Which of the following statements about valence electrons and the periodic table is true

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Date Assigned: Tuesday, Feb 23, 2016
Date Due: Tuesday, Feb 23, 2016

CK12

assignments - Unit 2 Scientists and Organization of Atoms -->valence electrons --> practice 100% and you don't have to do review questions

assignments - Unit 2 Scientists and Organization of Atoms -->atomic number --> practice 100% and you don't have to do review questions

assignments - Unit 2 Scientists and Organization of Atoms -->atomic nucleus --> practice 100% and you don't have to do review questions

IF you don't do the practice on ck12, you must do the review questions at the end of each section.

Valence Electrons (review questions only)

Electrons in the outer energy level of an atom that can participate in interactions with other atoms.

·         Define valence electron.

·         Show how to represent valence electrons with electron dot diagrams.

·         Describe variation in valence electrons in the periodic table.

·         Relate valence electrons to reactivity and electrical conductivity of elements.

Did you ever play the card game called go fish? Players try to form groups of cards of the same value, such as four sevens, with the cards they are dealt or by getting cards from other players or the deck. This give and take of cards is a simple analogy for the way atoms give and take valence electrons in chemical reactions.

What Are Valence Electrons?

Valence electrons are the electrons in the outer energy level of an atom that can participate in interactions with other atoms. Valence electrons are generally the electrons that are farthest from the nucleus. As a result, they may be attracted as much or more by the nucleus of another atom than they are by their own nucleus.

Electron Dot Diagrams

Because valence electrons are so important, atoms are often represented by simple diagrams that show only their valence electrons. These are calledelectron dot diagrams, and three are shown below. In this type of diagram, an element's chemical symbol is surrounded by dots that represent the valence electrons. Typically, the dots are drawn as if there is a square surrounding the element symbol with up to two dots per side. An element never has more than eight valence electrons, so there can’t be more than eight dots per atom.

Q: Carbon (C) has four valence electrons. What does an electron dot diagram for this element look like?

A: An electron dot diagram for carbon looks like this:

Valence Electrons and the Periodic Table

The number of valence electrons in an atom is reflected by its position in the periodic table of the elements (see the periodic table in the Figure below). Across each row, or period, of the periodic table, the number of valence electrons in groups 1–2 and 13–18 increases by one from one element to the next. Within each column, or group, of the table, all the elements have the same number of valence electrons. This explains why all the elements in the same group have very similar chemical properties.  

For elements in groups 1–2 and 13–18, the number of valence electrons is easy to tell directly from the periodic table. This is illustrated in the simplified periodic table in the Figure below. It shows just the numbers of valence electrons in each of these groups. For elements in groups 3–12, determining the number of valence electrons is more complicated. You can learn more about the valence electrons of elements in these groups at this URL:http://www.colorado.edu/physics/2000/periodic_table/transition_elements.html.

Q: Based on both periodic tables above (Figures above and above), what are examples of elements that have just one valence electron? What are examples of elements that have eight valence electrons? How many valence electrons does oxygen (O) have?

A: Any element in group 1 has just one valence electron. Examples include hydrogen (H), lithium (Li), and sodium (Na). Any element in group 18 has eight valence electrons (except for helium, which has a total of just two electrons). Examples include neon (Ne), argon (Ar), and krypton (Kr). Oxygen, like all the other elements in group 16, has six valence electrons.

Valence Electrons and Reactivity

The table salt pictured in the Figure below contains two elements that are so reactive they are rarely found alone in nature. Instead, they undergo chemical reactions with other elements and form compounds. Table salt is the compound named sodium chloride (NaCl). It forms when an atom of sodium (Na) gives up an electron and an atom of chlorine (Cl) accepts it. When this happens, sodium becomes a positively charged ion (Na+), and chlorine becomes a negatively charged ion (Cl-). The two ions are attracted to each and join a matrix of interlocking sodium and chloride ions, forming a crystal of salt.

Table salt (sodium chloride).

Q: Why does sodium give up an electron?

