Atomic Structure

Atomic Structure Timeline

Democritus (400 B.C.)

• Proposed that matter was composed of tiny indivisible particles

• Not based on experimental data

• Greek: atomos

Alchemy (next 2000 years)

• Mixture of science and mysticism. • Lab procedures were developed, but alchemists did not perform controlled experiments like true scientists.

John Dalton (1807)

• British Schoolteacher–based his theory on

others’ experimental data

• Solid Sphere Model–atom is a

uniform, solid sphere

John Dalton

Dalton’s Four Postulates1. Elements are composed of small

indivisible particles called atoms.

2. Atoms of the same element are identical. Atoms of different elements are different.

3. Compounds contain atoms of more than one element

4. In a compound, atoms of different elements always combine the same way.

Henri Becquerel (1896)

• Discovered radioactivity–spontaneous emission of

radiation from the nucleus• Three types:

–alpha () - positive–beta () - negative–gamma () - neutral

J. J. Thomson (1903)• Cathode Ray Tube

Experiments–beam of negative particles

• Discovered Electrons–negative particles within

the atom

• Plum-pudding Model

Thomson’s ExperimentThe Cathode Ray Tube

screen with hole Anode +

High Voltage

Cathode -

Gas at very low pressure(almost vacuum)

Direction of Cathode Rays

to vacuum pump

screen with hole Anode +

High Voltage

Cathode -

Gas at very low pressure(almost vacuum)

to vacuum pump

-

+

Negatively charged plate

Positively charged plate

Direction of cathode ray

screen with hole Anode +

High Voltage

Cathode -

to vacuum pump

Shadow formed

screen with hole Anode +

High Voltage

Cathode -

to vacuum pump

Thomson Model• Thomson studied the passage of an electric current through a gas.

• As the current passed through the gas, it gave off rays of negatively charged particles.

J. J. Thomson (1903)

Plum-pudding Model–positive sphere

(pudding) with negative electrons (plums) dispersed throughout

Eugen Goldstein (1886)

• Discovered proton (component of canal rays)– Positive

particles within the atom

screen with hole Anode +

High Voltage

Cathode -

Cathode rays (Electrons)

Direction of Cathode Rays

to vacuum pump

Direction of canal rays

Canal rays (positive particles

Robert Milikan (1909)

• Determined the electric charge of an electron

• The charge on a single electron: 1.602 × 10−19 coulomb

Milikan’s Oil Drop Experiment

Ernest Rutherford (1911)

• Gold Foil Experiment

• Discovered the nucleus–dense, positive

charge in the center of the atom

• Nuclear Model

Rutherford’s ExperimentGold Foil

Source ofalpha particles

Fluorescent screen

Gold foil

Atoms of gold

Alpha particles

NUCLEUS

Ernest Rutherford (1911)

• Nuclear Model–dense, positive nucleus surrounded by negative

electrons

James Chadwick (1932)

Discovered the neutron– neutral particles

within the atom

Subatomic Particles

POSIT IVECHARG E

PROTONS

NEUTRALCHARG E

NEUT RONS

NUCLEUS

NEG ATIVE CHARGE

ELECT RONS

ATOM

NUCLEUS ELECTRONS

PROTONS NEUTRONS NEGATIVE CHARGE

POSITIVE CHARGE

NEUTRAL CHARGE

ATOM

Atomic Structure

C126Mass

Number

Atomic Number

XAZMass

Number

Atomic Number

Element

Atomic Number (Z)

• Number of protons in an atom• Unique for each element

Z = p+

Mass Number (A)

• Number of nucleons in an atom– Nucleon is the numerical sum of the

protons and neutrons

A = p+ + n0

Subatomic Particles

POSIT IVECHARGE

PROT ONS

NEUT RALCHARG E

NEUTRONS

NUCLEUS

NEG ATIVE CHARG E

ELECTRONS

AT OM

Most of the atom’s mass. Atomic Numberequals the # of...

in a neutral atom

NUCLEUS ELECTRONS

PROTONS NEUTRONS NEGATIVE CHARGE

POSITIVE CHARGE

NEUTRAL CHARGE

ATOM

How do we compute for the number of protons, electrons and neutrons

in a neutral atom?

• Protonsp+ = Z

• Electronse- = p+

• Neutronsn0 = A – Z or n0 = A – p+

ExampleDetermine A, Z, p+, e-, n0 and net charge of

the neutral atom carbon.

C126

A = 12Z = 6p+ = 6e- = 6n0 = 6 Net Charge = 0

Ions

• Charged particle• Exist when an atom transfers or

gains one or more electrons• May be positive (cation) or negative

(anion)

Example

4126C

A = 12Z = 6p+ = 6e- = 6 – 4 = 2n0 = 6 Net Charge = +4

Determine A, Z, p+, e-, n0 and net charge of the charged atom carbon.

