Is CH3F a dipole

Elements and molecules, school book

31 2.4 THE ATOMIC BINDING MODEL The polarized atomic bond Why does water boil at 100 ° C and alcohol at 78 ° C? Why does ice melt at 0 ° C and sulfur only at 119 ° C? What does the boiling point and melting point depend on? a) From the forces between the particles: The stronger the attraction of the particles to one another, the higher are Kp and Fp. b) From the mass of the particles: The “heavier” a particle, the higher the Kp (boiling point) and Fp (melting point). In the case of non-polar particles - with low attractive forces - the mass is particularly decisive. The dependence of the boiling point on the particle mass can be seen from the boiling points of the noble gases. (Fig. 31.2) However, if you look at the boiling points of the hydrogen halides, you can see that hydrogen fluoride, the “lightest” molecule in this series, has the highest boiling point. Strong forces must therefore be active between hydrogen fluoride molecules. If different atoms are linked to one another, the electrons are unevenly distributed between the atoms involved due to the differences in the electrolessivity of the binding partners. The binding electron pair is more strongly attracted by the more electronegative atom. In doing so, electrons are not completely drawn into the area of ​​influence of the “stronger” one - this only happens when there is a very large difference in electronegativity, such as between metal and non-metal atoms, for example - but only more or less unevenly distributed. The atoms involved in the bond get partial charges because on average there is more or less negative electrical charge available than corresponds to the nuclear charge. The more electro-negative partner achieves a negative partial charge (symbol δ -), the other partner a positive partial charge (symbol δ +). Such polarizations occur in all molecules with different atoms. Really unpolarized bonds only exist when similar atoms are connected. However, the smaller the electronegativity difference, the weaker the polarization. Compare the substances H 2 O, H 2 S, H 2 Se as well as HF, HCl, HBr in Fig. 31.2! Whether the partial charges arising from the polarized atomic bond have an effect on the properties of the molecule and thus on the properties of the corresponding substance also depends on the spatial structure of the molecule. If the focal points of the negative and positive partial charges coincide, they cancel each other out in their effect. Such molecules are called non-polar. (Fig. 31.3) If the centers of gravity do not coincide, the molecule has a negative and a positive area. Such molecules are called dipole molecules and the corresponding substances are called polar substances. (Fig. 31.4) (Recognition aid for dipole molecules: In molecules made up of 2 types of atoms, the central atom carries one or more non-binding electron pairs.) Strong forces act between strongly polar substances; this also explains the high boiling point of hydrogen fluoride. However, there must also be interactions between non-polar substances that also bring about the solid state of these particles. ■ 31.1: Determine whether the following substances are polar or non-polar: CCl 4, CH 3 F, I 2, SO 2, HF, CS 2! EXERCISES Butane Water Small attraction Large attraction 100 H 2 OH 2 SH 2 Se H 2 Te CH 4 SiH 4 GeH 4 SnH 4 Ne Ar Kr Xe HF HCl HBr HI Temp. In ° C 2 3 4 5 Period 50 0 –50 –100 –150 –200 –250 CO O δ - δ - (2) δ + CH H δ + δ + H δ + H δ + (4) δ - H Cl δ + δ - OHH δ + δ + (2) δ - NH H δ + δ + (3) δ - H δ + Fig. 31.1: Forces of attraction between molecules Fig. 31.2: Boiling points of different substances Fig. 31.3: Charge distribution in different non-polar molecules Fig. 31.4: Charge distribution in different polar molecules Go ahead Testing purposes - owned by the publisher öbv

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