Quantum Physics

Dalton’s Atomic Theory• Dalton’s indivisible atom has not

been disregarded—it has been modified to explain new observations.

• Two important concepts,(1) All matter is composed of atoms and(2) Atoms of any one element differ in

properties from atoms of another element,

remain unchanged.

Electrons

• Discovered by Joseph John Thompson and Robert A. Millikan

• Discovered through experiments with cathode-ray tubes.

• Negative charge• Very small mass 9.109 x 10-31 kg• Plum pudding model-now had

subatomic particles

• Discovery of the electron led to 2 other inferences about atomic structure:– Because atoms are electrically neutral,

they must contain a positive charge to balance the negative electrons.

– Because electrons have so much less mass than atoms, atoms must contain other particles that account for most of their mass.

Atomic Nucleus• Ernest Rutherford, Hans Geiger, and

Ernest Marsden• Discovered through experiments with

alpha particles and gold foil.• Except for the nucleus of the hydrogen

atom, contains protons and neutrons.– Protons have a positive charge equal to the

negative charge of the electron.• Atoms are electrically neutral because they

contain equal #s of protons and electrons.

– Neutrons are electrically neutral.

http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther14.swf

Atoms• Defined as the smallest particle of

an element that retains the chemical properties of the element.

• Composed of:– Nucleus protons (+) & neutrons (no

charge)– Electrons (-)– Collectively referred to as subatomic

particles.

• Proton mass = 1.673 x 10-27 kg– 1,836 times greater than the mass of

the electron

• Neutron mass = 1.675 x 10-27 kg

Nuclear Forces

• Nuclear forces are short-range proton-neutron, proton-proton, and neutron-neutron forces hold the nuclear particles together.

Atomic Number

• The atomic number of an element is the number of protons in the nucleus of each atom of that element.

• Determines the identity of the atom.• Look at the periodic table, what is

the atomic number for hydrogen? Nitrogen? Sodium?

Atomic Mass

• Total number of protons and neutrons• Therefore, if you want to know

number of neutrons, you will have to subtract atomic # from the mass #.

• If you were not given a specific mass for your atoms, ONLY THEN should you use the average atomic mass on the periodic table.

Atomic #

Mass #

Symbol

Info on the Periodic Table

Isotopes• Isotopes are atoms of the same element

that have different masses.– The isotopes of a particular element all have

the same number of protons and electrons but different numbers of neutrons.

– Most of the elements consist of mixtures of isotopes.

– Although isotopes have different masses, they do not differ significantly in their chemical behavior.

– Nomenclature

• The number of protons defines the element.

• The number of neutrons varies.

H11 H21 H31 Protium Deuterium Tritium

Hydrogen-1 Hydrogen-2 Hydrogen-3

Isotopes of Hydrogen

• The total number of protons and neutrons in the nucleus of an atom is equal to the mass number.

He42

mass number

atomic number

Mass Number

• Or if it is shown asHelium-4

then it is showing element-mass #

Quantum numbers-Pauli’s exclusion principle

• N = energy level (row on periodic table)

• l = sublevel (s-0, p-1, d-2, f-3)

• ml = blank in sublevel

• s = spin (+1/2 for up -1/2 for down)

Photoelectric Effect

• Emission of electrons from a metal when light shines on the metal.

• This stream of electrons creates an electric current

• For a given metal, no electrons will be emitted IF the light’s frequency is below a certain minimum (no matter how long the light is shined on it)

Planck and Einsten

• Planck suggested that hot objects release EM energy (light) as small, specific amounts he called quanta

• Einstein said these particles of EM energy had no mass and carry a specific amount of energy (quantum)

• Different metals required different minimum frequency of energy

• E = h v• Energy = Planck’s x frequency of

constant radiation Joules 6.626 x 10-34 Hz Js

What does this have to do with atoms?

• When electricity is passed through a light bulb, the atoms are excited

• This means electrons jump to a higher energy level… but must release energy to return to their ground state

• If this light is passed through a prism, it separates into specific frequencies of light

He42

How many protons are in the nucleus of this helium atom?

How many neutrons?

Practice

2 protons and 2 neutrons

# of protons + # of neutrons = mass number

Practice Problems

• How many protons, electrons, and neutrons are there in an atom of chlorine-37?

17 protons

17 electrons

20 neutrons

More Practice

• How many protons, electrons, and neutrons are in each of the following atoms:– Bromine-90– Carbon-13

• What element contains 15 electrons and 15 neutrons?

IONS

• When an atom (or group of atoms) gains or loses an electron(s), the atom is no longer neutral– Cation-+ charged ion because the

atom LOST electron(s)– Anion-(-) charged because the atom

GAINED electron(s)– YES, that does seem backwards, but

remember electrons are negatively charged