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The lowest lying states of LiH have been widely used to develop and calibrate many different methods in quantum mechanics. In this paper, we show that the electron-transfer processes occurring in these two states are a difficult test for chemical bonding descriptors and can be used to assess new bonding descriptors on its ability to recognise the harpoon mechanism. To this aim, we study the bond formation mechanism in a series of diatomic molecules. In all studied electron reorganisation mechanisms, the maximal electron-transfer variation point along the bond formation path occurs when about half electron has been transferred from one atom to another. If the process takes place through a harpoon mechanism, this point of the reaction path coincides with the avoided crossing. The electron sharing indices and one-dimensional plots of the electron localisation function and the Laplacian of the electron density along the molecular axis can be used to monitor the bond formation in diatomics and provide a distinction between the harpoon mechanism and a regular electron reorganisation process. [ABSTRACT FROM PUBLISHER]

Getting students to understand the particulate nature of matter is a major challenge for chemistry education. In upper secondary school students commonly struggle to distinguish between intra- and intermolecular bonding and analyse chemical bonding in terms of electronegativity. In this study, we explore how creative drama may be used in chemistry education to facilitate students' explorations of electronegativity and chemical bonding in Swedish upper secondary chemistry education. The study was conducted as a design-based intervention in three cycles in two schools. The interventions (which lasted for 30–60 minutes) were single-lesson-interventions consisting of drama activities and discussions in whole-class and student groups. A qualitative content analysis produced themes that captured the ways in which the students explored electronegativity and chemical bonding and how creative drama enabled collective student agency. The findings indicate that creative drama enabled the students to link the electronegativity and polarity of molecules to formations of molecular grid structures using their own bodies to represent how molecules were organised in different states of matter. The results also indicate that creative drama in chemistry education may enhance student agency in their explorations of electronegativity and the linking of electronegativity to intramolecular and intermolecular bonding. [ABSTRACT FROM AUTHOR]

Chemical bonding is one of the key and basic concepts in chemistry. The learning of many of the concepts taught in chemistry, in both secondary schools as well as in the colleges, is dependent upon understanding fundamental ideas related to chemical bonding. Nevertheless, the concept is perceived by teachers, as well as by learners, as difficult, with teaching commonly leading to students developing misconceptions. Many of these misconceptions result from over-simplified models used in text books, by the use of traditional pedagogy that presents a rather limited and sometimes incorrect picture of the issues related to chemical bonding and by assessments of students' achievement that influence the way the topic is taught. In addition, there are discrepancies between scientists regarding key definitions in the topic and the most appropriate models to teach it. In particular, teaching models that are intended to have transitional epistemological value in introducing abstract ideas are often instead understood by students as accounts of ontological reality. In this review paper we provide science educators, curricula developers and pre-service and in-service professional development providers an up-to-date picture regarding research and developments in teaching about chemical bonding. We review the external and internal variables that might lead to misconceptions and the problematic issue of using limited teaching/learning models. Finally, we review the approaches to teaching the concept that might overcome some of these misconceptions. [ABSTRACT FROM AUTHOR]

Polycarbonate/aluminum alloy hybrids were prepared using a homemade ultrasound-assisted hot pressing molding (UAHPM) technique, and their properties and structures at the plastic-metal interface were also investigated. The aluminum alloy surface was chemically etched and anodized to form micron and nano-holes structures. The joint strength of 21.58 MPa was achieved at the optimum molding process by optimizing the process parameters of ultrasonic hot compression molding. The specific structure and morphology of the polycarbonate/aluminum hybrid material bonding layer were observed by field emission scanning electron microscopy (FE-SEM) using the ultrathin sectioning technique, and the plastic embedding depth reached about 3.8 µm under the optimal process conditions. X-ray photoelectron spectroscopy (XPS) analysis revealed that the bonding layer not only has a micro-nano interlocking structure but also forms an Al-O-C chemical bonding interaction. The plastic in the bonding layer was exposed by dissolving the aluminum alloy of the joint, and the filling form of the plastic in the micro-nanoholes was directly observed. [ABSTRACT FROM AUTHOR]

