C H A P T E R 4

ATOMIC STRUCTURE

STUDYING ATOMS4.1

ANCIENT GREEK MODELS OF ATOMS

• Democritus• Believed that all matter was made of small particles that

could not be divided• Called the particles atoms (atomos means uncut)

• Aristotle• Did not think there was a limit to number of times matter

could be cut

ARISTOTLE’S VIEW

JOHN DALTON

• Studying gases• Concluded gas was individual particles because of

the way they exerted pressure• Measured the masses of elements, discovered all

compounds have fixed compositions• Modeled atoms as solid spheres

DALTON’S THEORY

• All matter is made up of individual particles called atoms, which can’t be divided.• All elements made of atoms• Atoms of same element have same mass, atoms of

different elements have different masses• Compounds have more than one type of element• Atoms of different elements always combine the same

way in compound

JJ THOMSON

• Used electric current to study atoms• Thomson thought atoms had charged particles in

them• He set up a tube with positive plate and a

negative plate, the beam of light was either repelled or bent towards the plates• First evidence that atoms are made of even

smaller particles

THOMSON

THOMSON’S MODEL

• Had evenly scattered positively and negative charges in the atom• Called this the “plum pudding model”• Think chocolate chip ice cream

ERNEST RUTHERFORD

• His assistant aimed a beam of particles through a slit so that they would hit gold foil, he though that many would be so small they would go in a straight line and hit a screen that would light up• Many particles were deflected though

RUTHERFORD

RUTHERFORD’S MODEL

• Concluded that the positive charge of the atom is not evenly distributed• Concentrated in a very small, central area- called

the nucleus• Nucleus = a dense, positively charged mass at

center of atom• Negative charges (e-) traveled randomly outside

the nucleus

THE STRUCTURE OF THE ATOM

4.2

PARTS OF THE ATOM

• Proton• Neutron• Electron

SUB-ATOMIC PARTICLES

• Protons = Positively (1+) charged particle in the nucleus• Neutrons = neutral particle in nucleus• Electron = negatively (1-) charge particle in

nucleus

• Protons, electrons and neutrons can be distinguished by mass, charge and location in atom

ATOMIC NUMBER

• Atoms of an element always have the same number of p+• Different elements have different atomic numbers• Atomic Number = the # of protons in an atom • Can find on the periodic table• Equals the number of electrons (atoms have no

charge so + = -)

MASS NUMBER

• Mass Number = sum of protons and neutrons

• Number of Neutrons = Mass number – Atomic Number

ISOTOPES

• Atoms of the same element but have different numbers of and neutrons• Atomic mass is different because number of

neutrons is different

Property Ordinary Water Heavy Water

Melting Point 0.00 ⁰C 3.81 ⁰C

Boiling Point 100 ⁰C 101.42 ⁰C

Density .99701 g/cm3 1.1044 g/cm3

MODERN ATOMIC THEORY4.3

BOHR’S MODEL

• Atom had nucleus with p+ and N (agreed with Rutherford)• Electrons moved at constant speeds in fixed

orbits around the nucleus (like planets and sun)• If atoms gain or lose energy the energy of the

electrons changed• Electrons had energy levels • Electrons could move up electrons levels or down

electrons levels (like stairs)

BOHR’S MODEL

• Energy e- gain and lose can be measured• We can see some of the energy released by e- as

visible light• Fireworks:• Explosions cause e- to move to different energy levels,

when e- move back down energy levels they release light• No two elements have the same energy levels so we get

different colors of light

BOHR MODEL

ELECTRON CLOUD MODEL

• Atoms have energy levels, but do not move like planets• Electron cloud model = most likely location of e-

in an atom, denser the locations have higher probability of find an electron• Like a fan – when still you can count fan blades,

when it is on high they look like a blur, you know they are there but can’t not exactly sure where they are

ELECTRON CLOUD MODEL

ATOMIC ORBITALS

• Orbital = region of space around the nucleus where e- is likely to be• Example: Map of school, you place a dot every 10

minutes of where you would be, after a week you would have your orbital, a place where you would likely to be found • Electron cloud = good approximation of how e-

behave in orbitals• Lowest energy level has 1 orbital, higher energy

levels have more orbitals• Each orbital can have 2 e- at most

ENERGY LEVELS, ORBITALS AND E-

Energy Level # of Orbitals Max. # of E-

1 1 2

2 4 8

3 9 18

4 16 32

ELECTRON CONFIGURATIONS

• E- configuration = arrangement of e- in orbitals of an atom• Most stable e- configuration is the one in which e-

are in lowest energy level possible• Ground state = when all electrons in atom have

lowest possible energy levels• Excited state = when e- move to higher energy

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