Greek philosophers (300 BC) proposed matter was made of 4 elements: earth, air, fire, water.

Democritus coined the word “atom” meaning “cannot be broken.”Atom seen as a solid sphere

John Dalton (1766-1844)1. All matter is made of atoms

2. Atoms are indestructible and can’t be divided

3. All atoms of one element are exactly alike, but different from atoms of other elements.

4. Atoms of diff elements combine in simple, whole-number ratios to make compounds

5. In reactions, atoms are combined, separated, or rearranged.

1800s- scientists still thought atom was a tiny solid ball.

THEN…JJ Thomson (1897) discovers the electron (e-)

Vacuum tube (no air inside) w/ electrode on each end, attached to a terminal.

He send electricity through the tube and saw A bright ray travelling from the

negative end (cathode) to the positive end (anode). “cathode rays”

Cathode ray bends toward a + end of a magnet.

“ “ bends away from a - end of a magnet.

Cathode ray tubes pass electricity through a gas that is contained at a very low pressure.

Television Computer Monitor

Conclusion: 1. The cathode ray was actually ____

charged particles.2. The atom could not be ________ as

scientists had thought, but must contain charged particles.

1909-scientists now believe the atom is like chocolate chip cookie dough

In 1910 Thomson discovered that neon atoms have different masses.

Conclusion: there must be another particle that has no charge, called a neutron.

(In 1932, James Chadwick confirms existence of the neutron)

• 1911-Rutherford’s Gold Foil Experiment

• Shot “alpha Particles” (helium nuclei) at gold foil.

• Hypothesis: they would pass through unaffected.

Expected results

Actual results

a) The nucleus is smallb) The nucleus is densec) The nucleus is positively

chargedd) Most of the atom is empty

space

Most of the particles passed right through A few particles were deflected VERY FEW were greatly deflected

“Like howitzer shells bouncing off of tissue paper!”

Conclusions:

In 1920, Bohr proposes that electrons around the nucleus exist at only distinct distances from the nucleus

This is known as the Bohr model

ParticleParticle ChargeCharge Mass (g)Mass (g) LocationLocation

ElectronElectron

(e(e--)) -1-1 9.11 x 109.11 x 10-28-28 Electron Electron cloudcloud

ProtonProton (p(p++)) +1+1 1.67 x 101.67 x 10-24-24 NucleusNucleus

NeutronNeutron

(n(noo)) 00 1.67 x 101.67 x 10-24-24 NucleusNucleus

The + charge on a proton is equal to the - charge on an electron.

Atoms are neutral (have no overall charge)

Therefore, the # of protons = # electrons in an atom.

Slide 2.2

determines the identity of the atom.

tells us the # of protons in the atom.

also tells us the # of electrons (b/c an atom is neutral in charge.)

Ex: atomic number of carbon, C = 6

Question: how many protons? How many electrons? How many neutrons?...

Atomic number (Z) of an element is the number of protons in the nucleus of each atom of that element.

ElementElement # of protons# of protons Atomic # (Z)Atomic # (Z)

CarbonCarbon 66 66

PhosphorusPhosphorus 1515 1515

GoldGold 7979 7979

Mass number is the number of protons and neutrons in the nucleus of an isotope: Mass # = p+ + n0

NuclideNuclide pp++ nn00 ee-- Mass #Mass #

Oxygen Oxygen - - 1010

- - 3333 4242

- - 3131 1515

8 8 1818

Arsenic 75 33 75

Phosphorus 15 3116

(Def) Atoms of the same element w/different #s of neutrons.

The number of neutrons can vary from atom to atom in an element.

In order to know how many neutrons in an atom you must be told.

The mass number tells you how much mass the atom has. Since p+ and n0 are the heavy parts, mass # = # of p+’s + n0’s.

B5

11

Atomic #

Mass # Symbol for element

The atom in the prior slide can be called

“boron-11”Name of element Mass #

How many protons? How many electrons? How many neutrons?

LET’S PRACTICE! Whiteboard Marker Paper towel

Practice Problem #1Practice Problem #1 If an element has an atomic If an element has an atomic

number of 34 and a mass number of 34 and a mass number of 78, what is the: number of 78, what is the:

a)a) number of protonsnumber of protons

b)b) number of neutronsnumber of neutrons

c)c) number of electronsnumber of electrons

d)d) complete symbolcomplete symbol

Practice Problem #2Practice Problem #2 If an element has 78 If an element has 78

electrons and 117 neutrons electrons and 117 neutrons what is the what is the

a)a) Atomic numberAtomic number

b)b) Mass numberMass number

c)c) number of protonsnumber of protons

d)d) complete symbolcomplete symbol

Atoms are weighed in a.m.u. 1 a.m.u. is based on the mass of a

Carbon-12 atom. it has 6 p+ and 6 n0, 1 a.m.u = 1/12 the mass of a carbon-12 atom.