The ethylene epoxidation is a challenging catalytic process, and development of active and selective catalyst requires profound understanding of its chemical behaviour under reaction conditions. The systematic study on intermetallic compounds in the Ca–Ag system under ethylene epoxidation conditions clearly shows that the character of the oxidation processes on the surface originates from the atomic interactions in the pristine compound. The Ag-rich compounds Ca2Ag7 and CaAg2 undergo oxidation towards fcc Ag and a complex Ca-based support, whereas equiatomic CaAg and the Ca-rich compounds Ca5Ag3 and Ca3Ag in bulk remain stable under harsh ethylene epoxidation conditions. For the latter presence of water vapour in the gas stream leads to noticeable corrosion. Combining the experimental results with the chemical bonding analysis and first-principles calculations, the relationships among the chemical nature of the compounds, their reactivity and catalytic performance towards epoxidation of ethylene are investigated. [ABSTRACT FROM AUTHOR]

We have carried out a computational study of the behavior under pressure of three polymorphs of MgInS. Ab initio calculations are performed to determine the equation of state parameters of direct (DS) and inverse (IS) spinel phases, and a defect-LiTiO-type (LTO) structure observed at high pressure. It has been shown that the DS phase has higher enthalpy at zero pressure than the IS one, and the latter undergoes a pressure-induced transition to the LTO structure at 8 GPa, very close to the experimental value. Additionally, the topological analysis of the electron density of this IS phase indicates that the size and atomic-like bulk modulus associated with the sulfur atom are the key contributions to of the IS phase of MgInS. This result is in concordance with the anomalous trend found for at spinels (, S, Se) as X increases its atomic number. [ABSTRACT FROM AUTHOR]

During the last decade there has been on-going discussions about students’ declining interest and low achievement in science. One of the reasons suggested for this decline is that teachers and students have different frames of reference, whereby teachers sometimes communicate science in the classroom in a way that is not accessible to the students. There is a lack of research investigating the effects of coteaching with senior students in science in upper secondary schools. To improve teaching and to narrow the gap between teachers’ and students’ different frames of references, this study investigates how an experienced chemistry teacher gains and refines her pedagogical content knowledge (PCK) by cooperating with two grade 12 students (age 18) as coteachers. The teacher and the two coteachers coplanned, cotaught and coevaluated lessons in chemical bonding in a grade 10 upper secondary class. Findings indicate that the coteachers contributed with their own learning experiences to help the teacher understand how students perceive difficult concepts. In such way, the coteachers were mediating between the teacher and the students, thus bridging the gap between the teacher and the students’ frames of references. The teachers’ PCK was refined which in turn lead to improved teaching strategies. [ABSTRACT FROM AUTHOR]

Purpose:To investigate the effects of phosphoric acid esters (PAEs)-containing primers on the micro-tensile bond strengths (MTBS) of etched enamel, the micro-morphologies of the resin–enamel interfaces, and the enamel surfaces. Materials and Methods:Thirty-three bovine incisors were used in this study. After the labial enamel surfaces were highly polished, they were etched, water sprayed, and air-dried. Afterward, the enamel surfaces were treated with or without (control) one of three PAEs-containing primers (40%MDP, Ivoclar Vivadent; Resulcin AquaPrime A + B, DMG; Xeno V, Dentsply), water sprayed, and air-dried. Subsequently, an adhesive Heliobond (Ivoclar Vivadent) was applied and resin composite (Core Build-up, Bisco) was placed incrementally. They were prepared into multiple beams of about 1 × 1 × 8 mm for MTBS tests. The enamel surfaces and resin–enamel interfaces were analyzed by SEM and TEM. The data were analyzed by two-way ANOVA and LSD test. Results:Using PAEs-containing primers could increase the enamel MTBS (p < 0.05). The primer MDP produced higher MTBS than the primer Resulcin AquaPrime A + B and Xeno V. The SEM findings revealed the primed enamel surfaces were covered with a thin or glue-like layer of monomer-calcium salts of PAEs and the residual enamel crystallites, and various micro-porosities were detected within the hybrid layers. The TEM findings revealed the hydroxyapatite crystallites were tightly covered by the adhesive, the adhesive mixed with resin composite, or sparse irregular enamel crystallites. Conclusion:The application of PAEs-containing primers on the etched enamel substrate could significantly increase the immediate enamel bond strengths. However, this effect was dependent on the individual PAEs-containing primer used. [ABSTRACT FROM AUTHOR]

