elements are composed of atoms.

2. All atoms of the same element have the same mass, and atoms of different elements have different masses.

3. Compounds contain atoms of more than one element

4. In a particular compound, atoms of different elements • Air was a combination of hot and wet. always combine in the same way.1. What is the evidence • Water was a combination of wet and cold. for atoms?

• Earth was a combination of cold and dry. When some objects are rubbed, they gain the ability to attract or repel other materials. Based on their behavior, such • Fire was a combination of dry and hot. materials are said to have either a positive or negative electric charge. Objects with like charge or the same charge, repel or Until the 1800’s, most people accepted Aristotle’s model, until push apart. Objects with opposite charge or different charges, Dalton. Dalton, a teacher and amateur scientist, was attract or pull together. Some charge can flow from one interested in the behavior of gases in air. Based on the way location to another. A flow of charges particle is called an gasses exert pressure, he concluded that gas consist of electric current. If the particle has a positive charge, it will be individual particles, atoms. He gathered evidence for the represented by a +. If the particle has a negative charge, it will existence of atoms by measuring the masses of elements that be represented by a -. combine when compounds form, and noticed that all no matter how large or small the sample, the ratio of the masses J.J. Thomson used a devise below. When it was turned on, a of the elements in the compound is always the same. Thus beam formed following path A. He hypothesized that the compounds have a fixed composition. Dalton concluded that beam was a stream of charge particles. 9 gold foil would be too small to change the path of an .

The screen flashed when an alpha particle struck it. Some of the location of the flashes on the screen did not support Rutherford’s hypothesis. Some of the particles behaved as though they had struck an object and bounced straight back. The alpha particles whose paths were deflected must have come close to another charged object. The closer they came to In one experiment, Thomson tested his hypothesis by placing the other charge object, the greater the deflection was; charge plates on either side of the beam. The plates caused however, many alpha particles passed through the gold the beam to deflect, or bend, from its straight path. He without being deflected. observed that the beam was repelled by the negatively charged plate and attracted by the positively charge plate, thus the From these results, Rutherford concluded that the positive beam had negative charge. Thomson’s experiments provided charge of an atom is NOT evenly spread throughout the atom, the first evidence that atoms are made of even smaller instead it is in a very small, central area called the nucleus. particles. The nucleus is a dense, positively charge mass located in the center of the atom. An atom is neutral, meaning it has neither a negative or positive charge. In Thomson’s atomic model, the negatively charges were evenly scattered throughout an atom filled with a positively charge mass of matter. Think of it like chocolate chip ice cream, the chocolate chips are the negatively charges and the vanilla ice cream is the positively charge mass. The model is called the “plum pudding” model, after a traditional English dessert.

Earnest Rutherford discovered that uranium emits fast moving particles that have a positive charge. He called them alpha particles. Based on Thomson’s model, Rutherford hypothesized that the mass and charge at any location in a 10 Niels Bohr agreed with Rutherford’s model of the nucleus According the Rutherford’s model, all the of an atom’s surrounded by a large volume of space. But he added to positively charge is in its nucleus. Rutherford’s model and focused on the electrons.

Something else was missing, however, as the mass of the In Bohr’s model, electrons move with constant speed in fixed heavier nuclei is greater than could be explained by just orbits around the nucleus. Each electron in an atom has a protons alone. The mystery was solved when James specific amount of energy. The possible that Chadwick discovered the neutron, an almost “twin” of the electrons in an atom can have are called energy levels. An proton with roughly the same mass but no charge. The electron in an atom can move from one energy level to neutron had to be in the nucleus. another when the atom gains or loses energy.

When atoms absorb energy, three things happen, it increases its kinetic energy, a phase change, or emits light. The last option is important in understanding the atoms. The energy is temporarily absorbed by the atom and then emitted as light.

11 Different energy levels have different maximum number of electrons.

MAXIMUM ENERGY NUMBER OF LEVEL ELECTRONS

1 2

2 8

3 18

4 32

Like earlier model’s, Bohr’s model was improved as scientists made further discoveries. Bohr was incorrect in assuming that electrons traveled in fixed orbits. Today, scientists know that electrons move in a less predicted way. An electron configuration is the arrangement of electrons An electron cloud is a visual model of the most likely in the orbitals of an atom. The most stable electron locations for electrons in an atom. Scientists use the electron configuration is the one in which the electrons are in orbitals cloud to describe the possible locations of electron around the with the lower possible energies. nucleus, and it is a good approximation of how electrons When all the electrons in an atom have the lowest possible behave in their orbitals. An orbital is a region of space energies, the atom is said to be in its ground state. When an around the nucleus where an electron is likely to be found. electron moves to an orbital with a higher energy, it is in an excited state. An excited state is less stable than the ground state.

12 Section Review: Section Review Continued:

1. What did the philosopher Democritus believe about all 11.How will a positive charge be represented? matter? 12.What did Thomson observe about the beam? 2. What did Democritus call the particles? 13.In Thomson’s experiment, why was the glowing beam 3. What were Aristotle’s four elements? repelled by a negative charged plate?

4. What were Aristotle’s four properties which the elements 14.How did Thomson’s experiments change how people were combinations of? thought about the atom?

5. How combination of properties was water? 15.Describe Thomson’s atomic model.

6. What did Dalton conclude about the evidence he 16.What did Rutherford conclude about the structure of an gathered? atom?

7. In the spaces provided, write which part (number) of the 17.In the Rutherford experiment, why weren’t all the alpha atomic theory supports the given statement. particles deflected?

______a) The element helium is composed of atoms. 18.What is Rutherford’s model?

______b) Salt, a compound, contains sodium and 19.What is Bohr’s model of the atom? chlorine. 20.How does Bohr’s model differ from Rutherford’s model? ______c) Hydrogen atoms can combine with oxygen atoms to form the compound water. 21.What are energy levels?

______d) An atom of hydrogen weighs much less 22.How can electrons move from one energy level to than an atom of silver. another?

8. How do like electric charged particles behave? 23.How do scientists use the electron cloud model?

9. How do opposite electric charged particles behave? 24.What does it mean to say that an atom is in an excited state? 10.What is electric current?

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