CHEMICAL BONDING

So far, we’ve studied atoms and compounds and how they react with each other. Now let’s take a look at how these atoms and molecules hold together. Bonds hold atoms and molecules of substances together. There are several different kinds of bonds; the type of bond seen in elements and compounds depends on the chemical properties as well as the attractive forces governing the atoms and molecules. The three types of chemical bonds are Ionic bonds, Covalent bonds, and Polar covalent bonds. Chemists also recognize hydrogen bonds as a fourth form of chemical bond, though their properties align closely with the other types of bonds.

In order to understand bonds, you must first be familiar with electron properties, including valence shell electrons. The valence shell of an atom is the outermost layer (shell) of an electron. Though today scientists generally agree that electrons do not rotate around the nucleus, it was thought throughout history that each electron orbited the nucleus of an atom in a separate layer (shell). Today, scientists have concluded that electrons hover in specific areas of the atom and do not form orbits; however, the valence shell is still used to describe electron availability.

One can determine how many electrons an atom will have by looking at its periodic properties. In order to determine an element’s periodic properties, you will need to locate a periodic table. After you’ve found your periodic table, look at the roman numerals above each column of the table. You should see that above Hydrogen, there’s a IA, above Beryllium there’s a IIA, above Boron there’s a IIIA, and so on all the way to Fluorine, which is VIIA. Also, note that the metals are all in group B—their roman numerals have the letter B afterwards instead of the letter A. For now, we are going to ignore the columns with a B, and focus on the columns with an A (the non-metals, generally speaking). Once you have located the group-A elements, we are going to count across, giving each column a number, like this:

The first A-column is I (1), then counting across, 2-8 (skipping the B group, which consists of metals). In the periodic table we labeled the 8th column as 0, however when counting electrons, we’ll count it as 8. Now, we can determine how many valence electrons each element has in its outermost shell. The elements in the IA column have 1 valence electron. The elements in the IIA column have 2 bonding electrons, and so on. By the time we get to the noble gases (the column labeled 0), we are up to 8 bonding electrons. This means that these gases can stand on their own, or donate electrons to another element, but they cannot accept any more electrons. This is because the electrons they have satisfy the octet rule.

The Octet and Duet Rules

When it comes to bonding, everything is based on how many electrons an element has or shares with its compound partner or partners. The octet rule is followed by most elements, and it says that to be stable, an atom needs to have eight electrons in its outermost shell. Elements that do not follow the octet rule are H, He, B, Li and Be (sometimes). Lithium gives up an electron whereas the other elements listed here gain one. These elements instead follow the duet rule which says that the atoms only need two valence electrons to be stable. When bonding, stability is always considered and preferred. Therefore, atoms bond in order to become more stable than they already are.

Not all atoms bond the same way, so we need to learn the different types of bonds that atoms can form. There are three (sometimes four) recognized chemical bonds; they are ionic, covalent, polar covalent, and (sometimes) hydrogen bonds.

Ionic Bonds

Ionic bonds form when two atoms have a large difference in electronegativity. (Electronegativity is the quantitative representation of an atom’s ability to attract an electron to itself). Although scientists do not have an exact value to signal an ionic bond, the amount is generally accepted as 1.7 and over to qualify a bond as ionic. Ionic bonds often occur between metals and salts; chloride is often the bonding salt. Compounds displaying ionic bonds form ionic crystals in which ions of positive and negative charges hover near each other, but there is not always a direct 1-1 correlation between positive and negative ions. Ionic bonds can typically be broken through hydrogenation, or the addition of water to a compound.

Covalent Bonds

Covalent bonds form when two atoms have a very small (nearly insignificant) difference in electronegativity. The value of difference in electronegativity between two atoms in a covalent bond is less than 1.7. Covalent bonds often form between similar atoms, nonmetal to nonmetal or metal to metal. Covalent bonding signals a complete sharing of electrons. There is usually a direct correlation between positive and negative ions, meaning that because they share electrons, the atoms balance. Covalent bonds are usually strong because of this direct bonding.

Polar Covalent Bonds

Polar covalent bonds fall between ionic and covalent bonds. They result when two elements bond with a moderate difference in electronegativity moderately to greatly, but they do not surpass 1.7 in electronegativity difference. Although polar covalent bonds are classified as covalent, they do have significant ionic properties. They also induce dipole-dipole interactions, where one atom becomes slightly negative and the other atom becomes slightly positive. However, the slight change in charge is not large enough to classify it entirely as an ion; they are simply considered slightly positive or slightly negative. Polar covalent bonds often indicate polar molecules, which are likely to bond with other polar molecules but are unlikely to bond with non-polar molecules.

