Content Benchmark P.12.A.9
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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. (Note 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^- → S^2-

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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Performance 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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Performance 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

Sample Test Questions

1. Which number of subatomic particles can be found in an electrically neutral atom?
a. 3 protons, 3 neutrons, 2 electrons
b. 3 protons, 4 neutrons, 2 electrons
c. 3 protons, 3 neutrons, 3 electrons
d. 3 protons, 4 neutrons, 4 electrons

2. When one atom loses or gains electron(s), what is the resulting particle called?
a. ion
b. isotope
c. isobar
d. polyatomic

3. When a sodium atom (Na) loses one electron, what is the symbol for the substance formed?
a. Na⁺
b. Na^-
c. Na₂
d. 2 Na

4. What must be true for an atom to be electrically neutral?
a. The number of protons must equal the number of neutrons.
b. The number of protons must equal the number of electrons.
c. The number of neutrons must equal the number of electrons.
d. The total number of nucleons must equal the number of electrons.

5. The nucleus of an oxygen atom has 8 protons. What number of particles would make this become an ion?
a. 8 neutrons
b. 8 electrons
c. 10 neutrons
d. 10 electrons

6. What information must be known to determine if a substance is an atom or ion?
a. Number of nucleons and electrons
b. Number of neutrons and protons
c. Number of protons and neutrons
d. Number of protons and electrons

7. What are the tiny particles of which matter is made?
a. Isotopes
b. Atoms
c. Rays
d. Photons

8. Name what is diagrammed below.

a. atom
b. Ion
c. Nucleon
d. Allotrope

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

Answers to Sample Test Questions
1. (c)
2. (a)
3. (a)
4. (b)
5. (d)
6. (d)
7. (b)
8. (b)

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Performance 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.

The answers are:

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

The answers are:

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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