מודל דחיית זוגות אלקטרונים של קליפת הערכיות In the valence-shell electron-pair repulsion (VSEPR) theory, the electron groups around a central atom are arranged as far apart from each other as possible. have the least amount of electron-electron repulsion. are used to predict the molecular shape.

Shapes of Molecules צורות של מולקולות The three-dimensional shape of a molecule can be predicted by the number of bonding groups and lone-pair electrons around the central atom VSEPR theory (valence-shell-electron-pair repulsion).

Four Electron Groups ארבעה זוגות אלקטרוני קישור In a molecule of CH4, there are four electron groups around C. repulsion is minimized by placing four electron groups at angles of 109, which is a tetrahedral arrangement. the four bonded atoms form a tetrahedral shape.

Four Electron Groups with a Lone Pair שלושה זוגות אלקטרוני קישור וזוג אחד של אלקטרונים לא קושרים Four Electron Groups with a Lone Pair In a molecule of NH3, three electron groups bond to H atoms, and the fourth one is a lone (nonbonding) pair. repulsion is minimized with 4 electron groups in a tetrahedral arrangement. the three bonded atoms form a pyramidal (~109) shape.

Four Electron Groups with Two Lone Pairs שני זוגות אלקטרוני קישור ושני זוגות של אלקטרונים לא קושרים Four Electron Groups with Two Lone Pairs In a molecule of H2O, two electron groups are bonded to H atoms and two are lone pairs (4 electron groups). four electron groups minimize repulsion in a tetrahedral arrangement. the shape with two bonded atoms is bent (~109).

Molecular Shapes for 4 Bonded Atoms צורות מולקולריות עבור אטומים עם 4 קשרים Molecular Shapes for 4 Bonded Atoms טטרא-הדרית פירמידה זויתית

Two Electron Groups שני זוגות של אלקטרונים In a molecule of BeCl2, there are two electron groups bonded to the central atom, Be (Be is an exception to the octet rule). to minimize repulsion, the arrangement of two electron groups is 180, or opposite each other. the shape of the molecule is linear.

Three Electron Groups שלושה זוגות אלקטרונים In a molecule of BF3, three electron groups are bonded to the central atom B (B is an exception to the octet rule). repulsion is minimized with 3 electron groups at angles of 120. the shape is trigonal planar.

Three Electron Groups with a Lone Pair "שני זוגות" אלקטרוני קישור וזוג אחד של אלקטרונים לא קושרים Three Electron Groups with a Lone Pair In a molecule of SO2, S has 3 electron groups; 2 electron groups bonded to O atoms and one lone pair. repulsion is minimized with the electron groups at angles of 120, a trigonal planar arrangement. the shape is determined by the two O atoms bonded to S, giving SO2 a bent (~120) shape.

Molecular Shapes for 2 and 3 Bonded Atoms צורות מולקולריות של אטומים עם 2 ו 3 קשרים Molecular Shapes for 2 and 3 Bonded Atoms קוית משולש מישורי זויתית

כללים לחיזוי הגאומטריה של המולקולה

Predicting the Molecular Shape of H2Se Step 1 Draw the electron-dot formula. In the electron-dot formula for H2Se, there are four electron groups, including two lone pairs of electrons around Se.

המשך Step 2 Arrange the electron groups around the central atom to minimize repulsion. The four electron groups around Se would have a tetrahedral arrangement. Step 3 Use the atoms bonded to the central atom to determine the molecular shape. Two bonded atoms give H2Se a bent shape with a bond angle of ~109.

Learning Check תרגיל What is the shape of a molecule of NF3? A. linear B. trigonal pyramidal C. bent (120)

Solution פתרון What is the shape of a molecule of NF3? B. trigonal pyramidal Step 1 Draw the electron-dot formula. In the electron-dot structure for NF3, there are three electron groups and a lone pair.

Solution המשך פתרון What is the shape of a molecule of NF3? B. trigonal pyramidal Step 2 Arrange the electron groups around the central atom to minimize repulsion. The four electron groups around N would have a tetrahedral arrangement. Step 3 Use the atoms bonded to the central atom to determine the molecular shape. Three bonding groups and a lone pair give NF3 a trigonal pyramidal shape with a bond angle of 109.

מולקולות פולריות ולא פולריות מהי מולקולה פולרית? A polar molecule contains polar bonds. has a separation of positive and negative charge. called a dipole, indicated with + and –. has dipoles that do not cancel. + – H–Cl NH3 dipole Dipoles do not cancel.

מולקולות לא פולריות מהי מולקולה לא פולרית? A nonpolar molecule contains nonpolar bonds Cl–Cl H–H or has a symmetrical arrangement of polar bonds.

כללים לקביעת פולריות

Molecular Polarity, H2O האם מולקולת המים פולרית? Determine the polarity of the H2O molecule. Step 1 Determine if the bonds are polar covalent or nonpolar covalent. From the electronegativity table, O 3.5 and H 2.1 gives a difference of 1.4, which makes the O — H bonds, polar covalent.

המשך Determine the polarity of the H2O molecule. Step 2 If the bonds are polar covalent, draw the electron-dot formula and determine if the dipoles cancel or not. The four electron groups of oxygen are bonded to two H atoms. Thus, the H2O molecule has a net dipole, which makes it a polar molecule.

תרגיל Learning Check Determine the shape of each of the following molecules and whether they are polar or nonpolar. Explain. 1. PBr3 2. HBr 3. Br2 4. SiBr4

פתרון Solution Determine the shape of each of the following molecules and whether they are polar or nonpolar. Explain. 1. PBr3 pyramidal; polar; dipoles don’t cancel 2. HBr linear; polar; one polar bond (dipole) 3. Br2 linear; nonpolar; nonpolar bond 4. SiBr4 tetrahedral; nonpolar; dipoles cancel