Isotopes

• Atoms that have the same number of protons but different numbers of neutrons.Ex. 1

1H – hydrogen2

1H – deuterium3

1H – tritium

Atomic Mass

• The mass of an atom in atomic mass unit (amu).

• Atomic mass unit is defined as the mass exactly equal to 1/12 the mass of one carbon-12 atom

Example

The element copper has naturally occurring isotopes with mass numbers 63 and 65. The percent abundance and atomic masses are 69.2% for atomic mass=62.93 amu, and 30.8% for atomic mass=64.93. Calculate the relative atomic mass of copper.

Example

Calculate the relative/average atomic mass of bromine. The two isotopes of bromine have atomic masses and percent abundance of 78.92 amu (50.69%) and 80.92 amu (43.91%).

ExampleCompute for X.

IsotopeAtomic mass (amu)

% Abundance

Relative Atomic Mass (amu)

3517Cl 34.969 75.53

35.4537

17Cl X 24.47

ExampleCompute for X.

IsotopeAtomic mass (amu)

% Abundance

Relative Atomic Mass (amu)

32He X 0.0001

4.002642He 4.0026 99.9999

Modern Atomic Theory

Niels Bohr (1913)• Bright-Line Spectrum

–tried to explain presence of specific colors in hydrogen’s spectrum

• Energy Levels–electrons can only exist in

specific energy states

• Planetary Model

Niels Bohr (1913)

• Planetary Model

–electrons move in circular orbits within specific energy levels

Bright-line spectrum

Erwin Schrödinger (1926)• Quantum mechanics

–electrons can only exist in specified energy states

• Electron cloud model –orbital: region around the

nucleus where e- are likely to be found

Erwin Schrödinger (1926)

Electron Cloud Model (orbital)• dots represent probability of finding an e-

not actual electrons

Wave Model

James Chadwick (1932)• Discovered neutrons

–neutral particles in the nucleus of an atom

• Irene and Frederic Joliot-Curie –Chadwick based his

theory on their experimental evidence

James Chadwick (1932)

Neutron Model• revision of Rutherford’s Nuclear Model

Atomic Orbitals

• Orbital–Region of space

around the nucleus where an electron is likely to be found

–More energy = more orbitals

The Wave Model

• Today’s atomic model is based on the principles of wave mechanics.

• According to the theory of wave mechanics, electrons do not move about an atom in a definite path, like the planets around the sun.

The Wave Model

• In fact, it is impossible to determine the exact location of an electron. The probable location of an electron is based on how much energy the electron has.

• According to the modern atomic model, at atom has a small positively charged nucleus surrounded by a large region in which there are enough electrons to make an atom neutral.

Electron Cloud:

• A space in which electrons are likely to be found.

• Electrons whirl about the nucleus billions of times in one second

• They are not moving around in random patterns.

• Location of electrons depends upon how much energy the electron has.

Electron Cloud:

• Depending on their energy they are locked into a certain area in the cloud.

• Electrons with the lowest energy are found in the energy level closest to the nucleus

• Electrons with the highest energy are found in the outermost energy levels, farther from the nucleus.

Indivisible Electron Nucleus Orbit Electron Cloud

Greek X

Dalton X

Thomson X

Rutherford X X

Bohr X X X

Wave X X X

RADIOACTIVITYDefinition: spontaneous emission of

particles and.or radiationTYPES:• Alpha (α) Ray – consists of

positively charged particles • - has a low penetrating

ability (can be stopped by paper)

• Beta (β) rays – consists of electrons.

• has a medium penetrating ability (can be stopped by

heavy clothing)

• Gamma (γ) Rays – no charge and not affected by an external field.

• high penetrating ability (can be

stopped by lead)

Fundamental Subatomic ParticlesParticle Symbol Relative Mass Mass

Electrical (g) (amu)Charge

ELECTRON e- -1 9.11 x 10-28 0.00060

PROTON p+ +1 1.67 x 10-24 1.00731

NEUTRON n0 0 1.67 x 10-24 1.00871

Electron Configuration• Arrangement of electrons within the orbitals

of the atom–The most stable configuration is the one in

which electrons are in their lowest possible orbitals. This is called their Ground State.

• If energy is added, electrons can move to a higher energy orbital. The atom is then considered to be in an excited state.

Example• Lithium

–When lithium reacts with water, the atom goes to an excited state.

–When the electron returns to its ground state, it gives of energy in the form of fire.