A: An atom of a group 1 element such as sodium has just one valence electron. It is “eager” to give up this electron in order to have a full outer energy level, because this will give it the most stable arrangement of electrons. You can see how this happens in the animation at the following URL and in the Figure below. Group 2 elements with two valence electrons are almost as reactive as elements in group 1 for the same reason.

http://www.visionlearning.com/img/app/library/objects/Flash/VLObject-1349-030213040210.swf

Q: Why does chlorine accept the electron from sodium?

A: An atom of a group 17 element such as chlorine has seven valence electrons. It is “eager” to gain an extra electron to fill its outer energy level and gain stability. Group 16 elements with six valence electrons are almost as reactive for the same reason.

Atoms of group 18 elements have eight valence electrons (or two in the case of helium). These elements already have a full outer energy level, so they are very stable. As a result, they rarely if ever react with other elements. Elements in other groups vary in their reactivity but are generally less reactive than elements in groups 1, 2, 16, or 17.

Q: Find calcium (Ca) in the periodic table (see Figure above). Based on its position in the table, how reactive do you think calcium is? Name another element with which calcium might react.

A: Calcium is a group 2 element with two valence electrons. Therefore, it is very reactive and gives up electrons in chemical reactions. It is likely to react with an element with six valence electrons that “wants” to gain two electrons. This would be an element in group 6, such as oxygen.

Valence Electrons and Electricity

Valence electrons also determine how well—if at all—the atoms of an element conduct electricity. The copper wires in the cable in the Figure below are coated with plastic. Copper is an excellent conductor of electricity, so it is used for wires that carry electric current. Plastic contains mainly carbon, which cannot conduct electricity, so it is used as insulation on the wires.

Q: Why do copper and carbon differ in their ability to conduct electricity?

A: Atoms of metals such as copper easily give up valence electrons. Their electrons can move freely and carry electric current. You can see in detail how this occurs at the URL below. Atoms of nonmetals such as the carbon, on the other hand, hold onto their electrons. Their electrons can’t move freely and carry current.

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/conins.html

A few elements, called metalloids, can conduct electricity, but not as well asmetals. Examples include silicon and germanium in group 14. Both become better conductors at higher temperatures. These elements are called semiconductors.

Q: How many valence electrons do atoms of silicon and germanium have? What happens to their valence electrons when the atoms are exposed to an electric field?

A: Atoms of these two elements have four valence electrons. When the atoms are exposed to an electric field, the valence electrons move away from the atoms and allow current to flow.

Summary

·         Valence electrons are the electrons in the outer energy level of an atom that can participate in interactions with other atoms.

·         Because valence electrons are so important, atoms may be represented by electron dot diagrams that show only their valence electrons.

·         The number of valence electrons in atoms may cause them to be unreactive or highly reactive. For those atoms that are reactive, the number of valence electrons also determines whether they tend to give up or gain electrons in chemical reactions.

·         Metals, which easily give up electrons, can conduct electricity. Nonmetals, which attract electrons, generally cannot. Metalloids such as silicon and germanium can conduct electricity but not as well as metals.

Explore More

Watch the video at the following URL, and then answer the questions below.

http://www.youtube.com/watch?v=iuj_hw_MFN8 (9:41)

1.    What analogy does Mr. Sams use to show how atoms “view” each other?

2.    What is the octet rule? What is it based on?

3.    What are two ways atoms can achieve an octet of valence electrons?

Review

1.    What are valence electrons?

2.    Draw an electron dot diagram for an atom of nitrogen (N).

3.    Which of the following statements about valence electrons and the periodic table is true?

a.    The number of valence electrons decreases from left to right across each period.

b.    The number of valence electrons increases from top to bottom within each group.

c.    All of the elements in group 9 have nine valence electrons.

d.    Elements with the most valence electrons are in group 18.

4.    Which element would you expect to be more reactive: phosphorus (P) or fluorine (F)? Explain your answer.

5.    Why can’t nonmetals conduct electricity?

Atomic Number (explore more and review questions)

Explains the arrangement of the periodic table and the relationship between subatomic particles.

