Electronic Structure of Matter PDF

Electronic Structure of Matter Bohr’s Atomic Model Quantum Mechanical Model Definition of Atoms Atoms are the building blocks of matter that make up everything that we encounter every day. An atom is a particle of matter that uniquely defines a chemical element. An atom con...

Electronic Structure of Matter Bohr’s Atomic Model Quantum Mechanical Model Definition of Atoms Atoms are the building blocks of matter that make up everything that we encounter every day. An atom is a particle of matter that uniquely defines a chemical element. An atom consists of a central nucleus that is surrounded by one or more negatively charged electrons. The nucleus is positively charged and contains one or more relatively heavy particles known as protons and neutrons. Early Theory The word atom actually comes from ancient Greek and roughly translates as indivisible. The ancient Greek theory has been credited to several different scholars, but is most often attributed to Democritus (460-370 BC) and his mentor Leucippus. They outlined the idea that everything is made of atoms, invisible and divisible spheres of matter of infinite type and number. John Dalton In 1803, English chemist John Dalton started to develop a more scientific definition of the atom. He drew on the ideas of the ancient Greeks in describing atoms as small, hard spheres that are indivisible and that atoms of a given element are identical to each other. He also came up with theories about how atoms combine to make compounds and also came up with the first set of chemical symbols for the known elements. JJ Thomson In the late 1800s, an English physicist Joseph John Thomson discovered that the atom wasn’t as indivisible as previously claim. He had discovered the electron (though he referred to it as a corpuscle) and shown that atoms were not indivisible, but has smaller constituent parts. In 1904, he put forward his model of the atom based on his findings. JJ Thomson Dubbed ‘The Plum Pudding Model’, it envisaged the atom as a sphere of positive charge, with electrons dotted throughout like plums in a pudding. Ernest Rutherford According to Rutherford, the positively charge particles of the atom is not spread out throughout the atom, but concentrated in a small, dense empty space. With his discovery of the nucleus, he proposed a model where the electrons orbit the positively charged nucleus. Niels Bohr His model postulated the existence of energy level or shells of electrons. Electrons could only be found in these specific energy levels, their energy was quantized. Electrons could move between these energy levels. Arnold Johannes Wilhelm Sommerfeld Sommerfeld, a famous German physicist expanded and enhance Bohr’s model of the atom. He assume that electrons moved in elliptical rather than circular orbits, introducing the concepts of elliptical orbits, allowance for an orbiting motion of the electrons and the consideration of relativistic mass effects. Erwin Schrodinger In 1926 Schrodinger proposed that, rather than the electrons moving in fixed orbits or shells, the electrons behave as waves. His model shows the nucleus surrounded by clouds of electron density. These regions of space are referred to as electron orbitals. James Chadwick In 1932, James Chadwick discovered the existence of the neutron, completing the picture of the subatomic particles that make up the atom. Physicists have since discovered that the protons and neutrons that make up the nucleus are themselves divisible into particles called quarks - Quantum Mechanical Model Louise de Broglie and Erwin Schrodinger proposed that light and atoms might exhibit the same properties since electrons may also have wave characteristics at times. This is known as the quantum mechanical model of the atom. Werner Heisenberg formulated his uncertainty principle which states, “ It is impossible to determine both the momentum and the position of an electron at the same time accurately.” Quantum Mechanical Model The location of an electron depends on the amount of energy it has. An energy level represents a volume occupied by an electron cloud. The lowest energy level or energy state that an electron normally occupies is called ground state. When a substance is strongly heated, it then becomes excited and will go to the highest energy state farthest from the nucleus. This time, electron is said to be in an excited state. The highest occupied energy level are located is called valence shell, and the electrons occupying this shell are called valence electrons. Quantum Mechanical Model Valence electrons are important in determining how the atom reacts chemically with other atoms. Atoms with complete valence electrons tend to be stable. This is known as the closed shell, which is obtained when the shell is completely filled with the assigned electrons. Quantum Number A total of four quantum numbers are used to describe completely the movement and trajectories of each electron within an atom. Quantum numbers are important because they can be used to determine the electron configuration of an atom and the probable location of the atom’s electrons. Quantum numbers are also used to understand other characteristics of atoms, such as ionization energy and the atomic radius. Quantum Number In atoms, there are a total of four quantum numbers: the principal quantum number (n), the orbital angular momentum quantum number (l), the magnetic quantum number (ml) and the electron spin quantum number (ms). Quantum Number The principal quantum number (n), describes the energy of an electron and the most probable distance of the electron from the nucleus. In other words, it refers to the size of the orbital and the energy level an electron is placed in. The number of subshells (l), describes the shape of the orbital. It can