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NYS Chemistry Regents June 2010
Chemistry Regents June 2010 - Question 01 PDF Print E-mail
Written by The Chemistry Wizard   

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Correct Answer: 1

Why: The gold foil experiment (also known as the Gelger-Marsden experiment or Rutherford experiment), was carried out by Hans Geiger and Ernest Marsden in 1909 to help determine the structure of the atom. The prevailing theory of atomic structure at the time was the plum pudding model, proposed by J. J. Thomson. This model proposed that atoms were composed of a positively charged cloud in which electrons, or negatively charged particles, were suspended. The gold foil experiment was designed to determine the distribution of electrons in the plum pudding model.

The gold foil experiment involved firing alpha particles through gold foil and onto a sheet Zinc sulfide, which would light up when struck by the alpha particles. Alpha particles are positively charged particles composed of two protons and two neutrons. When these particles were fired at the gold foil, according to the plum pudding model of the atom, the alpha particles would pass right through the gold foil with minor deflections occurring. It was assumed that there were no particles with enough mass or charge to deflect accelerated alpha particles at an angle greater than 90 degrees. However, what happened was a small number of the alpha particles were deflected at much greater angles than expected.

This led to two significant findings: (1.) rather than the distributed negative charge in a positive cloud, the positive charge carried most of the mass of an atom (to be able to deflect an alpha particle, it had to have high mass and be positively charged as like charges repel), and (2.) the nucleus was actually very small (or more collisions would have occurred).

With this knowledge the alpha particles would have mostly passed through the golf foil with a few being deflected at greater than 90 degree angles as they collided with the small, high mass and positively charged nuclei of the atoms.

Answering the Question:

To correctly answer this question, it is important to read the question carefully. The question states that the gold foil experiment led to the conclusion that each atom is composed mostly of empty space. If the atom were composed mostly of empty space, then particles fired at it would not be trapped, as suggested by answer (2). Particles becoming trapped would not suggest mostly empty space, but that atoms were densely packed with electrons. The positively charged alpha particles would have been attracted to the densely packed, negatively charged electrons and would become trapped.

Answer (3) would suggest a very large positively charged nuclei taking up most of the volume of the atom. The positive nuclei would then repel the positively charged alpha particles.

Answer (4) would not be possible as electrons are negatively charged and alpha particles are positively charged. The presence of opposite charges would result in strong forces of attraction and not, as the answer suggests, deflect the alpha particles.
Last Updated on Tuesday, 07 June 2011 16:36
 
Chemistry Regents June 2010 - Question 02 PDF Print E-mail
Written by The Chemistry Wizard   

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Correct Answer: 3

Why: Carbon is an element located in group 14 of the periodic table. Like most other known elements, they are composed of a positively charged nucleus orbited by negatively charged electrons; therefore, the nucleus is the positively charged center of the atom. The nucleus is both positively charged and contains most of the mass of the atom, usually referred to as the atomic mass. The nucleus is composed of protons and neutrons for most elements, hydrogen being the exception, as its nucleus is composed of one proton and no neutrons.

Based on that information, the nucleus of carbon is composed of protons and neutrons. Carbon 12, the most common form of carbon, is composed of six (6) protons and six (6) neutrons, giving an atomic mass of approximately 12 (the mass of the electrons being negligible in comparison).

Answering the Question:

To answer this question, an understanding of atomic structure is necessary; although, knowledge of the structure of carbon is not necessary. The structure of the atom is such that it is composed of a central positively charged nucleus surrounded by orbiting negatively charged electrons. With that is mind, answer (4) and answer (2) would be immediately eliminated: answer (4) because the nucleus is not composed of protons and electrons, and answer (2) because neutrons have no charge and, therefore, would not be the only subatomic particles present in the nucleus of an atom. Answer (1) is also not possible as hydrogen is the only element with a nucleus that does not have neutrons associated with it. Therefore, the correct answer is answer (3), protons and neutrons.
Last Updated on Wednesday, 08 June 2011 11:18
 
Chemistry Regents June 2010 - Question 03 PDF Print E-mail
Written by The Chemistry Wizard   

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Correct Answer: 3

Why? Helium is an element found in group 18 of the periodic table along with the other noble or inert gasses.  The helium atom, like most other atoms, is composed of three primary subatomic particles. These are the positively charged protons, the neutral neutrons and the negatively charged electrons. Helium has two protons, two neutrons and two electrons, the nucleus, containing the protons and neutrons and having a positive charge, while the orbiting electrons are negatively charged.

Answering the Question:

The question asks "which part of helium is positively charged?" To answer this question, knowledge of general atomic structure is required. Helium is composed of the same atomic structure governing most other elements. Protons are the only positively charged particles in atoms and protons, along with neutrons, form the nucleus of most atoms.

