ch. 4 modern chemistry the arrangement of electrons in atoms

CH. 4 MODERN CHEMISTRY The Arrangement of Electrons in Atoms

A New Atomic Model Cathode Ray Tube Summary As of 1911, Rutherfords Gold Foil Experiment had shown scientists the nucleus of an atom was very small, dense, and positively charged. Why was Rutherfords model of the atom incomplete? Rutherfords model did not explain how electrons were distributed around the nucleus. Just assumed electrons located around nucleus

Wave-like Properties of Light Over the next decade, scientists began to discover the dual nature of electrons - particles and waves. Electromagnetic radiation is a form of energy that exhibits wavelike behavior as it travels through space. Together, all the forms of electromagnetic radiation form the electromagnetic spectrum. Visible spectrum: visible portion of spectrum But there are many other forms of energy

Electromagnetic Spectrum Electromagnetic radiation is the emission and transmission of energy in the form of electromagnetic waves.

Properties of Light Wavelength () : distance between corresponding points on adjacent waves Frequency (): number of waves that pass a given point per unit time time (usually 1 sec)

Properties of Light Frequency and wavelength are mathematically related to each other: c = c : speed of light (m/s) : wavelength (m) : frequency of wave (s 1 or 1/s).

Photoelectric Effect Particle Description of Light Electromagnetic radiation strikes the surface of the metal Electrons are ejected from the metal causing an electric current

Photoelectric Effect The photoelectric effect refers to the emission of electrons from a metal when light shines on the metal. Not all forms of electromagnetic radiation were powerful enough to eject electrons from the surface A minimum amount of energy was needed A quantum of energy - the minimum quantity of energy that can be lost or gained by an atom

Photoelectric Effect German physicist Max Planck proposed the following relationship: E = h E : quantum of energy (J joules) : frequency of the radiation emitted; s 1 h : fundamental constant called Plancks constant; h = 6.626 10 34 J s

Particle Description of Light photon : particle of electromagnetic radiation having zero mass and carrying a quantum of energy. The energy of a particular photon depends on the frequency of the radiation. E photon = h

The Hydrogen Line Emission Spectrum

When investigators passed electric current through a vacuum tube containing hydrogen gas at low pressure, they observed the emission of a characteristic pinkish glow. When a narrow beam of the emitted light was shined through a prism, it was separated into four specific colors of the visible spectrum. The four bands of light were part of what is known as hydrogens line-emission spectrum.

The Hydrogen Line Emission Spectrum What had scientists expected to observe? A continuous range of frequency (all bands of ER)

The Hydrogen Line Emission Spectrum The lowest energy state of an atom is its ground state. A state in which an atom has a higher potential energy than it has in its ground state is an excited state.

Bohr Model of the Atom By 1922, Niels Bohr proposed a hydrogen-atom model that linked the atoms electron to photon emission. electrons can circle the nucleus only in allowed paths, called orbits. The energy of the electron is higher when it is in an orbit farther from the nucleus.

Bohr Model of the Atom When electrons move from an excited state to a ground state or to a lower energy state, a photon of energy is emitted from an atom Process called emission (high to low orbit) absorption (low to high orbit): energy is added to an atom to move an electron from lower energy level to higher energy level 3) The energy of a photon can be directly calculated if you know the frequency of radiation being emitted

Photo Absorption & Emission

QUANTUM MODEL OF THE ATOM Section 4.2

Electrons as Waves French scientist Louis de Broglie suggested that electrons be considered waves confined to the space around an atomic nucleus. electron waves could exist only at specific frequencies. It followed that if each electron has a specific frequency, it also has a corresponding energy. Recall equation E = hv

Heisenberg Uncertainty Principle German physicist Werner Heisenberg proposed that any attempt to locate a specific electron with a photon knocks the electron off its course. The Heisenberg uncertainty principle: it is impossible to determine simultaneously both the position and velocity of an electron or any other particle.

The Schrdinger Wave Equation In 1926, Austrian physicist Erwin Schrdinger developed an equation that treated electrons as waves in an atom. Heisenberg & Schrdinger ideas laid the foundation for modern quantum theory. Quantum theory describes mathematically the wave properties of electrons and other very small particles.

The Schrdinger Wave Equation Electrons do not travel around the nucleus in neat orbits, as Bohr had postulated, but rather in certain regions called orbitals. An orbital: three-dimensional region around the nucleus - indicates the probable location of an electron. Quantum numbers specify the properties of atomic orbitals and the properties (location & spin) of electrons in orbitals. 4 quantum numbers

Schrodinger Equation Quantum Theory mathematically explains wave properties of electrons and similarly sized particles. The solutions described shapes in space that electrons were highly likely to be moving around in.

