4.2 THE STRUCTURE OF AN

ATOM

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

4.3 Modern Atomic Theory

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

Beryllium Atom

Drawing Atoms• 2. Sodium has an atomic # of 11 and a mass # of 23

Sodium Atom