Therefore, answer (1) could not be correct, as electrons are not positively charged. Answer (2) refers to the neutral nucleus, which is also not positively charged. Answer (4) does not referring to a charged particle at all, but refers to the general area in which electrons may be found around atoms. Answer (3) refers to the nucleus, which is composed of both protons and neutrons. Although neutrons do not have a charge, protons do have a positive charge, giving the nucleus a positive charge. Therefore, the answer is three (3) the nucleus.
Last Updated on Wednesday, 08 June 2011 11:19
 
Chemistry Regents June 2010 - Question 04 PDF Print E-mail
Written by The Chemistry Wizard   

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Correct Answer: 3

Why? An atom is composed of a number of sub-atomic particles, they are: protons, neutrons and electrons. The protons and neutrons make up the nucleus of the atom, while the electrons orbit the nucleus. Both the nucleus and electrons are charged particles and help to stabilize each other, often being compared to a sun being orbited by planets. The nucleus is responsible for almost all the mass of an atom and is used to categories atoms into specific elements.

Answering the Question:

Looking at each of the possible answers in turn:

Answer (1) suggests an alpha particle. An alpha particle is in effect a helium ion, which is composed of two (2) protons and two (2) neutrons; therefore, it has a higher mass than a single proton.

Answer (2), an electron, is negatively charged and has significantly less mass than a proton. According to the current models of the atom, most of the mass of an atom is contained within the nucleus and not in the orbiting electrons. As the proton is a major component of the nucleus, it would, therefore, not be possible for a proton and an electron to have approximately equal mass.

Answer (4), a positron, is the only particle of opposite charge that is also equal in mass to an electron. This means a positron's mass is also significantly less than a proton. Answer (3), a neutron, is the other component that makes up the nucleus of an atom, for all but hydrogen. The neutron and proton give the atom its mass. Though electrically neutral, the neutron is close in mass to that of a proton. Therefore, the best possible answer is (3).
Last Updated on Wednesday, 08 June 2011 11:19
 
Chemistry Regents June 2010 - Question 05 PDF Print E-mail
Written by The Chemistry Wizard   

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Correct Answer: 1

Why? Carbon is a group 14 element on the periodic table. It has six (6) protons and six (6) neutrons in its most common form, and is tetravalent, meaning it will make four (4) electrons available for the formation of covalent bonds. Carbon has many physical forms called allotropes, the most popular of which are graphite and diamond.

Both allotropes are very different. Diamond is the hardest naturally occurring material on earth. It is a poor electrical conductor but an excellent thermal conductor. It is clear (like glass) and used heavily in industry as a result of its hardness. In diamond, the carbon atoms form covalent bonds with each other, utilizing all available electrons to form a tetrahedron monomer for its lattice.

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Diagram 1. Showing the lattice structure of diamond

Graphite, on the other hand, is soft enough to rub off on paper, a good electrical conductor but a poor thermal conductor. Graphite is opaque and dark grey to black in appearance. Covalently bonded hexagonal sheets form it, each sheet bonding to the next through weak Van Der Waals forces.

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Diagram 2.showing the lattice structure of graphite

Although chemically identical, graphite and diamond display very different structural, chemical and physical properties.

Answering the Question:

Answering this question does not require much knowledge of carbon. Graphite and diamond are very common and popular allotropes of carbon. It would be unlikely that graphite and diamond, both known to be very different, would have the same molecular structure. If they did indeed have the same structure, they would be categorized as isotopes and would share many properties. Therefore, answers (3) and (4) would be eliminated as they both state "the same molecular structure" as a part of the answer. Also, it is unlikely that given the difference in structures, that the properties would remain the same. Therefore, answer (2) would be eliminated, leaving answer (1), different molecular structure and different properties, as the correct answer.

 

 

Last Updated on Wednesday, 08 June 2011 11:20
 
Chemistry Regents June 2010 - Question 06 PDF Print E-mail
Written by The Chemistry Wizard   

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Answer: 4

Why? The group 14 elements are made up of carbon, silicon, germanium, tin and lead. As you go down the periodic table, the elements tend to go from non-metal to metal. Carbon is at the top of the group and is a non-metal. It is then followed by silicon and germanium, which are metalloids, and then followed by tin and lead, which are both metal. A metal has specific chemical properties that help define it as a metal. Metals will more readily lose an electron and they will react with acids by displacing hydrogen. As you go from left to right along the periodic table, elements become less metal-like in their chemical properties. Going from left to right along the periodic table, there is a decrease in atomic size and an increase in the attraction between the nucleus and electrons. Therefore, there is an increase in the likelihood that electrons will be gained. However, as you go down the groups of the periodic table, there is an increase in the number of orbitals of electrons. Each orbital will act as an insulator, reducing the effect of the nucleus on subsequent orbitals. This reduces the attraction between the nucleus and electrons and results in a tendency to lose electrons, hence making it more metal-like in behavior.

Answering the Question:

To answer the question, knowledge of the trends found in the periodic table ,along with group 14 elements, helps but is not absolutely essential. Group 14 elements tend to be relatively well known. Elements such as tin and carbon are very common. Most persons would have heard of tin and categorized it as a metal.