·         Define atomic number.

·         Explain what mass number represents.

Athletes wearing the same-colored jerseys are all on the same team. In addition, each player’s jersey has a unique number to distinguish him from his teammates. Imagine how confusing it would be if members of both teams wore the same-colored jerseys or all the members of a team had the same number on their jerseys. How could you tell the athletes apart?

Telling Atoms Apart

It’s often useful to have ways to signify different people or objects like athletes on teams. The same is true of atoms. It’s important to be able to distinguish atoms of one element from atoms of other elements. Elements are puresubstances that make up all other matter, so each one is given a unique name. The names of elements are also represented by unique one- or two-letter symbols, such as H for hydrogen, C for carbon, and He for helium. You can see other examples in the Figure below.

Q: The table shown in the Figure above is called the periodic table of theelements. Each symbol stands for a different element. What do you think the symbol K stands for?

A: The symbol K stands for the element potassium. The symbol comes from the Latin name for potassium, which is kalium.

The symbols in the Figure above would be more useful if they revealed more information about the atoms they represent. For example, it would be useful to know the numbers of protons and neutrons in the atoms. That’s where atomic number and mass number come in.

Atomic Number

The number of protons in an atom is called its atomic number. This number is very important because it is unique for atoms of a given element. All atoms of an element have the same number of protons, and every element has a different number of protons in its atoms. For example, all helium atoms have two protons, and no other elements have atoms with two protons. In the case of helium, the atomic number is 2. The atomic number of an element is usually written in front of and slightly below the element’s symbol, like in the Figure below for helium.

Atoms are neutral in electrical charge because they have the same number of negative electrons as positive protons. Therefore, the atomic number of an atom also tells you how many electrons the atom has. This, in turn, determines many of the atom’s properties.

Mass Number

There is another number in the Figure above for helium. That number is themass number, which is the mass of the atom in a unit called the atomic mass unit (amu). One atomic mass unit is the mass of a proton, or about 1.67 × 10-27kilograms, which is an extremely small mass. To better appreciate how small anatomic mass unit is, go to this URL:http://www.colorado.edu/physics/2000/periodic_table/atomic_mass.html.

A neutron has just a tiny bit more mass than a proton, so its mass is often assumed to be one atomic mass unit as well. Because electrons have virtually no mass, just about all the mass of an atom is in its protons and neutrons. Therefore, the total number of protons and neutrons in an atom determines its mass in atomic mass units.

Consider helium again. Most helium atoms have two neutrons in addition to two protons. Therefore the mass of most helium atoms is 4 atomic mass units (2 amu for the protons + 2 amu for the neutrons). However, some helium atoms have more or less than two neutrons. Atoms with the same number of protons but different numbers of neutrons are called isotopes. Because the number of neutrons can vary for a given element, the mass numbers of different atoms of an element may also vary. For example, some helium atoms have three neutrons instead of two. Therefore, they have a different mass number than the one given in the Figure above.

Q: What is the mass number of a helium atom that has three neutrons?

A: The mass number is the number of protons plus the number of neutrons. For helium atoms with three neutrons, the mass number is 2 (protons) + 3 (neutrons) = 5.

Q: How would you represent this isotope of helium to show its atomic number and mass number?

A: You would represent it by the element’s symbol and both numbers, with the mass number on top and the atomic number on the bottom:

52He

Summary

·         Elements are pure substances that make up all matter, so each one is given a unique name. The names of elements are also represented by unique one-, two-, or three- letter symbols.

·         The number of protons in an atom is called its atomic number. This is also unique for each element.

·         An atom’s mass number is its mass in atomic mass units (amu), which is about equal to the total number of protons and neutrons in the atom. Differentisotopes of an element have different mass numbers because they have different numbers of neutrons.

Explore More

Use the periodic table of the elements at the first URL below to fill in the blanks in the worksheet at the second URL.

