Lesson 5 Atomic Theory Anything in black letters = write it in your notes (‘knowts’)

Section 1 – Defining the Atom Section 2 – The Spectrum of Light Section 3 – Development of Modern Atomic Theory Section 4 – The Nucleus Section 5 – Electrons in Atoms

How do we know that atoms exist?

Democritus (460 B.C. – 370 B.C.) one of the first to propose the idea of the atom; based on pure speculation comes from the Greek word atomos which means uncuttable or indivisible 5.1 – Defining the Atom

John Dalton (~1800) proposed 1 st atomic theory

Daltons Atomic Theory (~1800) 1.All elements are composed of tiny indivisible particles called atoms. Atoms of element A Atoms of element B 2.Atoms of the same element are identical. The atoms of any one element are different from those of any other element. Mixture of atoms of elements A and B 3.Atoms of different elements can physically mix together or can chemically combine in simple whole-number ratios to form compounds. Compound made by atoms of elements A and B 4.Chemical reactions occur when atoms are separated from each other, joined, or rearranged in different combinations. Atoms of one element are never changed into atoms of another element as a result of a chemical reaction.

What we know now of Dalton’s Atomic Theory 1.All elements are composed of tiny indivisible particles called atoms. 2.Atoms of the same element are identical. The atoms of any one element are different from those of any other element. 3.Atoms of different elements can physically mix together or can chemically combine in simple whole-number ratios to form compounds. 4.Chemical reactions occur when atoms are separated from each other, joined, or rearranged in different combinations. Atoms of one element are never changed into atoms of another element as a result of a chemical reaction. Atoms are not indivisible – they are made of subatomic particles Every atom has at least one isotope; one atom’s isotope is NOT identical to another isotope of the same atom. This is known as the Law of Definite Proportions – very important. Atoms of one element can change into an atom of another element as a result of a nuclear reaction.

Law of Definite Proportions - Atoms combine in fixed whole number ratios. Law of Multiple Proportions - Sometimes, atoms combine in more than one ratio. CO, CO 2 H 2 O, H 2 O 2 CO 2 H 2 O

Atom - smallest particle of an element that still has the properties of the element

Light waves can be described by their wavelength or frequency. 5.2 – The Spectrum of Light

Wavelength– length of the wave! Frequency– number of waves per second. measured in meters measured in waves per second or Hertz (Hz) How frequent the waves are

Low energy ( = 700 nm) High energy ( = 380 nm) Frequency (s -1 ) 3 x x x The Electromagnetic Spectrum

Visible light ranges from 380 – 700 nm 1 nm = m

Light behaves as a wave but consists of particles called photons. Photon - particle of light

Incandescence – emission of light by hot objects. Incandescent light bulbs give off more heat energy than light energy and so are not very efficient.

Fluorescence – absorption of light of high freq. and re-emit at lower freq.

When Hg is energized, it produces UV light. The white powder in a fluorescent light bulb will fluoresce and visible white light is emitted. Fluorescent Lights VIDEO – half-coated fluorescent tube

Blacklight – emits UV light and small amount of visible light.

A fluorescent material will absorb the high energy UV light and re-emit lower energy visible light – the material then appears to “glow” in the dark.

Phosphorescence – absorption of light followed by a delay in the re-emission of light. Phosphorescent materials will keep emitting light for a while in the dark.

Phosphorescent pigments ZnSSrAl 2 O 4 In visible light

ZnSSrAl 2 O 4 Phosphorescent pigments In the dark

ZnSSrAl 2 O 4 Phosphorescent pigments In the dark after 4 minutes

Chemiluminescence – emission of light by a chemical reaction.