Physical Science 9: Atomic Structure

WILLMAR PUBLIC SCHOOL 2013-2014 EDITION

HIGH SCHOOL SCIENCE

CHAPTER 9

Atomic StructureIn this chapter you will:1. Compare and contrast quarks, leptons, and bos-

ons. 2.Describe the standard model. Identify three

subatomic particles and compare their proper-ties.

3.Distinguish the atomic number of an element from the mass number of an isotope, and use these numbers to describe the structure of at-oms.

4.Describe ancient Greek models of matter.5.Describe Dalton’s atomic theory and his evi-

dence for the existence of atoms.6.Explain Thomson’s, Rutherford’s, and Bohr’s

atomic models as well as the electron cloud model.

7. Distinguish the ground state from excited states of an atom based on electron configurations.

OBJECTIVES:

1. Compare and contrast quarks, leptons, and bosons.

2. Describe the standard model.

Vocabulary:

fermion

quark

lepton

boson

standard model

SECTION 9.1

Standard ModelFor decades scientists have known that atoms consist of electrons and other particles called protons and neutrons. 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 fermions. A fermion is the true building blocks of matter. Scientists theorize that fermions are held together by yet another type of fundamental particles called bosons.

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A quark is a tiny fundamental particle or fermion that make up protons and neutrons. There are six types of quarks. In ordinary matter, virtually all quarks are of the types called up and down quarks. All quarks have mass, and they have an electric charge of either +2/3 or -1/3. For example, up quarks have a charge of +2/3, and down quarks have a charge of -1/3. Quarks also have a different type of charge, called color charge, although it has nothing to do with the colors that we 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 "Higgs Boson", which gives objects the masses 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 universe 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

of matter. According to the model, all known matter consists of quarks and leptons that interact by exchanging bosons, which transmit fundamental forces. The standard model is a good theory because all of its predictions have been verified by experimental data. However, the model doesn’t explain everything, including the force of gravity and why matter has mass. Scientists continue to search for evidence that will allow them to explain these aspects of force and matter as well.

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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?

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OBJECTIVE:

1. Identify three subatomic particles and compare their properties.

2. Distinguish the atomic number of an element from the mass number of an isotope, and use these numbers to describe the structure of atoms.

Vocabulary:

atoms proton

neutron electron

atomic number mass number

isotope

SECTION 9.2

Atomic StructureAtoms are the smallest particles of an element that still have the element’s properties. Individual atoms are extremely 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 an atom.

A proton is a positive charged subatomic particle that is found in the nucleus of an atom. Protons have two up quarks and one down quark. Each nucleus must contain at least one particle with a positive charge.

A neutron is a neutral subatomic particle that is found in the nucleus of an atom. It has a mass almost exactly equal to that of a proton. Neutrons have one up quark and two down quarks.

An electron is a negatively subatomic particles that is found in the space outside the nucleus. It would take 1836 electrons

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Symbol Relative Charge

Relative Mass Location

Electron e- 1- 1/1836 Outside the Nucleus

Proton p+ 1+ 1 Nucleus

Neutron n 0 1 Nucleus

to equal to the mass of a proton. Thus people say it does not have enough mass to be considered or almost 0. Electrons are one type of lepton thus have very difference properties than protons and neutrons.

The atoms of any given elements always have the same number of protons. For example, there is one proton in the nucleus of each and every hydrogen atom. Atoms of different elements have different number of protons, therefore, the number of protons determines the element.

Atomic number of an element equals the number of protons in an atom of that element. Atoms of different elements have different atomic numbers. The number of protons is the atomic number.

Each positive charge in an atom is balanced by a negative charge because atoms are neutral. So the atomic number of an element also equals the number of electrons in an atom.

Electrons have almost no mass. Instead, almost all the mass of an atom is in its protons and neutrons in the nucleus. The nucleus is very small, but it is densely packed with matter. The SI unit for the mass of an atom is the atomic mass unit (amu). One atomic mass unit equals the mass of a proton, 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.

The mass number of an atom is the sum of the protons and neutrons in the nucleus on that atom. If you know the atomic number and the mass number of an atom, you can find the number of neutrons by subtracting.

Number of Neutrons = Mass number – Atomic number

Every atom of a given element has the same number of protons and electrons. Some atoms of the same element may have different numbers of neutrons. For example, some carbon atoms have seven or eight neutrons instead of the usual six. Isotopes are atoms of the same element that have different numbers of neutrons and different mass numbers. Isotopes of an element have the same atomic number but different mass numbers because they have different numbers of neutrons.

Some atoms of hydrogen have no neutrons and a mass number of one. Some atoms of hydrogen have one neutrons and a mass number of two. Some atoms of hydrogen have two neutrons and a mass number of three. When it is important to distinguish one hydrogen isotope from another, the isotopes are referenced to as hydrogen-1, hydrogen-2, and hydrogen-3. The number after the element is the mass number.

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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?

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OBJECTIVES:

1. Describe ancient Greek models of matter.

2. Describe Dalton’s atomic theory and his evidence for the existence of atoms.

3. Explain Thomson’s, Rutherford’s, and Bohr’s atomic models as well as the electron cloud model.

4. Distinguish the ground state from excited states of an atom based on electron configurations.

Vocabulary:

energy levels electron cloud

orbital electron configuration

ground state excited state

SECTION 9.3

Atomic ModelsThe history of the atom begins around 450 B.C. with a Greek philosopher named Democritus. Democritus wondered what 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 atom comes from. Additionally, he thought each type of atom had a specific set of properties. For example, solid atoms were rough and prickly while liquid atoms were round and smooth.