Hydrogen Bonds

Hydrogen bonds only form between hydrogen and oxygen (O), nitrogen (N) or fluorine (F). Hydrogen bonds are very specific and lead to certain molecules having special properties due to these types of bonds. Hydrogen bonding sometimes results in the element that is not hydrogen (oxygen, for example) having a lone pair of electrons on the atom, making it polar. Lone pairs of electrons are non-bonding electrons that sit in twos (pairs) on the central atom of the compound. Water, for example, exhibits hydrogen bonding and polarity as a result of the bonding. This is shown in the diagram below.

Because of this polarity, the oxygen end of the molecule would repel negative atoms like itself, while attracting positive atoms, like hydrogen. Hydrogen, which becomes slightly positive, would repel positive atoms (like other hydrogen atoms) and attract negative atoms (such as oxygen atoms). This positive and negative attraction system helps water molecules stick together, which is what makes the boiling point of water high (as it takes more energy to break these bonds between water molecules).

In addition to the four types of chemical bonds, there are also three categories bonds fit into: single, double, and triple. Single bonds involve one pair of shared electrons between two atoms. Double bonds involve two pairs of shared electrons between two atoms, and triple bonds involve three pairs of shared electrons between two atoms. These bonds take on different natures due to the differing amounts of electrons needed and able to be given up.

Now, let’s look at determining what types of bonds we see in different compounds. We’ve already looked at the bonds in H₂O, which we determined to be hydrogen bonds. However, now let’s look at a few other types of bonds as examples.

Compound: HNO₃ (also known as Nitric acid)

There are two different determinations we can make as to what these bonds look like; first we can decide whether the bonds are covalent, polar covalent, ionic, or hydrogen. Then, we can determine if the bonds are single, double, or triple.

In order to decide whether the bonds are covalent, polar covalent, ionic or hydrogen, we need to look at the types of elements seen and the electronegativity values. We look at the elements and see hydrogen, nitrogen, and oxygen—no metals. This rules out ionic bonding as a type of bond seen in the compound. Then, we would look at electronegativity values for nitrogen and oxygen. Oftentimes, this information can be found on a periodic table, in a book index, or an educational online resource. The electronegativity value for oxygen is 3.5 and the electronegativity value for nitrogen is 3.0. The way to determine the bond type is by taking the difference between the two numbers (subtraction). 3.5 – 3.0 = 0.5, so we can determine that the bond between nitrogen and oxygen is a covalent bond. We can also determine, from past knowledge, that the bond between oxygen and hydrogen is a hydrogen bond as it was in water.

Now, we need to count the electrons and draw the diagram for HNO₃. For more help counting electrons, please see the page onElectron Configuration. For more help drawing the Lewis structures, please see the page on Lewis Structures. This process combines both of these in order to determine the structure and shape of a molecule of the compound.

First, we determine that N follows the octet rule, so it needs eight surrounding electrons. This is important to keep in mind as we move forward. Next we count up how many valence electrons the compound has as a whole. H gives us 1, N gives us 5, and each O gives us 6. We can discern this from looking at the tops of the columns in the periodic table (see above). We then add these numbers together (3 x 6 = 18, + 1 = 19, + 5 = 24), and we get 24 electrons that we need to distribute throughout the molecule. First, we need to draw the molecule to see how many initial bonds we’ll be putting in. Our preliminary structure looks like this:

Now, we can count how many electrons we have used by counting 2 electrons for each bond placed. We see that we have placed 4 bonds, so we have used 8 electrons. 24 – 8 = 16 electrons that we need to distribute. In order to correctly place the rest of the electrons, we need to determine how many electrons each atom needs to be stable.