·         http://periodic.lanl.gov/index.shtml

·         http://images.pcmac.org/SiSFiles/Schools/AL/BaldwinCounty/SpanishFortMiddle/Uploads/Forms/ANMNWorksheet.pdf

Review

1.    What is the atomic number of an atom? Why is this number important?

2.    Describe the atomic mass unit. What does it represent and what does it equal?

3.    The symbol below represents an isotope of helium. How many protons and neutrons does it have?

52He

4.    All carbon atoms have six protons. Most also have six neutrons, but some have seven or eight neutrons. What is the mass number of a carbon isotope that has seven neutrons?

Atomic Nucleus  (review questions only)

Defining characteristics of center of the atom.

·         Identify the nucleus of the atom.

·         Describe the size and mass of the nucleus.

·         Explain what holds the nucleus together.

An atomic bomb explodes and generates a huge mushroom cloud. The tremendous energy released when the bomb explodes is incredibly destructive. Where does all the energy come from? The answer is the nucleus of the atom.  

At the Heart of It All

The nucleus (plural, nuclei) is a positively charged region at the center of the atom. It consists of two types of subatomic particles packed tightly together. The particles are protons, which have a positive electric charge, and neutrons, which are neutral in electric charge. Outside of the nucleus, an atom is mostly empty space, with orbiting negative particles called electrons whizzing through it. TheFigure below shows these parts of the atom.

Size and Mass of the Nucleus

The nucleus of the atom is extremely small. Its radius is only about 1/100,000 of the total radius of the atom. If an atom were the size of a football stadium, the nucleus would be about the size of a pea! You can see another sports analogy comparing the size of the nucleus and atom at this URL:

http://www.youtube.com/watch?v=1o-FsxAkvZk (1:06)

Electrons have virtually no mass, but protons and neutrons have a lot of mass for their size. As a result, the nucleus has virtually all the mass of an atom. Given its great mass and tiny size, the nucleus is very dense. If an object the size of a penny had the same density as the nucleus of an atom, its mass would be greater than 30 million tons! You can learn more about the size and mass of the nucleus at this URL:

http://www.youtube.com/watch?v=Tfy0sIVfVOY (2:03)

Holding It All Together

Particles with opposite electric charges attract each other. This explains why negative electrons orbit the positive nucleus. Particles with the same electric charge repel each other. This means that the positive protons in the nucleus push apart from one another. So why doesn’t the nucleus fly apart? An even stronger force—called the strong nuclear force—holds protons and neutronstogether in the nucleus. You can learn more about these forces in the nucleus by watching the video at this URL:

http://www.youtube.com/watch?v=PdFsb2sWrW4 (6:27)

Q: Can you guess why an atomic bomb releases so much energy when it explodes?

A: When an atomic bomb explodes, the nuclei of atoms undergo a process called fission, in which they split apart. This releases the huge amount of energy that was holding together subatomic particles in the nucleus.

Summary

·         The nucleus is a small, dense region at the center of the atom. It consists of positive protons and neutral neutrons, so it has an overall positive charge.

·         The nucleus is just a tiny part of the atom, but it contains virtually all of the atom’s mass.

·         The strong nuclear force holds together protons and neutrons in the nucleus and overcomes the electric force of repulsion between protons.

Explore More

Watch this short video about how the nucleus was discovered, and then answer the questions below.

http://www.youtube.com/watch?v=Q8RuO2ekNGw (0:48)

1.    Describe the scientific procedure that was used to discover the nucleus.

2.    What evidence led scientists to conclude that atoms consist mostly of empty space with a very small, positively charged mass at the center?

3.    Reflect on the method used in the experiment. Why was it important to send positive—as opposed to neutral or negative—particles toward the gold foil?

Review

1.    Describe the nucleus of the atom.

2.    Why is the nucleus positive in charge?

3.    Explain why the nucleus is very dense.

4.    Outline the forces that act on particles in the nucleus.

5.    If you made a three-dimensional model of an atom and its nucleus, how would you represent the atom? How would you represent nucleus? Explain your choices.