If the order of group 14 were not known, tin would have been a very educated guess.
Last Updated on Wednesday, 08 June 2011 11:20
 
Chemistry Regents June 2010 - Question 07 PDF Print E-mail
Written by The Chemistry Wizard   

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Answer: 3

Answering the Question:

Based on the information present in the question, light is being emitted. For this to occur there must be a release of energy, which eliminates answers (1) and (2). Light is a form of energy and can neither be created nor destroyed but changed from one form to another. If light is being emitted (and light is a form of energy), it stands to reason that energy is being emitted or released.

Electrons are negatively charged particles usually orbiting the nucleus of an atom. When electrons are supplied with energy, they become excited and move to a higher energy state, or higher orbit. This means that the distance between the electrons and the nucleus will increase as more energy is supplied to overcome the forced of attraction between the electrons and the nucleus. As energy is released, the electrons no longer have sufficient energy to overcome the force of attraction from the nucleus. This results in an electron entering a lower energy state, hence the answer is (3).
Last Updated on Wednesday, 08 June 2011 11:21
 
Chemistry Regents June 2010 - Question 08 PDF Print E-mail
Written by The Chemistry Wizard   

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Answer: 4

Why? The periodic table of elements has many trends as you move across or down. As you move down any groups of the periodic table, the atoms of the elements will become larger. However, as you move across the period, from left to right, the atoms of the elements will become smaller. The decrease in size as you move across the period is a result of the increased forces of attraction between the nucleus and the orbiting electrons. As you move across the periodic table, the number of protons, and hence electrons, increases. This increase is responsible for greater forces of attraction and, hence, a reduction in the distance of the outermost electrons from the nucleus.

Answering the Question:

To answer this question, knowledge and use of the periodic table are very important. It is important to identify where each is placed in the table. All these chemicals belong to period 4. This means that as you go from left to right, the attraction for electrons increases. Identifying which element is farthest to the right will determine the element with the greatest attraction for electrons. The order going from left to right is: Ga → Ge → As → Se. As Se is the leftmost element, it will have the greatest attraction for electrons in a chemical bond. Therefore, the answer is (4).
Last Updated on Wednesday, 08 June 2011 11:22
 
Chemistry Regents June 2010 - Question 09 PDF Print E-mail
Written by The Chemistry Wizard   

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Answer: 2

Why? It is important to have an understanding of what polarity is and how it occurs. When atoms of an element form a molecule, they share electrons. The electrons become distributed but are not always shared equally; this causes the formation of a dipole or areas of charge across the molecule. The other way in which a molecule will form a dipole is as a result of the molecule being asymmetrically arranged and causing polarity to occur. The attraction that an atom in a molecule has for electrons is termed electronegativity and is a measure of the attraction one atom has over another towards the electrons. If the electrons present on each atom are shared equally, then none of the atoms forming the molecule will have a net charge present, resulting in the molecule being non-polar. If, however, one or more atoms in the molecule have a stronger affinity for the electrons, they will pull the electrons closer to their nuclei. This causes a net charge to develop around the molecule. At some point along the molecule, there will be a net negative charge and, at another, a net positive charge resulting in the creation of a dipole. The molecule is then said to be polar.

Answering the Question:

As a general rule, the following molecules are said to be polar:

(1)     Molecules that are linear (carbon monoxide)

(2)     Molecules that have a single hydrogen or hydroxide group at one end

(3)     Molecules with an oxygen at one end

(4)     Molecules with a nitrogen at one end

On the other hand, diatomic molecules and most carbon compounds tend to be non-polar.

Analyzing all the given answers to the question: (1) H2 is a diatomic molecule composed of two hydrogen atoms and falls into the non-polar group. (2) H2O has an oxygen molecule at one end and is categorized as polar. (3) CO2 is a carbon compound, and like most carbon compounds, is non-polar, especially in this case since it is very symmetrical. (4) CCl4 , carbon tetrachloride is a carbon compound and would be categorized as non-polar. It does have polar bonds, but is symmetrical. As a result of its symmetry, it is a non-polar molecule. Therefore, the only possible answer is (2), H2O.
Last Updated on Tuesday, 07 June 2011 07:09
 
Chemistry Regents June 2010 - Question 10 PDF Print E-mail
Written by The Chemistry Wizard   

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Answer: 3 & 4

Why? To answer this question, we can use the process of elimination. The question is asking for two categories of compounds, firstly we need to understand what a compound is. A compound is any chemical species formed from two or more atoms chemically bonding together. The chemical bonds are covalent or ionic and are formed depending on the nature of the atoms and the conditions of the reaction that formed them.

Answering the Question:

Applying this information to the question, (1) has covalent and molecular, covalent is one of the bonds that can be found in compounds, but is not in itself a category of compound, this eliminates answers (1) and (2). Answers (3) and (4) are both correct, compounds can be placed into three basic categories based on the reacting species. As a general rule, if a metal and a non-metal react they form an ionic compound, if two non-metal elements react they form a molecular compound and if two or more metals react they form a metallic compound. This means both (3), ionic and molecular, and (4) ionic and metallic, are correct.
 
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