ATOMIC STRUCTUREATOMIC STRUCTURE

Atomic StructureAtomic Structure All matter is composed of atoms.

Understanding the structure of atoms is critical to understanding the properties of matter

HISTORY OF THE ATOMHISTORY OF THE ATOM

1808 John Dalton

suggested that all matter was made

up of tiny spheres that were able

to bounce around with perfect

elasticity and called themATOMSATOMS

8 X2Y16 X 8 Y+

DALTONS ATOMIC THEORY

Subatomic Particles

Particle Mass (g)

Charge (Coulombs)

Charge (units)

Electron (e-) 9.1 x 10-28 -1.6 x 10-19 -1

Proton (p) 1.67 x 10-24 +1.6 x 10-19 +1

Neutron (n) 1.67 x 10-24 0 0

mass p = mass n = 1840 x mass e-

HISTORY OF THE ATOMHISTORY OF THE ATOM

1898 Joseph John Thompson

found that atoms could sometimes

eject a far smaller negative

particle which he called an

ELECTRONELECTRON

J.J. Thomson, measured mass/charge of e-

(1906 Nobel Prize in Physics)

A = alphaB = gammaC = beta

gold foil

helium nuclei

CHARGE OF AN ELECTRON

Millikan oil drop experiment

HISTORY OF THE ATOMHISTORY OF THE ATOM

1910 Ernest Rutherford

oversaw Geiger and Marsden carrying

out his famous experiment.

they fired Helium nuclei at a piece

of gold foil which was only a few

atoms thick.

they found that although most of

them passed through. About 1 in

10,000 hit

Rutherford’s experiment.

Plum Pudding model of an atom.

Results of foil experiment if Plum

Pudding model had been correct.

Actual Results.

A nuclear atom viewed in cross section.

atomic radius ~ 100 pm = 1 x 10-10 m

nuclear radius ~ 5 x 10-3 pm = 5 x 10-15 m

Rutherford’s Model of the Atom

Atomic Structure

Atoms are composed of-protons – positively charged particles-neutrons – neutral particles-electrons – negatively charged particles

Protons and neutrons are located in the nucleus. Electrons are found in orbitals surrounding the nucleus.

HELIUM ATOM

+N

N

+-

-

proton

electron

neutron

Shell

Atomic StructureAtomic StructureEvery different atom has a characteristic number of protons in the nucleus.

atomic number = number of protons

Atoms with the same atomic number have the same chemical properties and belong to the same element.

Atomic StructureAtomic StructureEach proton and neutron has a mass of approximately 1 dalton.

The sum of protons and neutrons is the atom’s atomic mass.

Isotopes – atoms of the same element that have different atomic mass numbers due to different numbers of neutrons.

ATOMIC STRUCTUREATOMIC STRUCTURE

the number of protons in an atom

the number of protons and neutrons in an atom

HeHe

22

44Atomic mass

Atomic number

number of electrons = number of protons

ATOMIC NUMBER (Z) = number of protons in nucleus

MASS NUMBER (A) = number of protons + number of neutrons

= atomic number (Z) + number of neutrons

ISOTOPS are atoms of the same element (X) with different numbers of neutrons in the nucleus

XAZ

H11 H (D)21 H (T)3

1

U23592 U238

92

Mass Number Atomic Number Element Symbol

Atomic Structure

Atomic StructureAtomic Structure

Two isotopes of sodium.

HISTORY OF THE ATOMHISTORY OF THE ATOM

1913 Niels Bohr

studied under Rutherford at the

Victoria University in

Manchester. Bohr refined Rutherford's idea

by adding that the electrons

were in orbits. Rather like

planets orbiting the sun. With

each orbit only able to contain

a set number of electrons.

MULTIELECTRON ATOMSMULTIELECTRON ATOMS

ELECTRONS IN ORBIT ABOUT ELECTRONS IN ORBIT ABOUT

THE NUCLEUSTHE NUCLEUS

1. e- can have only specific (quantized) energy values

2. light is emitted as e- moves from one energy level to a lower energy level

Bohr’s Model of the Atom (1913)

En = -RH ( )1n2

n (principal quantum number) = 1,2,3,…RH (Rydberg constant) = 2.18 x 10-18J

The Bohr Model of the Atom

Atomic StructureAtomic Structure

The Bohr Model of the Atom:

Ground and Excited States• In the Bohr model of hydrogen, the lowest

amount of energy hydrogen’s one electron can have corresponds to being in the n = 1 orbit. We call this its ground state.

• When the atom gains energy, the electron leaps to a higher energy orbit. We call this an excited state.

• The atom is less stable in an excited state and so it will release the extra energy to return to the ground state.– Either all at once or in several steps.

Line Emission Spectrum of Hydrogen Atoms

Every element has a unique emission spectrum

The Bohr Model of the Atom:

Hydrogen Spectrum• Every hydrogen atom has identical orbits, so every hydrogen atom can undergo the same energy transitions.

• However, since the distances between the orbits in an atom are not all the same, no two leaps in an atom will have the same energy.– The closer the orbits are in energy, the lower the energy of the photon emitted.

– Lower energy photon = longer wavelength.• Therefore, we get an emission spectrum that has a lot of lines that are unique to hydrogen.

The Bohr Model of the Atom:

Hydrogen Spectrum

Ephoton = E = Ef - Ei

Ef = -RH ( )1n2f

Ei = -RH ( )1n2i

i fE = RH( )

1n2

1n2

RH is the Rydberg constant

n is the principal quantum number

En = -RH ( )1n2

Bohr showed the energy a H atom can have is equal to:

Line spectrum ofsome elements

LIGHT EMISSION OF SODIUM ATOM

Line spectrum

Atomic StructureAtomic StructureNeutral atoms have the same number of protons and electrons.

Ions are charged atoms.-cations – have more protons than electrons and are positively charged-anions – have more electrons than protons and are negatively charged

An ion is formed when an atom, or group of atoms, has a net positive or negative charge (why?).If a neutral atom looses one or more electrons

it becomes a cation.

If a neutral atom gains one or more electronsit becomes an anion.

Na 11 protons11 electrons Na+ 11 protons

10 electrons

Cl 17 protons17 electrons Cl-

17 protons18 electrons