also be used to determine the number of angular nodes. Quantum Number The magnetic quantum number (ml), describes the energy levels in a subshell, and ms refers to the spin on the electron which can either be up and down. Principal Quantum Number (n) The principal quantum number (n), designates the principal electron shell. Because n describes the most probable distance of the electrons from the nucleus, the larger the number n is, the farther the electron is from the nucleus, the larger the size of the orbital and the larger the atom is. n can be any positive integer starting at 1. The principal shell is also called the ground state or lowest energy state. n = 1, 2, 3, 4 … Orbital Angular Quantum Number (l) The orbital angular momentum quantum number l determines the shape of an orbital, therefore the angular distribution. The number of angular nodes is equal to the value of the angular momentum quantum number l. Each value of l indicates a specific s, p, d, f subshell (each unique in shape). l = 0, 1, 2, 3, 4…, (n-1) s- sharp, p – principal, d- diffuse, f - fundamental Magnetic Quantum Number (ml) The magnetic quantum number ml, determines the number of orbitals and their orientation within a subshell. Consequently, its value depends on the orbital angular momentum quantum number l. Given a certain l, ml is an interval ranging from –l to +l, so it can be zero, a negative integer or a positive integer. ml = -l, (-l+1), (-l+2),…, -2, -1, 0, 1, 2… Electron Spin Quantum Number (ms) The electron spin quantum number m , s does not depend on another quantum number. It designates the direction of the electron spin and may have a spin of +1/2, represented by or -1/2 represented by. This means that when ms is positive the electron has an upward spin, when it is negative, the electron has a downward spin, so it is spin down. The significance of the electron spin quantum number is its determination of an atom’s ability to generate a magnetic field or not. m = + 1/2 s Atomic Orbitals Orbitals are usually illustrated as electron clouds. It is a region around the nucleus or an atom with the greatest probability of finding the electrons. It is the actual house of the electrons. Atomic orbitals, sometimes called as sublevel, have characteristics shape, size and energy. The different orbitals are labeled with different letters. Atomic Orbitals 1. S orbital (sharp)- It is a spherical cloud that becomes less dense as the distance from the nucleus increases. 2. P orbital (principal) – it is a dumbbell-shaped cloud, having two lobes on opposite sides of the nucleus. This orbital has three types based on their orientation Px (its orientation is along the x-axis), Py (its orientation is along the y-axis) and Pz (its orientation is along the z-axis). Atomic Orbitals 3. D orbital (diffused) – It is like a four leaf clover, as an hour and as a ring. Its shapes are more complicated than the orbitals in the s and p orbitals. 4. F orbital (fundamental) – The f orbitals are difficult to represent and too complex to visualize among them. Atomic Orbitals Atomic Orbitals Electron Configuration The probable distribution of electrons around the nucleus among the orbitals is called the electron configuration. Electron configuration is the shorthand representation on how each electron is arranged among the orbitals, levels and sublevels. The electron configuration is represented by a number, a letter and a superscript. The number corresponds to the main energy level or shell, the letter can be any of the different sublevels while the superscript represents the number of electrons occupying an assigned orbital. Electron Configuration Three rules serve as guides in order for the electron’s location to be easily predicted: Pauli’s exclusion principle – states that no more than two electrons in an atom can occupy an orbital. They must spin in opposite directions. Electrons are said to be paired if two electrons with opposite spins occupy an orbital and unpaired if a single electron is present in the orbital Electron Configuration Three rules serve as guides in order for the electron’s location to be easily predicted: Hund’s rule– states that for a set of orbitals, when electrons occupy orbital of equal energy, one electron enters each orbital until all the orbital contains one electron with parallel spins. Then the second electron will be added to each orbital pairing the spins of he first electrons. Hund’s rule is also called the principle of minimum pairing and the principle of maximum multiplicity. Electron Configuration Three rules serve as guides in order for the electron’s location to be easily predicted: The spin of the unpaired electrons and the orbital motion contribute to paramagnetism- it is the atom that contains unpaired electrons that is drawn to a magnetic field. On the other hand, the atom is diamagnetic if all electrons in an atom are paired. Aufbau principle– a German word that means “building up or construction”, states that electrons fill first the orbitals of the lowest energy until any added electrons occupy the available orbital of higher energy. It is the building up of an atom by progressively adding of electrons. Electron Configuration Rules for the Quantum Numbers The quantum numbers are integers except for ms. The lowest value that the principal quantum number, n, may have is 1. The highest value that the azimuthal quantum number, l, may depend on n. It can have a value from 0 to n-1. The magnetic quantum number, ml, can be any integer from -l to +l The spin quantum number, ms, can only be +1/2 or -1/2. No other values are acceptable. Seatwork (1 whole) Illustrate the electron configuration of the following elements and indicate the quantum numbers of the designated electron. Mn 25 - 11th electron Si 14 - 4th electron Na 11 - 2nd electron