Atomic Orbitals and Quantum Numbers An electrons quantum numbers describe: primary energy level ( n ) How close to the nucleus it is shape of momentum ( l ) direction of momentum ( m l ) spin direction ( m s )

Quantum Numbers Quantum numbers describe the behavior of an atoms electrons

Quantum Numbers n represents the main energy level an electron occupies The bigger n gets, the further away from the nucleus the electron gets If more than 1 electron has the same value for n they are in the same shell n can only be in integer values; n1

Angular Momentum Quantum Number l l represents the sublevel (or shape) of the electron cloud Each number value corresponds to a shape Value for lOrbital lettershape 0sSphere 1pDumbbell 2dButterfly 3fComplex

Magnetic Quantum Number m m represents the orientation of the orbital about the nucleus The values for m can be -l, 0, +l The amount of m values correspond to the number of orientations of that shape Value for m # orbitals of that shape Sublevel lettershape 01sSphere -1, 0, 13pDumbbell -2, -1, 0, 1, 25dButterfly -3, -2, -1, 0, 1, 2, 37fComplex

Magnetic Quantum Number m.

Spin Quantum Number Like charges repel therefore, electrons repel each other Electrons want to always move in opposite directions if they have to share an orbital Spin quantum number can be + 1 / 2 or - 1 / 2 m s = +m s = -

Quantum Numbers

APPLICATION OF QUANTUM NUMBERS ELECTRON CONFIGURATION ORBITAL NOTATION Section 4.3

Electron Configuration Shows the arrangement of electrons in an atom Each element has a unique electron configuration An element will have the orbitals of the elements preceding it plus any additional orbitals to account for its extra electrons Ex: He has a level 1 s orbital, Li has both a level 1 s orbital and a level 2 s orbital

Electron-Configuration Notation Electron-Configuration Notations uses the principle quantum number, the orbital letter, and the number of electrons in superscript How to write e-configurations? Read the PT as you would read a book, from left to right. Start at H and travel to the desired element writing down principal quantum number, sublevel and number of electrons. H = 1s 1 He = 1s 2 B = 1s 2 2s 2 2p 1

Orbital Diagram AKA Orbital notation another application of quantum numbers Orbital notation is a visual notation using arrows to represent electrons and lines to represent orbitals Write the electron configuration for the element Draw horizontal line to represent every orbital in each sublevel for every energy level Arrange orbitals from lowest to highest energy from left to right Group like orbitals together ex: put all p orbitals of one level closer together

Orbital Notation H ___ 1s He ___ 1s B ___ ___ ___ ___ ___ 1s 2s 2p

Electron Configuration & Orbital Diagram Three RULES! 1. Aufbau principle: an electron occupies the lowest-energy orbital available If we have 5 electrons, how can we fit them so the lowest energy orbitals fill first? 1s 2s 2px 2py 2pz Energy

Electron Configuration 2. Pauli Exclusion Principle: no two electrons of one atom can have the same set of 4 quantum numbers If we have 4 electrons, but they only fill 2 energy levels, how can we arrange them so they are different? 1s 2s 2px 2py 2pz Energy

Electron Configuration 3. Hunds Rule: orbitals of equal energy all fill with one electron before a second electron may be added. If we have 7 electrons, how can we fill the orbitals by energy level? 1s 2s 2px 2py 2pz Energy

Nobel Gas Notation Nobel Gas Notation is a short cut notation We know a Nobel gas will have all the orbitals up to that Nobel gas entirely filled Therefore, we can write a Nobel Gas plus any new orbitals corresponding to the new element Ca = 1s 2 2s 2 2p 6 3s 1 3p 6 4s 2 Ar precedes Ca; Ar = 1s 2 2s 2 2p 6 3s 1 3p 6 Therefore: Ca = [Ar]4s 2

Valence Electrons & Inner Electrons Every element has a set of valence electrons. For the s and p block, there are 8 valence electrons Electrons in the valence shell are typically in the highest occupied energy level For Argon (1s2 2s2 2p6 3s2 3p6) this is 3 For Berylium (1s2 2s2) this is 2 All non-valence electrons are inner-shell electrons If we know an electron configuration of a neutral atom, we can figure out which element it is

Exceptions There are several exceptions to the rules we learned. We will study two of these exceptions; Chromium, Cr and Copper, Cu The d-block elements are most stable when there are 10 electrons present (2 per orbital) The second most stable arrangement is with 5 electrons (one per orbital) To achieve this, an electron can be borrowed from the s- block.

Exceptions Write the noble gas configuration for copper following the rules we have learned so far. [Ar]4s 2 3d 9 Now, to make more stable, borrow an electron from the s-block and write a new configuration [Ar]4s 1 3d 10 Now write the noble gas configuration for chromium. [Ar]4s 2 3d 4 Apply the new exception [Ar]4s 1 3d 5

How should we look at the atom? Models of the Atom An Atomic Rant Models of the Atom

ch. 4 modern chemistry the arrangement of electrons in atoms

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