Modern Atomic Theory

Modern Atomic Theory Why do metals glow when heated? Models of the Atom • A model should explain not just what the material is made of (composition) but also how it is going to behave (changes). • Rutheford’s atomic model could not explain the chemical properties of elements. • Basically, it couldn’t explain why things change color when heated. Atomic Model Timeline Atomic Model Timeline The Bohr Model • Niels Bohr (1885-1962) was a Danish physicist and a student of Rutherford’s. • In 1913, Bohr introduced his atomic model based on the simplest atom, hydrogen (only 1 electron) – Bohr proposed that an electron is found only in specific circular paths, or orbits, around the nucleus. Bohr Model The Bohr Model • Each electron has a fixed energy = an energy level. – Electrons can jump from one energy level to another. – Electrons can not be or exist between energy levels. • A quantum of energy is the amount of energy needed to move an electron from one energy level to another energy level. The The degree degree to to which which they they move move from from level level to to level level determines determines the the frequency frequency of of light light they they give give off. off. Bohr Model • To move from one level to another, the electron must gain or lose the right amount of energy. • The higher the energy level, the farther it is from the nucleus. – Gain energy to move to higher energy levels (away from nucleus) – Lose energy to move to lower energy levels (closer to nucleus) The Bohr Model • The amount of energy required to go from one energy level to another is the not same for the electrons. • Higher energy levels are closer together. This means it takes less energy to change levels in the higher energy levels. • The Bohr model was tested with the hydrogen element but failed to explain the energies absorbed and emitted by atoms with more than one electron. Did you know that an element can be identified by its emission spectra? – When atoms absorb energy, electrons move into higher energy levels. These electrons then lose energy by emitting light when they return to lower energy levels. Mercury Nitrogen Fingerprints of certain atoms 5.3 Atomic Spectra – When atoms absorb energy, electrons move into higher energy levels. These electrons then lose energy by emitting light when they return to lower energy levels. 5.3 An Explanation of Atomic Spectra • In the Bohr model, the lone electron in the hydrogen atom can have only certain specific energies. –When the electron has its lowest possible energy, the atom is in its ground state. –Excitation of the electron by absorbing energy raises the atom from the ground state to an excited state. –A quantum of energy in the form of 5.3 An Explanation of Atomic Spectra – The light emitted by an electron moving from a higher to a lower energy level has a frequency directly proportional to the energy change of the electron. 5.3 An Explanation of Atomic Spectra • The three groups of lines in the hydrogen spectrum correspond to the transition of electrons from higher energy levels to lower energy levels. Bohr’s Model The Quantum Mechanical Model • Rutherford’s and Bohr’s model focused on describing the path of the electron around the nucleus like a particle (like a small baseball). • Austrian physicist Erwin Schrödinger (1887–1961) treated the electron as a wave. – The modern description of the electrons in atoms, the quantum mechanical model, comes from the mathematical solutions to the Schrödinger equation. Electrons as Waves EVIDENCE: DIFFRACTION PATTERNS VISIBLE LIGHT ELECTRONS 5.1 The Quantum Mechanical Model • The propeller blade has the same probability of being anywhere in the blurry region, but you cannot tell its location at any instant. The electron cloud of an atom can be compared to a spinning airplane propeller. – The quantum model determines the allowed energies an electron can have and how likely it is to find the electron in various locations around the nucleus. 5.1 The Quantum Mechanical Model • The probability of finding an electron within a certain volume of space surrounding the nucleus can be represented as a fuzzy cloud. – The cloud is more dense where the probability of finding the electron is high. 5.1 Atomic Orbitals • (fuzzy cloud) = An atomic orbital is often thought of as a region of space in which there is a high probability of finding an electron. Quantum Mechanics • Orbital (“electron cloud”) – Region in space where there is 90% probability of finding an e- Orbital Radial Distribution Curve Probability cloud Atomic orbital Atomic orbital 90% s-orbitals are spherically shaped. Smaller atom Smaller atom— Fewer electrons take up less space. Larger atom— More electrons take up more space. p-orbitals are “dumbell” shaped. z-axis p-orbitals are “dumbell” shaped. x-axis p-orbitals are “dumbell” shaped. y-axis p-orbitals together x, y, & z axes. Shells and Orbitals and Atomic Structure s p d f •Shells of an atom contain a number of stacked orbitals 4 3 2 1 1st and 2nd level s-orbitals and the p-orbitals all together. Why are Atoms Spherical? Tro, Chemistry: A Molecular Approach 5.1 Atomic Orbitals • Different atomic orbitals are denoted by letters. The s orbitals are spherical, and p orbitals are dumbbellshaped. • Four of the five d orbitals have the same shape but different orientations in space. 