4.2 THE STRUCTURE OF AN ATOM
Dec 14, 2015
Atomic Structure
•Atoms are composed of 2 regions:• Nucleus: center of atom that contains mass of atom
• Electron cloud: region that surrounds nucleus that contains most of space in atom
NucleusElectron
Cloud
What’s in the Nucleus?
•Nucleus contains 2 of 3 subatomic particles:• Protons: subatomic particle w/ 1+ charge (p+)• Rutherford - 1911
• Neutrons: subatomic particle w/ no charge (no)• James Chadwick - 1932
What’s in the Electron Cloud?
• The 3rd subatomic particle resides outside nucleus in electron cloud• Electron: subatomic particle w/ 1- charge (e-) and virtually no mass
• JJ Thomson - 1897
How do these particles interact?• Protons and neutrons live compacted in tiny nucleus
• most atom’s mass
• electrons small and reside outside nucleus • small mass (2000 e- = 1 p+ or no) • occupy large volume of space outside nucleus
Atoms
How do the subatomic particles balance each other?• In atoms:
• protons = electrons• If 20 protons are present in atom then 20 electrons balance overall charge of atom—atoms are neutral
• The neutrons have no charge; therefore they do not need to (and often times don’t) equal protons or electrons
How do we know the number of subatomic particles in an atom?
• Atomic #: indicates # of protons in atom• Ex: Hydrogen’s atomic # is 1
• hydrogen has 1 proton• Ex: Carbon’s atomic # is 6
• carbon has 6 protons
**Number of protons identifies elementsimilar to how your fingerprint ID’s
you. Ex. 2 protons = He, 29 protons = Cu
ALWAYS!!ALWAYS!!
How do we know the number of subatomic particles in an atom?• Mass number: number of protons and neutrons in nucleus (p+ + no)• Ex: hydrogen can have a mass # of 3. Since it has 1 proton it must have 2 neutrons
• # of neutrons = mass # - atomic #
What are Isotopes?• Atoms of same element with different # of neutrons• Same atomic #• Different mass # (b/c neutrons are different)
• Ex. Carbon 12, Carbon 13, and Carbon 14 all naturally occurring isotopes of Carbon.
• Each has 6 p+ and 6 e-, but each has different # of neutrons (therefore, different mass#)
Determining the number of protons and neutrons
• Li has mass # of 7 and atomic # of 3• Protons = 3 (same as atomic #)• Neutrons= 7-3 = 4 (mass # - atomic #)
• Ne has a mass # of 20 and an atomic # of 10• Protons = 10• Neutrons = 20 - 10= 10
What about the electrons?• electrons are equal to protons
• So e- = p+ = atomic #• Ex: He has mass # of 4 and atomic # of 2
• p+ = 2• no = 2• e- = 2
Basic Atomic Structure 1:57
Determine the number of subatomic particles in the following:• Chlorine has a mass # of 35 and an atomic # of 17• p+ = 17, no = 18, e- = 17
• Potassium has a mass # of 39 and an atomic # of 19• P+ = 19, no = 20 e- = 19
Candy Atoms• Atom #1 - mass # of 5 and an atomic # of 3.
• Atom #2 – 5 protons and 7 neutrons.
• Atom #3 – Atomic # of 7 and 8 neutrons.
Candy Atoms• Atom #4 – mass # 18 and 9 electrons
• Atom #5 – build your own candy atom using the candies that you have. You should be able to accurately determine:• Atomic #• Mass #• # of protons, neutrons, and electrons
Bohr Model of the Atom• Agreed with Rutherford
• Small nucleus w/ lots of space
• Devised planetary model• trying to show why e- were not sucked into p+ in nucleus of atom.
• e- in specific energy levels
Misconceptions from the Bohr Model• Bohr model good for diagramming atoms and energy levels
• e- do NOT move like planets in predictable orbits
• Mathematics determine probable location of e-
Energy Levels• Possible energies e- can have• Like floors in hotel
• Floor nearest nucleus - ground floor
The Electron Hotel
• Levels nearer nucleus have lower energy (ground floor of hotel)
• Electrons fill energy levels from inside - outside. (ground floor - top floor of Electron Hotel)
EN
ER
GY
Energy Level
3
2
1
1s
2s
2p
3s
3p3d
Electron distribution in an AtomElectron distribution in an Atom
NUCLEUSNUCLEUS
Energy Levels
• Can’t stand “in between” steps in hotel stairwell
• e-’s can’t exist “in between” energy levels• Must absorb energy to move up energy levels
• Must lose to move down
Evidence of Energy Levels
• Energy gains & loses can be measured
• As e- drop orbitals, energy released in form of light/heat• Like in fireworks (2:34)
Electron Cloud Model• Electrons travel around nucleus in random orbits.
• cannot predict location at any given moment.
• fast, appear to form “cloud” around nucleus.• Ex. - Airplane propeller
Atomic Orbitals• Rooms in “Electron Hotel”
• Region of space where e- likely located• Each orbital can have 2 e- max
• Denser region = higher probability
Energy Levels, Orbitals, and Electrons
Energy Level
(floors in hotel)
Number of Orbitals
(hotel rooms)
Maximum number of electrons
(occupants)
1 1 2
2 4 8
3 9 18
4 16 32
Electron Configuration
• Arrangement of e-’s (occupants) in orbitals (rooms)
• Each orbitals holds 2 e-’s max (1 double bed)• Stable when e-’s in orbitals w/ least energy
• Ground state• i.e. Lithium (atomic # = 3) has 1st 2 e-’s in the 1st energy level (fills up 1 room w/ double bed)
• 3rd e- goes to 2nd energy level
Electron Configuration• If Lithium absorbs enough energy, 3rd e- jumps energy levels• Excited state• Less stable (like gymnast on beam)
• Eventually releases energy (often as light)
• Returns to ground state
How exactly are the particles arranged?
• Bohr Model of the atom:Electrons move in orbits at fixed distances from the nucleus (planetary
model)
All of the protons and the neutrons
The 1st ring can hold up to 2 e-
The 2nd ring can hold up to 8 e-
The 3rd ring can hold up to 18 e-
The 4th ring and any after can hold up to 32 e-
What does carbon look like?Mass # = 12 atomic # = 6
p+ = 6 no = 6 e- = 6
6 p and 6 n live in the nucleus
Drawing Atoms• Draw the following atoms in your notes: • 1. Beryllium has an atomic # of 4 and a mass # of 9