Do Now:

1.On the blank side of an index card, draw a picture of an atom.

2.On the other side of the index card, write down things that you know about atoms.

YWBAT

• Explain how Democritus and John Dalton described atoms

• Identify instruments used to observe atoms.• Identify three types of subatomic particles.• Describe the structure of atoms according to

the Rutherford atomic model.

Sizing Up The Atom

• A pure copper coin the size of a penny contains about 2.4 x 1022 atoms

24,000,000,000,000,000,000,000 atoms

* Note – Earth’s population is ~ 7 x 109 people

Sizing Up The Atom

• Individual atoms are observable with a Scanning Electron Microscope (SEM).

SEM Pollen grains

Democritus

•Greek philosopher (460– 370BC)•Among the first to suggest the existence of atoms.

atomos – Greek word for “not to be cut”

•Reasoned that atoms were indivisible and indestructible.

Dalton

• English chemist & school teacher• 1766 – 1864• Used experimental methods to transform

Democritus’ ideas into scientific theory• Studied ratios in which elements combine in

chemical reactions.

Dalton’s Atomic Theory

1. All elements are composed of tiny indivisible particles called atoms.

Atoms of element A

Dalton’s Atomic Theory

2. Atoms of the same element are identical. The atoms of any one element are different from those of any other element.

Atoms of element A

Atoms of element B

Dalton’s Atomic Theory

3. Atoms of different elements can physically mix together or can chemically combine in simple whole-number ratios to form compounds.

Mixture of atoms of elements A and B

Dalton’s Atomic Theory

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.

Compound made by chemically combining atoms of elements A and B

Dalton’s Atomic Theory

• Do you think all parts of Dalton’s Atomic Theory are still believed to be true today?

Thomson

• In 1897, physicist J.J. Thomson discovered the electron.

• Electrons are negatively charged subatomic particles

Subatomic Particles

How are these three subatomic particles (protons, neutrons, and

electrons) put together in an atom?

Plum Pudding Model

• Thompson’s atomic model• Electron’s stuck in a lump of positive

charge, similar to raisins stuck in dough.

Rutherford’s Gold Foil Experiment

• In 1911, Ernest Rutherford, a former student of Thomson’s, tested the plum-pudding model.

• A narrow beam of alpha particles was directed at a very thin sheet of gold

Rutherford’s Gold Foil Experiment

• Most alpha particles went straight through, or were slightly deflected.

• A small fraction of the alpha particles bounced off the gold foil at very large angles.

Rutherford’s Atomic ModelThe Nuclear Atom

• Rutherford suggested a new theory of the atom based on the experimental results.– The atom is mostly empty space.– All the positive charge and amost all of the mass are

concentrated in a small positively charged region (nucleus)

– Protons & neutrons are in the positively charged nucleus

– Electrons are distributed around the nucleus and occupy almost all the volume of the atom.

Rutherford’s Atomic ModelThe Nuclear Atom

If an atom were the size of a football stadium, the nucleus would be the size of a marble.

The Bohr Model

• Niels Bohr (1913) • Incorporated discoveries about how the

energy of atoms changes when the atom absorbs or emits light.

• Stated that the electrons orbit the nucleus like planets orbit the Earth.

Quantum Mechanical Model

• The probability of finding an electron is represented as a fuzzy cloudlike region.

• The cloud is more dense where the probability of finding the electron is high.

Atomic Orbital – region of spacewhere there is a high probabilityof finding an electron