Chemistry Lesson #1

What is a model

A model is the use of familiar ideas to explain unfamiliar facts observed in nature. They can be changed as new information is collected

Who is Democritus and what is his significance to chemistry

A Greek philosopher who began the search for a description of matter

Democritus’ question

Could matter keep being divided into smaller and smaller pieces or is there a limit?

Democritus’ theory

Matter could not be divided into smaller and smaller pieces and eventually the smallest piece would be obtained

How did Democritus describe atoms?

Hard, small particles that were made of the same material but were different shapes and sizes

Atomos

To not be cut

Why was his theory rejected?

The eminent philosophers Aristotle and Plato favoured the 4 element approach to the nature of matter

John Dalton

English chemist who revived the idea of atoms after conducting various experiments.

The 5 Postulates Dalton’s Atomic Theory

1. All matter is made up of invisible particles called atoms

2. Atoms of the same element are similar to each other (in terms of shape and mass)

3. Atoms of different elements differ from each other in shape and mass

4. Atoms can neither be created or destroyed

5. Atoms combined in whole ratios to form compounds

What did Thomson’s Plum Pudding Model suggest

Thomson’s model suggested that atoms were made out of positively charged substances with negatively charged electrons scattered about

Thomason’s significance

Thomason was the first to identify the negative charges within an atom

Experiments leading to the Thomson model

• He passed a current through gas

• Demonstrated that the “cathode rays” coming from the gas were negatively charged particles

• The negatively charged particles must be apart of atoms

Thomson’s conclusion

Thomson concluded that atoms were divisible, and a particle smaller than the atom existed. He also concluded that negative charges came from within the atom.

Rutherford’s Gold Foil Experiment

Rutherford fired positively charged alpha particles from a radioactive source at thin metal foil sheets, expecting them to pass through. However, the experiment revealed that not all light rays were deflected, with most passing through gold atoms and some bouncing away.

Rutherford’s Conclusions

• Atoms are mostly open space

• Atoms have a small dense positively charged centre (nucleus) that repelled the positively charged bullets (alpha particles)

• Negatively charged particles were scattered outside the nucleus around the atoms edge

Bohr’s Experiment

• When a sample of hydrogen is heated, it glows

• If the light is passed through a prism, only certain colours can be seen (line emission spectrum)

• Lines of light have different energy and correspond to different “energy levels” where you can find electrons

Bohr’s Model

Electrons are in orbits, or energy levels, which are located at specific distances from the nucleus

Subatomic Particle

Matter consists of small, invisible particles called atoms, which are the most fundamental particles in elements. Subatomic particles are within and smaller than atoms.

• Protons

• Neutrons

• Electrons

Proton

• p+

• 1 amu

• nucleus

• +1

Electron

• e-

• ~0 amu

• shells

• -1

Neutron

• nº

• 1 amu

• nucleus

• 0

X

Atomic Symbol

An abbreviation of the name of the element (max 2 letters, capital first)

A

Atomic Mass

The mass of the nucleus (protons + neutrons)

Z

Atomic number

How many protons in the nucleus

if the atom is neutral, # of p+ = # of nº

N

Neutron number

N = A - Z

How to find charge

charge = # of protons - # of electrons

Bohr-rutherford diagrams

Used to model the structure of atoms and predict the bonding of atoms (ionic or covalent)

Only works with the first 20 elements on the periodic table

Nucleus in Bohr-rutherford diagrams

• Do NOT include the chemical symbol

• do NOT circle the nucleus

Rings

• Where to find the electrons

• If there are no electrons, there are no rings

Electrons

1. Exist only in one of 4 spatial orientations (N, S, E, W)

2. Do NOT pair them off until each orientation already has an electron

The periodic table

• Organized periodically

• Every period (row) is roughly a repeat of the same pattern of the period above (periodic law)

• As a result, elements in the same group (column) have similar electron arrangements and properties

Groups

• Go across the periodic table

• Are numbered 1-18

• Represent “families” that share similar properties/trends

Periods

• Go down the periodic table

• Numbered from 1-7

Staircase

• Separates metals from non-metals

• Elements touching the staircase are metalloids (B, Si, As, Sb, Te)

Group 1: Alkali Metals

• Very reactive

• Common ion has a charge of +1

• Does not include hydrogen

Group 2: Alkaline Earth Metals

• Reactive, but not as much as Alkali metals

• Common ion has a charge of +2

Group 17: Halogens

• The most reactive non-metals

• Common ion has a charge of -1

Group 18: Noble Gases

• Mostly non-reactive with other elements

• No common ion

Transition metals

• Many are multivalent elements

• NOT Scandium (Sc3+) and Zinc (Zn2+)

• Silver (Ag): 1+

Rare Earth Metals

• Lanthanide and Actinide Series

• pulled out of the table