Democritus was an important philosopher. However, he was less influential than the Greek philosopher Aristotle, who lived about 100 years after Democritus. Aristotle rejected Democritus’s idea of atoms. In fact, Aristotle thought the idea of atoms was ridiculous. He did not think there was a limit to the number of times matter could be divided. He thought that all substances were built up from only four elements – earth, 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. During that time, Democritus’s ideas were more or less forgotten.

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• Air was a combination of hot and wet.

• Water was a combination of wet and cold.

• Earth was a combination of cold and dry.

• Fire was a combination of dry and hot.

Until the 1800’s, most people accepted Aristotle’s model, until Dalton. Dalton, a teacher and amateur scientist, was interested in the behavior of gases in air. Based on the way gasses exert pressure, he concluded that gas consist of individual particles, atoms. He gathered evidence for the existence of atoms by measuring the masses of elements that combine when compounds form, and noticed that all no matter how large or small the sample, the ratio of the masses of the elements in the compound is always the same. Thus compounds have a fixed composition. Dalton concluded that

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 always combine in the same way.1. What is the evidence for atoms?

When some objects are rubbed, they gain the ability to attract or repel other materials. Based on their behavior, such materials are said to have either a positive or negative electric charge. Objects with like charge or the same charge, repel or push apart. Objects with opposite charge or different charges, attract or pull together. Some charge can flow from one location to another. A flow of charges particle is called an electric current. If the particle has a positive charge, it will be represented by a +. If the particle has a negative charge, it will be represented by a -.

J.J. Thomson used a devise below. When it was turned on, a beam formed following path A. He hypothesized that the beam was a stream of charge particles.

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In one experiment, Thomson tested his hypothesis by placing charge plates on either side of the beam. The plates caused the beam to deflect, or bend, from its straight path. He observed that the beam was repelled by the negatively charged plate and attracted by the positively charge plate, thus the beam had negative charge. Thomson’s experiments provided the first evidence that atoms are made of even smaller particles.

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

gold foil would be too small to change the path of an alpha particle.

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 the other charge object, the greater the deflection was; however, many alpha particles passed through the gold without being deflected.

From these results, Rutherford concluded that the positive charge of an atom is NOT evenly spread throughout the atom, instead it is in a very small, central area called the nucleus. The nucleus is a dense, positively charge mass located in the center of the atom.

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According the Rutherford’s model, all the of an atom’s positively charge is in its nucleus.

Something else was missing, however, as the mass of the heavier nuclei is greater than could be explained by just protons alone.  The mystery was solved when James Chadwick discovered the neutron, an almost “twin” of the proton with roughly the same mass but no charge. The neutron had to be in the nucleus.

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.

Niels Bohr agreed with Rutherford’s model of the nucleus surrounded by a large volume of space. But he added to Rutherford’s model and focused on the electrons.

In Bohr’s model, electrons move with constant speed in fixed orbits around the nucleus. Each electron in an atom has a specific amount of energy. The possible energies that electrons in an atom can have are called energy levels. An electron in an atom can move from one energy level to another when the atom gains or loses energy.

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Different energy levels have different maximum number of electrons.

ENERGY LEVEL

MAXIMUM NUMBER OF 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 cloud is a visual model of the most likely locations for electrons in an atom. Scientists use the electron cloud to describe the possible locations of electron around the nucleus, and it is a good approximation of how electrons behave in their orbitals. An orbital is a region of space around the nucleus where an electron is likely to be found.

An electron configuration is the arrangement of electrons in the orbitals of an atom. The most stable electron configuration is the one in which the electrons are in orbitals with the lower possible energies.

When all the electrons in an atom have the lowest possible energies, the atom is said to be in its ground state. When an 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.

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Section Review:

1. What did the philosopher Democritus believe about all matter?

2. What did Democritus call the particles?

3. What were Aristotle’s four elements?

4. What were Aristotle’s four properties which the elements were combinations of?

5. How combination of properties was water?

6. What did Dalton conclude about the evidence he gathered?

7. In the spaces provided, write which part (number) of the atomic theory supports the given statement.

_______a) The element helium is composed of atoms.

_______b) Salt, a compound, contains sodium and chlorine.

_______c) Hydrogen atoms can combine with oxygen atoms to form the compound water.

_______d) An atom of hydrogen weighs much less than an atom of silver.

8. How do like electric charged particles behave?

9. How do opposite electric charged particles behave?

10.What is electric current?

Section Review Continued:

11.How will a positive charge be represented?

12.What did Thomson observe about the beam?

13.In Thomson’s experiment, why was the glowing beam repelled by a negative charged plate?

14.How did Thomson’s experiments change how people thought about the atom?

15.Describe Thomson’s atomic model.

16.What did Rutherford conclude about the structure of an atom?

17.In the Rutherford experiment, why weren’t all the alpha particles deflected?

18.What is Rutherford’s model?

19.What is Bohr’s model of the atom?

20.How does Bohr’s model differ from Rutherford’s model?

21.What are energy levels?

22.How can electrons move from one energy level to another?

23.How do scientists use the electron cloud model?

24.What does it mean to say that an atom is in an excited state?

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