General Chemistry I Lecture Notes PDF

General Chemistry I Dr. Mahmoud A. A. Ibrahim Chemistry Department, Faculty of Science, Minia University Email: [email protected] 1 Mahmoud A. A. Ibrahim...

General Chemistry I Dr. Mahmoud A. A. Ibrahim Chemistry Department, Faculty of Science, Minia University Email: [email protected] 1 Mahmoud A. A. Ibrahim CH101  Lecture No. 3 Atomic orbitals & Atomic properties (I) 2 Mahmoud A. A. Ibrahim CH101 4. Atomic orbitals Shapes of atomic orbitals The electron orbitals presented here represent a volume of space within which an electron would have a certain probability of being based on particular energy states and atoms. For example, in a simple lowest- energy state hydrogen atom, the electrons are most likely to be found within a sphere around the nucleus of an atom. 3 Mahmoud A. A. Ibrahim CH101 4. Atomic orbitals Shapes of atomic orbitals The following figure shows the geometry of orbitals. s- orbitals are spherical, p-orbitals are shaped like a dumbbell. 4 Mahmoud A. A. Ibrahim CH101 4. Atomic orbitals Shapes of atomic orbitals For s orbital, we find that if we go out a given distance from the nucleus in any direction, the probability of finding the electron is the same. The size of s orbital increases with increasing n, i.e. 1s < 2s < 3s < 4s 5 Mahmoud A. A. Ibrahim CH101 4. Atomic orbitals Shapes of atomic orbitals In the case of s-orbitals, there are a number of radial nodes that separate the largest, outer, component from the inner components. The number of nodes is related to the principal quantum number, n. The ns orbital has (n-1) radial nodes, so the 3s-orbital has (3-1) = 2 nodes, as shown in the above plot. 6 Mahmoud A. A. Ibrahim CH101 4. Atomic orbitals Shapes of atomic orbitals The shape of p orbital is dumb bell in which the two lobes on opposite side separated by the nodal plane. A nodal plane is an imaginary flat surface on which every point has an electron density of zero. p-orbital has directional properties. The size of p orbital also increases with increasing n. A p subshell consists of three orbitals whose directions lie at 90 to each other along the axes of an imaginary coordinate system. 7 Mahmoud A. A. Ibrahim CH101 5. Atomic properties There are many chemical and physical properties that vary in a more or less systematic way according to an element’s position in the periodic table. These properties correlate with an atom’s electron configuration which also related to the location of an element in the periodic table. 8 Mahmoud A. A. Ibrahim CH101 5. Atomic properties Magnetic properties Because an electron spins, it creates a magnetic field, which can be oriented in one of two directions. For two electrons in the same orbital, the spins must be opposite to each other; the spins are said to be paired. These substances are not attracted to magnets and are said to be diamagnetic. Atoms with more electrons that spin in one direction than another contain unpaired electrons. These substances are weakly attracted to magnets and are said to be paramagnetic. 9 Mahmoud A. A. Ibrahim CH101 5. Atomic properties Effective nuclear charge Generally speaking, effective nuclear charge is the charge felt by the valence electrons after you have taken into account the number of shielding electrons that surround the nucleus. This positive charge is responsible for the attractions felt by the outer electrons toward the nucleus and , except for hydrogen, is always less than the full nuclear charge. This is because electrons in inner shells partially offset, or neutralize, the positive charge of the nucleus, thereby exposing the outer electrons to only a fraction of the full nuclear charge. 10 Mahmoud A. A. Ibrahim CH101 5. Atomic properties Effective nuclear charge Let's take a look at a fluorine atom. The nucleus itself has a +9 charge and anything in its vicinity will feel that charge. The two electrons in the first energy level as they look at the nucleus feel a +9 charge because that is the charge on the nucleus. But the electrons that are in the valence energy level would be shielded from the nucleus by the 2 shielding electrons. The +9 nuclear charge is shielded by 2 electrons to give an effective nuclear charge of +7 that is felt by the valence electrons. If you get out beyond the valence electrons, then the effective charge is 0 simply because the +9 charge of the nucleus is surrounded by 9 electrons. 11 Mahmoud A. A. Ibrahim CH101 5. Atomic properties Effective nuclear charge Let's take a look at a fluorine atom. The nucleus itself has a +9 charge and anything in its vicinity will feel that charge. The two electrons in the first energy level as they look at the nucleus feel a +9 charge because that is the charge on the nucleus. But the electrons that are in the valence energy level would be shielded from the nucleus by the 2 shielding electrons. The +9 nuclear charge is shielded by 2 electrons to give an effective nuclear charge of +7 that is felt by the valence electrons. If you get out beyond the valence electrons, then the effective charge is 0 simply because the +9 charge of the nucleus is surrounded by 9 electrons. 12 Mahmoud A. A. Ibrahim CH101 5. Atomic properties Effective nuclear charge 13 Mahmoud A. A. Ibrahim CH101 5. Atomic properties Effective nuclear charge Effective nuclear charge is determined primarily by the difference between the charge on the nucleus and the charge on the core. As you go across the table from left to right, the effective nuclear charge felt by the valence electrons increases. 14 Mahmoud A. A. Ibrahim CH101  End of Lecture 3 15 Mahmoud A. A. Ibrahim CH101