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Content Benchmark P.12.A.9
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Physical Science
Matter
  P.12.A.1
  P.12.A.2
  P.12.A.3
  P.12.A.4
  P.12.A.5
  P.12.A.6
  P.12.A.7
  P.12.A.8
  P.12.A.9
Force and Motion
Energy
Content Areas
Nature of Science (NOS)
Life Science
Earth Science
Physical Science

Students know the number of electrons in an atom determines whether the atom is electrically neutral
or an ion. E/S

All around us are objects. In school, we see desks, chairs, books, students and teachers. Perhaps you ride to and from school on a school bus that drives over a road paved in asphalt, and you see buildings, clouds, sky, cars and trees on your ride. All of these objects are composed of matter. These different objects are made of many different types of materials (or matter). Matter is all around us and is made of many small particles which are too small to be seen. These small particles are called atoms. An atom is the smallest representative particle of an element. It can bond with other atoms of the same element or with atoms of different elements. Elements are substances that cannot be separated into simpler substances by chemical means, and all atoms of one element have the same number of protons.

To learn more about atomic theory, go to
http://www.visionlearning.com/library/module_viewer.php?mid=50
.

Atoms consist of two major regions, the nucleus and the electron cloud. The nucleus is a very small region located in the center of the atom. Every atom has at least one proton and may contain at least one neutron in its nucleus. Protons are positively charged particles and neutrons have no electrical charge. The electron cloud is a region surrounding the nucleus, and it is occupied by electrons, which are negatively charged. Protons and neutrons have about the same mass, with neutrons being slightly higher in mass. Electrons are about 1/2000 the mass of the protons. These three particles are called subatomic particles. Most of the atom is composed of the electron cloud, and the nucleus occupies a very, very small region of the atom.

Figure 1: Diagram of a helium atom-planetary atom model. (Not to scale: from http://www.aboutnuclear.org/i/the_atom/helium-anim.gif)

The helium atom in Figure 1 has two protons and two neutrons in its nucleus. There are two electrons outside of the nucleus. This schematic diagram of a helium atom is not drawn to scale. The nucleus is much, much smaller, and the electrons occupy a volume that is much, much larger. It appears that the electrons have a definite pathway on which they will be located. This planetary atom model was first proposed by Niels Bohr.

The Bohr model effectively explained many observations that had been made about the hydrogen atom. To learn more about the Bohr model, go to http://csep10.phys.utk.edu/astr162/lect/light/bohr.html.

Figure 2. Quantum mechanical diagram of a helium atom. Note the two protons and two neutrons in the nucleus and two electrons in the electron cloud.
(From http://www.chem4kids.com/files/atom_intro.html)

Figure 2 shows the electrons in the first energy level, and other, unoccupied energy levels further from the nucleus. The vague, fuzzy energy levels are more consistent with current theory of atomic structure.

To see another representation of the electron cloud, go to
http://regentsprep.org/Regents/physics/phys05/catomodel/cloud.htm.

When atoms react with each other, they can transfer electrons in an ionic bond or share electrons in a covalent bond. The number of protons and neutrons in an atom stays the same during bonding. The number of electrons in an atom determines if that atom is electrically neutral or if it has a charge. In a neutral atom, the number of electrons is the same as the number of protons. If an atom loses one or more electrons, it becomes a positive ion. If it gains one or more electrons, it becomes a negative ion. For example, a sodium atom (Na) has 11 protons and 11 electrons, and it is electrically neutral. When sodium forms compounds, it loses one electron (e-). It now has 11 protons and 10 neutrons and is a sodium ion (Na1+). This can be represented as:

Na → Na1+ + e-

Another example can be seen in atoms of sulfur. A sulfur atom (S) has 16 protons and 16 electrons. When sulfur forms a sulfide ion (S2-), it will gain two electrons, represented as:

S + 2 e- → S2-

To learn more about atomic structure and how the number of electrons in an atom determines whether the atom is electrically neutral or an ion, go to
http://web.jjay.cuny.edu/~acarpi/NSC/3-atoms.htm.

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Content Benchmark P.12.A.9

Students know the number of electrons in an atom determines whether the atom is electrically neutral or an ion. E/S

Common misconceptions associated with this benchmark:

1. Students incorrectly think that atoms can be seen with a microscope.

The radii of atoms of all elements are measured in 10-10 meters, and therefore, atoms are much too small to be seen with an optical microscope. Even the more powerful electron tunneling microscopes can be only used to infer the position of atoms on a surface. (Note: this misconception is relevant for P.5.A.6).

To see an image of an atomic surface, go to http://www.answers.com/topic/scanning-tunneling-microscope. This misconception is relevant for P.5.A.6.