We present a generalisation of the electron localisation function (ELF) to current-density-functional theory as a descriptor for the properties of molecules in the presence of magnetic fields. The resulting current ELF (cELF) is examined for a range of small molecular systems in field strengths up toB0= 235 kT (one atomic unit). The cELF clearly depicts the compression of the molecular electronic structure in the directions perpendicular to the applied field and exhibits a structure similar to that of the physical current densities. A topological analysis is performed to examine the changes in chemical bonding upon application of a magnetic field. [ABSTRACT FROM PUBLISHER]

Researchers have shown a growing interest in science teachers’ professional knowledge in recent decades. The article focuses on how chemistry teachers impart chemical bonding, one of the most important topics covered in upper secondary school chemistry courses. Chemical bonding is primarily taught using models, which are key for understanding science. However, many studies have determined that the use of models in science education can contribute to students’ difficulties understanding the topic, and that students generally find chemical bonding a challenging topic. The aim of this study is to investigate teachers’ knowledge of teaching chemical bonding. The study focuses on three essential components of pedagogical content knowledge (PCK): (1) the students’ understanding, (2) representations, and (3) instructional strategies. We analyzed lesson plans about chemical bonding generated by 10 chemistry teachers with whom we also conducted semi-structured interviews about their teaching. Our results revealed that the teachers were generally unaware of how the representations of models they used affected student comprehension. The teachers had trouble specifying students’ difficulties in understanding. Moreover, most of the instructional strategies described were generic and insufficient for promoting student understanding. Additionally, the teachers’ rationale for choosing a specific representation or activity was seldom directed at addressing students’ understanding. Our results indicate that both PCK components require improvement, and suggest that the two components should be connected. Implications for the professional development of pre-service and in-service teachers are discussed. [ABSTRACT FROM PUBLISHER]

This study investigated the effects of priming time (5, 30 or 60 s), 10-MDP (10-methacryloyloxydecyl dihydrogen phosphate)-containing primer’s concentrations (0.5, 1, 5, 10 and 20% (w/w)) and two neutral primers (5% (w/w) of 10-MDP-Na or EDTA-2Na) on bovine enamel microtensile bond strength (μTBS) using a novel prime-and-rinse approach. After priming, the enamel surfaces were thoroughly water-sprayed and dried before resin composite was placed. There was a significant influence of primer’s concentrations and priming time on bovine enamel μTBS when 10-MDP-containing primers were used (p < 0.001), revealing the optimal priming time (30 s) and the best combination (priming with 20% of 10-MDP for 30 s). The prime-and-rinse approach might be an innovative supplement to contemporary dental adhesive procedures. The neutral primer (5% of 10-MDP-Na) produced moderate enamel μTBS on highly polished enamel surfaces, while another neutral primer (5% of EDTA-2Na) failed, suggesting that chemical bonding of 10-MDP to enamel hydroxyapatite crystallites could yield detectable bond strengths on smooth enamel surfaces. [ABSTRACT FROM PUBLISHER]

The availability of the coupling agent bis (3-triethoxysilylpropyl)-tetrasulfide (TESPT) has provided an opportunity for enhancing the reinforcing capabilities of precipitated amorphous white silica in rubber. Styrene-butadiene rubber, synthetic polyisoprene rubber (1R), acryloni-trile-butadiene rubber, and natural rubber (NR) containing the same loading of a precipitated silica filler were prepared. The silica surface was pretreated with TESPT, which is a sulfur-bearing bifunctional organosilane to chemically bond silica to the rubber. The rubber com-pounds were subsequently cured by reacting the tetrasulfane groups of TESPT with double bonds in the rubber chains and the cure was optimized by adding sulfenamide accelerator and zinc oxide. The 1R and NR needed more accelerators for curing. Surprisingly, there was no obvious correlation between the internal double bond content and the accelerator require-ment for the optimum cure of the rubbers. Using the TESPT pretreated silanized silica was a very efficient method for cross-linking and reinforcing the rubbers. It reduced the use of the chemical curatives significantly while maintaining excellent mechanical properties of the cured rubbers. Moreover, it improved health and safety at work-place, reduced cost, and min-imized damage to the environment because less chemical curatives were used. Therefore, TESPT was classified as "green silane" for use in rubber formulations. [ABSTRACT FROM AUTHOR]

Analogies are parts of human thought. From them, we can acquire new knowledge or change that which already exists in our cognitive structure. In this sense, understanding the analogical reasoning process becomes an essential condition to understand how we learn. Despite the importance of such an understanding, there is no general agreement in cognitive science literature about this issue. In this study, we investigated students' analogical reasoning as a creative process where an environment was set up to foster the students' generating and explaining their own analogies. Data were gathered from pre- and post-teaching interviews, in which the 13–14-year-old students were asked to make comparisons that could explain how atoms are bound. Such data supported the discussion about how students reasoned analogically. Our results made it evident that the task aims and the students' salient knowledge exerted a great influence on the drawing of analogies. [ABSTRACT FROM PUBLISHER]