(definition) Weighted average of all the isotopes of an element. See p 68 of text.

calculating atomic mass

Located below element symbol on periodic table.

Isotopes are atoms of the same element having different masses, due to varying numbers of neutrons.

IsotopeIsotope ProtonsProtons ElectronsElectrons NeutronsNeutrons NucleusNucleus

Hydrogen–1Hydrogen–1

(protium)(protium) 11 11 00

Hydrogen-2Hydrogen-2

(deuterium)(deuterium) 11 11 11

Hydrogen-3Hydrogen-3

(tritium)(tritium)

11 11 22

Elements occur in nature as mixtures of isotopes.

Isotopes are atoms of the same element that differ in the number of neutrons.

Multiply the atomic mass of each isotope by it’s abundance (expressed as a decimal), then add the results.

If not told otherwise, the mass of the isotope is expressed in atomic mass units (amu)

IsotopeIsotope SymbolSymbol Composition of Composition of the nucleusthe nucleus

% in nature% in nature

Carbon-12Carbon-12 1212CC 6 protons6 protons

6 neutrons6 neutrons

98.89%98.89%

Carbon-13Carbon-13 1313CC 6 protons6 protons

7 neutrons7 neutrons

1.11%1.11%

Carbon-14Carbon-14 1414CC 6 protons6 protons

8 neutrons8 neutrons

<0.01%<0.01%

Atomic mass is the average of all the naturally occurring isotopes of that element.

Carbon = 12.011

Copper has the following isotopes ISOTOPE MASS # % ABUNDANCE Copper-63 63 69.15 Copper-65 65 30.85

Calculate the atomic mass (average mass)

Finding Atomic Mass on Periodic Table

Ch 5 – Chem IHCh2.2 & 7 – Chem I

• Energy can travel in waves. • There are high energy and low energy

waves.• The ones we can see are called “the

visible spectrum.” ROY G BIV• Red is the low energy end: violet is the

high energy end.

1. Wavelength: distance between crests of a wave. Ex: radio waves = 102 m

2. Frequency: number of wave cycles to pass a point per unit time.

Energy of Electrons Why electrons don’t crash into the

nucleus: they have enough energy to keep them away.

Why e-s (usually) don’t fly off of atoms: they have enough attraction to the nucleus to keep them in “orbit.”

(Kind of like planets in orbit around the sun.)

(Don’t write this!)

DISCUSS WITH YOUR NEIGHBOR: You are an electron. If you have a lot of

energy, will you stay close to the nucleus or will you move further from it?

Answer: you may still stay in “orbit” but you will be able to move further away from the nucleus.

Neils Bohr studied w/Rutherford

His model is also called the planetary model

He discovered that e-s could only exist at certain distances from the nucleus. (Energy Levels)

"The opposite of a correct statement is a false statement. But the opposite of a profound truth may well be another profound truth." Neils Bohr

See p 75 of text: electron energy levels are like rungs of a ladder.

Ladder To climb to a higher level, you can’t put your

foot at any level, you must place it on a rung

Electron energy levels e-s must move to higher or lower e.l.’s in

specific intervals

Interactive Bohr Model

Atoms are arranged in energy levels (e.l.’s), at different distances from nucleus

Close to nucleus = low energy Far from nucleus = high energy e-s in highest occupied level are “valence

e-s” Only so many e-’s can fit in energy levels e-s fill lower e.l.’s before being located in

higher e.l.’s*(* There are exceptions we will learn later!)

Only so many e-’s can fit in energy levels

Energy Level # of electrons1st 22nd 8

3rd 18* 4th 32*

VALENCE ELECTRONS

DETERMINE HOW ELEMENTS BEHAVE!!!

Let’s practice drawing some atoms/ions

In your teams, pick up enough of the following for your team:

1 white board per person 1 marker per person1 paper towel per team (Please save a tree

& share!)

Electrons aren’t in perfect orbits.

Energy levels are regions of space in which an e- is likely to be found most of the time.

The area in which they move is like a cloud, an area of space surrounding the nucleus.

Show # of protons and neutrons in the nucleus

Draw e.l.’s and show each electron in the proper e.l.

Ex: Bohr Model of BORON-11

Hydrogen-2 (Practice together) Helium-4 Lithium-6 Beryllium-8 Carbon-12 Magnesium-24

Have 2 parts1. Chemical symbol of element2. Valence e-s, represented by dots

Are placed in one of four locations Above Below Right left

Are not paired unless there is 1 e- in each location.

Ex: Oxygen

TEACHER DEMONSTRATION Hydrogen Helium Lithium

STUDENT PRACTICE Beryllium Boron Carbon

Bohr Models Lewis dot diagrams