2. Students incorrectly think that when an atom loses an electron, it becomes negative.

Students tend to forget that electrons have a negative charge. Losing a negatively charged particle (an electron) results in forming a positive ion. As a general rule of thumb, metals will lose electrons to get the same number of electrons as the nearest noble gas, and therefore, metal ions are positively charged. On the other hand, nonmetals gain electrons to get the same number of electrons as the nearest noble gas, and therefore, nonmetal ions have are negatively charged.

For more misconceptions about atomic structure, go to http://educ.queensu.ca/~science/main/concept/chem/c07/C07CDTL1.htm.

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Content Content P.12.A.9

Students know the number of electrons in an atom determines whether the atom is electrically neutral or an ion. I/S

Sample Test Questions

1st Item Specification: Calculate the number of protons and electrons to determine the electrical charge of an atom.

Depth of Knowledge Level 1

  1. A sodium ion has 11 protons, 12, neutrons, and 10 electrons. How many electrons does the sodium ion need to gain to have a neutral charge?
    1. 0
    2. 1
    3. 2
    4. 11
  1. Analyze the table below showing information about four common elements.
Element Symbol Mass Number Protons Neutrons Electrons Charge
Helium He 4 2 2 2 0
Sodium Na 23 11 12 10 +1
Chlorine Cl 35 17 18 18 -1
Carbon C 12 6 6 2 +?

Since the carbon ion has lost 4 electrons, what is the magnitude of the charge?

  1. 4
  2. 6
  3. 10
  4. 12
  1. Examine the information about four common elements in the table below.
Element Symbol Mass Number Protons Neutrons Electrons Charge
Helium He 4 2 2 2 0
Lithium Li 7 3 4 2 +1
Chlorine Cl 35 17 18 18 -1
Nitrogen N 14 7 7 ? -3

How many electrons does the nitrogen atom contain?

  1. 3
  2. 4
  3. 7
  4. 10

Depth of Knowledge Level 2

  1. A sodium ion has 11 protons, 12, neutrons, and 10 electrons. For the sodium ion to become a neutral atom it will need to
    1. lose one electron.
    2. lose one neutron.
    3. gain one electron.
    4. gain one proton.
  1. Examine the model of ionic bonding below. The diagram shows sodium transferring an electron to chlorine to form sodium chloride or table salt. When sodium transfers an electron to chlorine both atoms become charged and are called ions.


(From: http://www.visionlearning.com/library/module_viewer.php?mid=55)

Choose the statement below that correctly identifies the positive and negative ions and the reason for those charges.

  1. Na is neutral because it lost one electron to Cl which became Cl-.
  2. Na becomes Na+ because it lost one electron to Cl
    which became neutral.
  3. Na+ because it lost one electron and Cl- because it
    gained one electron.
  4. Na- because it lost one electron, and Cl- because it
    gained one electron.
  1. Examine the models of atoms below.

Which of the following correctly identifies the electrically neutral atom and the reason that it is neutral?

  1. Atom A is neutral because it has a greater number of
    neutrons than electrons.
  2. Atom A is neutral because it has a fewer number
    electrons than Atom B.
  3. Atom B is neutral because it has an equal number of
    neutrons and electrons.
  4. Atom B is neutral because at has an equal number of
    protons and electrons.

2nd Item Specification: Calculate the magnitude and sign of the charge on an ion given the number of protons and electrons.

Depth of Knowledge Level 1

  1. An oxygen atom has 8 protons, 8 neutrons and 8 electrons. If the oxygen atom gains two electrons to form an oxygen ion what will be the magnitude and sign of the charge?
    1. -2
    2. +2
    3. -8
    4. +8
  1. Examine the information about four common elements in the table below.
Element Symbol Mass Number Protons Neutrons Electrons Charge
Helium He 4 2 2 2 0
Sodium Na 23 11 12 10 +1
Chlorine Cl 35 17 18 18 -1
Oxygen O 16 8 8 10 -?

The oxygen ion has a negative charge. What is the magnitude of the charge on the oxygen ion?

  1. 2
  2. 8
  3. 10
  4. 16

Depth of Knowledge Level 2

  1. Examine the model of ionic bonding below. The diagram shows sodium giving an electron to chlorine to form sodium chloride or table salt. When sodium gives an electron to chlorine both atoms become charged and are called ions.


(From: http://www.visionlearning.com/library/module_viewer.php?mid=55)

Choose the statement below that correctly identifies the magnitude and sign of the charge on each ion.