In complex perovskite-type oxides, which have been studied as cathode materials, the thermal expansion coefficient (TEC) increases with the increase in the oxygen ionic conductivity. The aim of the present study is to explain such a behavior from a chemical bond point of view. For the perovskite oxides expressed as A1-xA'xB1-yB'yO3-δ and ABO3, the ionicity of A-O and B-O bonds and the thermal expansion coefficient were evaluated theoretically by using semiempirical methods. It is found that the thermal expansion coefficient and the oxygen ionic conductivity decrease with the increase in the difference of the ionicity between A-O and B-O bonds. [ABSTRACT FROM AUTHOR]

Chemical bonding of neutral rhenium trichloride Re3Cl9 and doubly charged [image omitted] has been brought into question in the current work. Despite recently reported propositions of π-aromaticity of neutral Re3Cl9 based on negative nuclear independent chemical shift (NICS) values[1,2] to the contrary, it is shown to be a purely classical chemical molecule, according to the results of the recently developed Adaptive Natural Density Partitioning (AdNDP) analysis.[3-53, 4, 5] The formation of three consecutive double bonds in a triatomic cycle formed by rhenium atoms that was initially predicted by Cotton et al.[6-86, 7, 8] is confirmed. The chemical bonding picture changes dramatically upon addition of two excess electrons to Re3Cl9. The rhenium atoms of the resulting doubly charged cluster are shown to be held together via three single Re-Re σ-bonds, three Re-Cl-Re σ-bonds, and a totally delocalized π-bond based on valence d atomic orbitals (d-AO) of rhenium atoms with minor contributions coming from the density associated with six apical chlorine atoms, which makes [image omitted] π-aromatic. [ABSTRACT FROM AUTHOR]

A study of pressure effects on the vibrational frequencies of ice X is performed to shed light on the existence and stability of this phase. The analysis reveals: (i) its stability range, (ii) the soft phonon nature of the transition at high pressures to the Pbcm structure proposed by Benoit et al. (M. Benoit, M. Bernasconi, P. Focher, and M. Parrinello, New high-pressure phase of ice, Phys. Rev. Lett. 76 (1996), pp. 2934-2936.) and (iii) the phonon collapse associated with the dynamical disordered structure at low pressures. Additionally, a topological analysis of the electron localization function and the electron density through the atoms-in-molecules formalism clarifies the chemical nature of ice X. [ABSTRACT FROM AUTHOR]

Atomic and electronic structures of interfaces in an internally oxidized Pd/ZnO system were studied by high-resolution transmission electron microscopy (HRTEM) and electron energy-loss spectroscopy (EELS). {0001}ZnO polar planes tend to be parallel to the interface. A ZnO precipitate was terminated by an oxygen [image omitted] surface at one end of the precipitate and by a zinc (0001) surface at the other end of the precipitate. A pre-edge peak was detected only in O-K energy-loss near-edge spectra (ELNES) acquired from the oxygen-terminated interface. Ab initio calculations indicated that the origin of the pre-edge peak could be attributed to the strong chemical bonding and hybridization of Pd-d and O-p orbitals at the oxygen-terminated polar interface. [ABSTRACT FROM AUTHOR]

Phase change materials (PCMs) of cellulose diacetate (CDA) modified with polyethylene glycol (PEG) prepared by chemical bonding and physical blending methods exhibit different characteristics: Chemically bonded materials exhibit typical solid-solid phase change characteristics, but the blended materials exhibit solid-liquid phase change characteristics. In all these materials, CDA is the framework, and PEG is a working substance that gives and takes in latent heat during its melting and crystallizing process. For the chemically bonded materials, the bonding strength is strong enough to ensure that no PEG is detached, so even at a high temperature (i.e., above the melting temperature of PEG), the material still remains in the solid state. In contrast, for the blends, at high temperature the amorphous PEG can migrate freely within the CDA framework and may escape to the surface of the blends through a diffusion mechanism. *This work was subsidized by the Special Funds for Major State Basic Research Projects (95-12 and G1999064800). [ABSTRACT FROM AUTHOR]