The central atom, N, has three bonds attached (equivalent of 6 electrons) so it needs 2 more electrons to be stable. The O to the right has one bond (two electrons) so it needs 6 more to be stable. The O above the N has one bond (two electrons) so it also needs 6 electrons to be stable. The O to the left of the N is bonded both to N and to H, so it has two bonds (4 electrons); therefore, it needs 4 more electrons to be stable. We add up the total amount of electrons needed, 2 + 6 + 6 + 4 = 18, and see that we need 18 electrons to stabilize the compound. We know this is not possible, since we only have 16 available electrons. When this happens, we need to insert a double bond in order to resolve the problem of lack of electrons. This is because, although we count each bond as 2 electrons, the elements joined together in the bond are actually sharing the electrons. Therefore, when we count out the bonds, we are counting some electrons twice because they are shared. This is normal and expected, and resolves not having enough valence electrons. Now, we need to decide where to put the double bond in this compound. We know that the double bond cannot go between O and H, because H does not have enough room to accept another electron. Therefore, we know we must place the bond between N and O. You might be thinking, how do I decide where to put the bond? In this particular example, we can place the bond either between the top O and N, or the right O and N. This is because HNO₃ displays resonance.

Here are the ways you can place the double bond:

or

We are going to keep the bond between N and the right O in our example. After we add in the bond, we subtract two more electrons from our available electrons (16) and are left with 14 electrons to distribute. Now we need to make sure we have the correct number of electrons. After placing in the double bond, N is now stable because it has 4 bonds (8 electrons) surrounding it. It does not need any additional electrons. The top O (above N) needs 6 electrons, the right O now only needs 4 electrons (because it has a double bond now, which is 4 electrons), and the left O still needs 4 electrons to become stable. We add these numbers together, 6 + 4 + 4 = 14, and we see that 14 is the number of electrons we have, so we can go ahead and distribute them, like this:

Now, our compound is stable with appropriately distributed valence electrons. We can see that there are three single bonds (H—O, Ne bond (N==O).—O, and N—O) and one double bond

INNOVATIVE WORK

SCIENCE LABORATORY

INTRODUCTION

The laboratory is central to science instruction. It is in the laboratory that the students learn to handle apparatus, think independently and to draw conclusions on the basis of experiments and observation. Science laboratory is an essential component of science education. It makes scientific understandings in students. Science educators frequently turn to the research literature for support of their requests for funds for supplies and equipment for laboratory activities. Science education researcher have examined the role of the laboratory on many variables, including achievement, attitudes, critical thinking, cognitive style, understanding science, the development of science process skills, manipulative skills, interests, retention in science courses, and the ability to do independent work.

SCIENCE LABORATORY

Laboratory work is an essential component of science education. Scientific theories and practical work in science are the two sides of a coin. These two aspects of science education should supplement and complement each other. Without experiments, the students cannot experience the reality of science. Practical work provides an activity which can be profitable and emotionally satisfying. The development of powers of observation, measurement, drawing inferences is all dependants on laboratory work. Laboratory work helps to realize the process oriented objectives of science teaching .The following are the educationally significant objectives of laboratory work.

· Making abstract scientific understandings concrete.

· Development of scientific concepts and principles.

· Development of scientific skills ,attitudes, interests and appreciation

· Training in scientific method.

· Awakening the maintenance of curiosity in the environment

LEARNING GOALS FOR LABORATORY ACTIVITIES

   

      Laboratory activities must be designed to engage students’ minds, so that students may acquire skill and confidence in their:    

· Measurement of physical quantities with appropriate accuracy        

· recognition of factors that could affect the reliability of their    measurements        

· manipulations of materials, apparatus, tools, and measuring instruments       

· clear descriptions of their observations and measurements      

· representation of information in appropriate verbal, pictorial, graphical, and mathematical terms. 

· inference and reasoning from their observations.

· ability to rationally defend their conclusions and predictions        

· effective and valued participation with their peers and their teacher in a cooperative intellectual enterprise 

· articulate reporting of observations, conclusions, and predictions in formats ranging from

· informal discussion to a formal laboratory report 

· ability to recognize those questions that can be investigated through experiment and to plan, carry out, evaluate, and report on such experiments.      

FOR LEARNING FROM LABORATORY ACTIVITIES TEACHING CONDITIONS

  Theory and research suggest that meaningful learning is possible in laboratory activities if all students are provided with opportunities to manipulate equipment and materials while working cooperatively with peers in an environment in which they are free to pursue solutions to problems that interest them.

The following teaching conditions enable this to occur.    

   

· For students to acquire the manual and mental skills associated with learning physics, it is essential that they be fully engaged in laboratory activities. This requires sufficient equipment and laboratory stations for laboratory groups containing only two or three students. 

· The number of students and of laboratory stations in a classroom must be small enough for the teacher to supervise the safety of student activities and to have sufficient time to actively work with each laboratory group.