  1. Na and Cl-
  2. Na+ and Cl-
  3. Na- and Cl+
  4. Na- and Cl
  1. Examine the table below.
Element Symbol Mass Number Protons Neutrons Electrons Charge
Hydrogen H 1 1 0 1 0
Calcium Ca 40 20 20 18 +2
Flourine F 19 9 10 10 -1
Iron Fe 56 26 30 23 +3
Oxygen O 16 8 8 10 ?

Identify the charge and magnitude of the charge on the oxygen ion.

  1. 0
  2. 2
  3. +2
  4. -2
  1. Examine the models of the lithium atom and lithium ion below.

Which of the following correctly identifies the amount and sign of the charge on the lithium ion?

  1. Li-
  2. Li+
  3. Li2+
  4. Li2-
  1. Analyze the information about five common elements on the table below.
Element Symbol Mass Number Protons Neutrons Electrons Charge
Helium He 4 2 2 2 0
Sodium Na 23 11 12 10 +1
Chlorine Cl 35 17 18 18 -1
Silicon Si 28 14 14 10 +4
Aluminum Al 27 ? 14 10 ?

Which of the following correctly identifies the number of protons and the charge on the aluminum atom?

  1. 10 protons, charge 0
  2. 10 protons, charge +3
  3. 13 protons, charge +3
  4. 13 protons, charge -3

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Content Benchmark P.12.A.9

Students know the number of electrons in an atom determines whether the atom is electrically neutral or an ion. I/S

Answers to Sample Test Questions

  1. B, DOK level 1
  2. A, DOK level 1
  3. D, DOK level 1
  4. C, DOK level 2
  5. C, DOK level 2
  6. D, DOK level 2
  7. A, DOK level 1
  8. A, DOK level 1
  9. B, DOK level 2
  10. D, DOK level 2
  11. B, DOK level 2
  12. C, DOK level 2

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Content Benchmark P.12.A.9

Students know the number of electrons in an atom determines whether the atom is electrically neutral or an ion. E/S

Intervention Strategies and Resources

The following is a list of intervention strategies and resources that will facilitate student understanding of this benchmark.

1. Periodic Table Use
Make sure that students know that an atom in which the number of protons equals the number of electrons is electrically neutral. The number of protons in any atom of an element is its atomic number (symbolized Z). This is found on a Periodic Table of the Elements. Consult the key on the periodic table to determine which number is the atomic number.

To see a periodic table, go to http://www.chemicool.com/ or http://www.webelements.com/webelements/scholar. If the atom has more or less electrons than the atomic number, the resulting particle is an ion.

2. Atom or Ion? Activity
To determine if an atom is electrically neutral, give the students a chart of elements, number of protons, number of electrons, and atom or ion columns, as below. Have the students decide if this is an atom or ion.

Element Number of Protons Number of Electrons Atom or Ion
Lithium 3 2  
Aluminum 13 13  
Oxygen 8 10  
Calcium 20 18  
Hydrogen 1 0  
Phosphorus 15 18  
Magnesium 12 10  

The answers are:

Element Number of Protons Number of Electrons Atom or Ion
Lithium 3 2 Ion
Aluminum 13 13 Atom
Oxygen 8 10 Ion
Calcium 20 18 Ion
Hydrogen 1 1 Atom
Phosphorus 15 15 Atom
Magnesium 12 10 Ion

3. Periodic Table Activity
Give the students a periodic table. Have them fill in the blanks on the table below.

Element, Symbol Number of Protons Number of Electrons Atom or Ion
Zinc, Zn 30 28  
Carbon, C 6 6  
Fluorine, F 9 10  
Barium, Ba 56 54  
Iron, Fe 26 25  
Sulfur, S 16 18  
Nickel, Ni 28 26  

The answers are:

Element, Symbol Number of Protons Number of Electrons Atom or Ion
Zinc, Zn 30 28 Ion
Carbon, C 6 6 Atom
Fluorine, F 9 10 Ion
Barium, Ba 56 54 Ion
Iron, Fe 26 28 Atom
Sulfur, S 16 18 Ion
Nickel, Ni 28 26 Ion

4. Determining Number of Protons
For further discussion on determining the number of protons or electrons in an atom, go to http://education.jlab.org/qa/pen_number.html. However, this website oversimplifies how to calculate the number of neutrons in an atom. Since this benchmark does not need to determine the number of neutrons in an atom, the neutron section can be eliminated.

5. Atomic Structure Information
This Web site shows the subatomic particles found in neutral atoms of hydrogen and helium. It shows an atom of both elements with their electrons in motion, and it shows the quantum mechanical representation of a hydrogen atom.

To access this simple, excellent discussion of atomic structure, go to
http://web.jjay.cuny.edu/~acarpi/NSC/3-atoms.htm.

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