ELECTRONS IN THE ATOM UNIT 4

OBJECTIVES Explain how atomic emission spectra can be used to identify elements Describe Bohr’s model of the atom. Describe the Quantum Mechanical model of the atom Write elements’ electron configurations.

HOW DO WE KNOW WHAT THE STARS ARE MADE OF?

ATOMIC EMISSION SPECTRA When an element is heated, its atoms absorb energy and become excited To become stable again, these excited and unstable atoms then release the energy as light If this light is passed through a prism the element’s atomic emission spectrum is produced

ATOMIC EMISSION SPECTRA An element’s atomic emission spectrum is the set of wavelengths (colors) of light given off when atoms of that element are excited (e.g. heated) Each element’s emission spectrum is unique and can be used to identify the element It is the element’s “fingerprint”

HOW DO WE KNOW WHAT THE STARS ARE MADE OF? Scientist analyze the light from a star using spectroscopes (similar to powerful prisms) Match the frequencies of light to the known spectra of the elements Stars are made of the same stuff as the rest of the Universe: 73% hydrogen, 25% helium, and the last 2% is all the other elements

LIGHT Visible light is a type of electromagnetic radiation All other electromagnetic radiation is invisible Electromagnetic (EM) radiation is energy that travels through space in the form of electromagnetic waves The electromagnetic spectrum encompasses all forms of electromagnetic radiations

increasing energy

BOHR’S MODEL OF THE ATOM Bohr studied the emission spectrum of hydrogen and developed his model of the atom The Bohr model describes the atom as a small, positively charged nucleus surrounded by electrons that travel in circular orbits around it

THE BOHR MODEL OF THE ATOM Each orbit or “ring” has a distinct energy levels or quantum number (n) the bigger the number the higher the energy Electrons in smaller orbits closer to the nucleus have less energy than electrons found in larger orbits farther from the nucleus

BOHR’S ATOM CONTINUED The lowest energy state of an atom is its ground state When an atom gains energy (through heating for example) it is in an excited state in an excited state the electron absorbs the energy & jumps to higher energy level when it falls back down to its ground state it releases excess energy in the form of light

BOHR MODEL CONTINUED Because electrons jump between orbitals that have specific energy levels only certain colors can be given off This is how Bohr explained hydrogen’s emission spectrum Transition color of light emitted n = 3 to n = 2 red n = 4 to n = 2 blue-green n = 5 to n = 2 blue n = 6 to n = 2 violet

Wait! Bohr’s model explained the emission spectrum of Hydrogen, but it did not explain the emissions of any other element!

THE QUANTUM MECHANICAL MODEL OF THE ATOM Electrons behave like waves It is impossible to know the exact location or the velocity of an electron in an atom (they don’t travel in circular orbits around the nucleus) Although it’s impossible to describe the exact location or describe how they are moving, the model describe the probability that electrons will be found in certain locations around the nucleus

ATOMIC ORBITALS An atomic orbital is a three-dimensional pocket of space around the nucleus that the electron is most likely to be found An electron has a 90% chance of being found within that space That is the best we can do!

ATOMIC ORBITALS

ORGANIZATION OF ATOMIC ORBITALS 1. Principal energy level (n) 2. Energy Sublevel 3. Orbitals value: n = 1-7 s, p. d, f 1, 3, 5, 7 description: -(n) indicates relative size and energy of orbital -As (n) increases so do energy and size -sublevels are labeled according to shape: s: spherical p: dumbbell d/f: varied -each sublevel has a certain number of orbitals: s = 1 p =3 d =5 f = 7 -each orbital can hold two electrons

ELECTRON CONFIGURATION An atoms electron configuration is the way an atom’s electrons are distributed among the orbitals of an atom The most state stable electron configuration is an atom’s ground state Ground state: all electrons are in the lowest possible energy state Electron configuration represented by writing symbol for the orbital and a superscript to indicate the number of electrons in the orbital Li: 1s2 2s1

increasing energy

Each orbital can hold two electrons 4p Energy 3d 4s 3p 3s H Hydrogen 1 1.008 He Helium 2 4.003 Li Lithium 3 6.941 C Carbon 6 12.01 B Boron 5 10.81 Be Beryllium 4 9.012 O Oxygen 8 16.00 Ne Neon 10 20.18 N Nitrogen 7 14.01 F Fluorine 9 19.00 2p       2s   1s  

The Pauli Exclusion Principle The two electrons in an orbital must spin in opposite directions   1s  2s  2p       4s 3d 3s 3p

HUND’S RULE Negatively charged electrons repel each other, so: Electrons won’t pair up unless they have to Once there is one electron in every orbital…the pairing will begin! 2s 1s  2p  1. Add an electron: 2s 1s  2p  2. Add an electron: 2s 1s  2p  3. Add an electron: 2s 1s  2p  4.

DRAW THE ORBITAL DIAGRAM AND WRITE THE ELECTRON CONFIGURATION FOR: Carbon Helium Potassium

ELECTRON CONFIGURATION The periodic table can be divided into four distinct blocks based on valence electron configuration electron configuration explain the recurrence of physical and chemical properties

SHORTHAND (NOBLE GAS) NOTATION Shows electron filling starting from previous noble gas: Na: 1s22s22p63s1 Noble gas configuration: [Ne]3s1

WRITE THE FOLLOWING ELECTRON CONFIGURATIONS IN NOBLE GAS NOTATION: Fluorine Titanium Beryllium