· Schools and teachers must ensure equal access to laboratory activities under appropriate supervision for all students, with provision made for adapting activities for students with a disability.

· Where appropriate, laboratory activities should include equipment and phenomena that relate to the students’ world, such as toys, sports equipment, tools, household items, etc. 

· The integration of laboratory activities with classroom work requires that students be able to move smoothly between their desks and the laboratory area and that there be sufficient space for equipment to remain set up. A classroom arrangement with space for desks, computers, and ample space for laboratory stations and equipment in the same room is ideal. At the high school level, it is especially desirable for the laboratory area to be integrated with the classroom. 

· Computers and modern instruments should be part of the laboratory equipment. Although excellent physics learning can take place using the simplest equipment, computers and measuring instruments incorporating modern technology can be powerful tools for learning physics concepts and developing skills of measurement, analysis, and processing information. 

· Computer simulations should not substitute for laboratory experience, but may be used to supplement and extend such experience.

· Evaluation of student learning in physics should include assessment of skills developed in laboratory activities as well as the knowledge acquired during these activities. Test questions relating directly to laboratory work act not only to assess laboratory learning, but also communicate  the importance of laboratory work to students.

· Effective employment of laboratory activities requires that teachers have adequate and convenient storage for equipment; a workspace with tools to repair, maintain, or construct equipment; and enough planning time in their schedule to maintain, set up, and try out laboratory equipment prior to classes.  

· Safe laboratory work for students and teachers requires adequate, up-to-date safety equipment; an emphasis on safe practice in all activities; and the availability of resources and references on safety, such as the AAPT publication, Teaching Physics Safely.

· To maintain their skills and keep abreast of new developments in physics teaching, teachers need time, money, support, and encouragement to participate in appropriate professional activities. These may include attendance at workshops and professional conferences; examining new laboratory equipment, curricula, texts and resource materials; and working and consulting with colleagues in schools and colleges and in the physics and engineering research community.      

   

The role of the laboratory is central in high school physics courses since students must construct their own understanding of physics ideas. This knowledge cannot simply be transmitted by the teacher, but must be developed by students in interactions with nature and the teacher. Meaningful learning will occur where laboratory activities are a well-integrated part of a learning sequence. The separation of laboratory activities from lecture is artificial, and not desirable in high school physics.    

   

LABORATORY RULES AND DISCIPLINE

              Laboratory safety is positive undertaking which the science teacher is experience to take up at the time of engaging in any activity in the laboratory in the presence of his students. This would ensure acquisition of safety conscious attitudes among pupils.  Creation of which is a very important duty of the science teacher. The sign of good discipline creating an atmosphere of healthy work. The following are some suggestions or rules for maintenance of discipline in the laboratory.

No pupil should be allowed to enter the laboratory in the absence of the teacher.

Every student should have a place assigned to him for his experiment.

Pupils should perform only those experiments assigned by the teacher.

No equipments/chemicals should be used until proper instructions are received from the teacher.

Reagent bottles should be returned to the shelf immediately after use and these should not be misplaced.

Consider the safety of fellow students. A scientific atmosphere should be kept up in the laboratory.

CONCLUSION

The school science laboratory encompasses a broad variety of forms, including disciplinary breadth (physics, life sciences, chemistry, earth sciences, interdisciplinary investigations) as well as historical breadth. The practical details of scientific work have continued to evolve. New technological infrastructures have been incorporated, interdisciplinary investigations have become more common, the applications of scientific knowledge continue to expand and intersect with the workings of society. School science investigations have not, by and large, kept pace. Efforts should be made to bring school science more in step with contemporary scientific practice. This includes showing how empirical investigation fits into the broader fabric of knowledge work associated with specific disciplines—engaging with the primary literature, communicating the research through 28 presentations and publications, as well as applying laboratory-derived knowledge to societal issues as appropriate. The overriding educational goals can be framed as providing citizens with: (a) images of scientific inquiry that help them understand the role of science in society and (b) experiences that help them develop sufficient disciplinary expertise such that it is personally relevant to their everyday activities.

REFERENCE

1. http://sites.nationalacademies.org/cs/groups/dbassesite/documents/webpage/dbasse_073328.pdf

2. https://www.narst.org/publications/research/labs.cfm

3. Essentials of educational technology-J C Agarwal

PRESENTATION

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