5.1 Atomic Orbitals • The numbers and kinds of atomic orbitals depend on the energy sublevel. Energy Level, n # of sublevels Letter of sublevels # of orbitals per sublevel # of electrons in each orbital Total electrons in energy level 5.1 Atomic Orbitals • The numbers and kinds of atomic orbitals depend on the energy sublevel. Energy Level, n # of sublevels Letter of sublevels # of orbitals per sublevel # of electrons in each orbital Total electrons in energy level 1 1 s 1 2 2 2 2 s p 1 3 2 6 8 3 s p d 1 3 5 2 6 10 18 4 s p d f 1 3 5 7 2 6 10 14 32 3 4 5.1 Atomic Orbitals • The number of electrons allowed in each of the first four energy levels are shown here. – A maximum of 2 electrons per orbital Use this to find the # of electrons in an energy level 2n2 5.2 Electron Configurations • The ways in which electrons are arranged in various orbitals around the nuclei of atoms are called electron configurations. – Three rules—the aufbau principle, the Pauli exclusion principle, and Hund’s rule—tell you how to find the electron configurations of atoms. 5.2 Electron Configurations • Aufbau Principle – According to the aufbau principle, electrons occupy the orbitals of lowest energy first. In the aufbau diagram below, each box represents an atomic orbital. • Pauli Exclusion Principle – According to the Pauli exclusion principle, an atomic orbital may describe at most two electrons. To occupy the same orbital, two electrons must have opposite spins; that is, the electron spins must be paired. • Hund’s Rule – Every orbital in a sublevel is singly occupied before any orbital is doubly occupied. – All of the electrons in singly occupied orbitals have the same spin (to maximize total spin). Filling Diagram for Sublevels Aufbau Principle Filling Diagram for Sublevels Electron Configurations • The electron configuration of an atom is a shorthand method of writing the location of electrons by sublevel. • The sublevel is written followed by a superscript with the number of electrons in the sublevel. – If the 2p sublevel contains 2 electrons, it is written 2p2 Writing Electron Configurations • First, determine how many electrons are in the atom. Iron has 26 electrons. • Arrange the energy sublevels according to increasing energy: – 1s 2s 2p 3s 3p 4s 3d … • Fill each sublevel with electrons until you have used all the electrons in the atom: – Fe: 1s2 2s2 2p6 3s2 3p6 4s2 3d 6 • The sum of the superscripts equals the atomic number of iron (26) Electron Configuration Practice • Write a ground state electron configuration for a neutral atom K Ne Electron Configuration Practice • Write a ground state electron configuration for these ions. O2Na+ Electron Configuration Practice • An excited atom has an electron or electrons which are not in the lowest energy state. Excited atoms are unstable energetically. The electrons eventually fall to a lower level. * is used to indicate an excited atom. For example: *Li 1s2 3p1. (The ground state for Li is 1s2 2s1.) • Write an excited electron configuration for the following atoms. • *Al • *K 5.2 Electron Configurations • Orbital Filling Diagram Electron Configurations and the Periodic Table • The periodic table can be used as a guide for electron configurations. • The period number is the value of n. • Groups 1A and 2A have the s-orbital filled. • Groups 3A - 8A have the p-orbital filled. • Groups 3B - 2B have the d-orbital filled. • The lanthanides and actinides have the f-orbital filled. Blocks and Sublevels • We can use the periodic table to predict which sublevel is being filled by a particular element. Noble Gas Core Electron Configurations • Recall, the electron configuration for Na is: Na: 1s2 2s2 2p6 3s1 • We can abbreviate the electron configuration by indicating the innermost electrons with the symbol of the preceding noble gas. • The preceding noble gas with an atomic number less than sodium is neon, Ne. We rewrite the electron configuration: Na: [Ne] 3s1 Electron Configurations • • • • • • Condensed Electron Configurations Neon completes the 2p subshell. Sodium marks the beginning of a new row. So, we write the condensed electron configuration for sodium as Na: [Ne] 3s1 [Ne] represents the electron configuration of neon. Core electrons: electrons in [Noble Gas]. Valence electrons: electrons outside of [Noble Gas]. 5.2 Exceptional Electron Configurations • Some actual electron configurations differ from those assigned using the aufbau principle because half-filled sublevels are not as stable as filled sublevels, but they are more stable than other configurations. 5.2 Exceptional Electron Configurations • Exceptions to the aufbau principle are due to subtle electronelectron interactions in orbitals with very similar energies. • Copper has an electron configuration that is an exception to the aufbau principle.