Unit 9: Atomic Theory and Periodicity

Section 1: Structure of the Atom

Structure of the Atom

• The organization of the periodic table is based on the structure of the atom for each element

• The 20th century is when most discoveries about the structure of atoms took place– The atom was first viewed as a small, solid unit– Rutherford found that the atom has a dense, positive center

(nucleus) that is quite small compared to the entire atom– Thompson found the electron to be a negatively charged particle of

almost no mass– Urey found that neutrons (neutral charged particles) resided in the

nucleus– Einstein’s experiments with color and light led to the understanding

of electron behavior

Wavelength

• The distance between similar parts of the curve, between crests (tops) or between troughs (bottoms) of adjacent waves

• Frequency: the number of waves that pass an observation point per second

Electromagnetic Spectrum• Has both electrical and magnetic properties• Is comprised of AC power, radio waves, microwaves,

infrared radiation, visible light, ultraviolet radiation, X rays, and gamma rays – The lists above is in order

of decreasing wavelength• The smaller the wavelength,

the higher the frequencyand energy

Visible Light• Part of the electromagnetic spectrum that we are

able to see• Made up of colors: ROYGBIV– Red, orange, yellow, green, blue, indigo, violet– The array of colors is due to slight variations in the

wavelengths• Red has the longest wavelength and violet has the shortest

wavelength

Light and the Structure of the Atom

• Different colors of fireworks are due to the actions of electrons within their compounds when ignited– Before fireworks are set off, the electrons are in ground

state• Ground state: the normal location for electrons within the

atom

Light and the Structure of the Atom

–Once the fireworks are ignited, some of the electrons acquire enough energy to go to the excited state• Excited state: some electrons jump towards the

outermost orbital within the atom

Light and the Structure of the Atom

• Electrons cannot stay at the elevated position, so they fall back to their normal position– Just like a jumper on a trampoline, the jumper falls back

down to the starting point• When electrons move to the

excited state they absorb energy, and when they move back to ground state they giveoff energy

Light and the Structure of the Atom

• Each color in the visible light spectrum has a certain energy level

• When electrons emit energy (go from excited state to ground state), they also emit a color– The color depends on the energy level being emitted

Niels Bohr• Because excited atoms give off specific colors and not a

rainbow of colors, he understood that electrons are allowed in only certain locations within the atom, called energy levels

• Each element behaves in its own unique way when excited by heat or electricity, and produces a very specific pattern of lines of color, called the atomic spectrum– This unique chemical fingerprint is because each element has

a different number of electrons, different sized nucleus that attracts the electrons back to ground state, and different distances from the nucleus to the outer electrons