Unit 2(Understanding the Properties of Elements)

Question 1

400 B.C – Democritus

Answer 1

• atomos = “indivisible” / “uncuttable”
• tiny geometric particles
• can not be broken down further
• different elements are made of different
  particles

Democritus

Question 2

350 B.C. – Aristotle

Answer 2

• famous philosopher
• believed all substances
  are made of 4 elements
  – water, earth, fire, air
  – mix to yield different
  substances
• four-element” model
  was accepted for
  almost 2000 years!

Question 3

1808 – Dalton

Answer 3

H2O → H2 + O2
water → hydrogen + oxygen

oxygen atom=smaller ball
hydrogen atom=bigger ball

Question 4

Dalton’s Atomic Theory

Answer 4

1. All matter is made of tiny indivisible atoms
   (“small, hard, indestructible sphere”), which
   are particles too small to see.
2. Atoms of the same element are identical in
   mass and size. Atoms of one element are
   different in mass and size form the atoms of
   other elements.
3. Compounds are created when atoms of
   different elements link together in definite
   proportions.
4. Chemical reactions involve the
   rearrangement of atoms. Atoms cannot
   be created, destroyed, or subdivided in
   chemical changes.

Question 5

1904 – Thomson (cathode ray tube)

Answer 5

J.J Thomson
* Made cathode ray tube
* It is a vacuum tube – all
the air has been
pumped out
* ray travels from negative
cathode to positive
anode
* thus ray is made up of
negative particles from
inside atoms

Question 6

1904 – Thomson

Answer 6

• J.J. Thomson “raisin bun” model

1. Atoms contain particles called
   electrons.
2. Electrons have a small mass and a
   negative charge.
3. The atom is a sphere of positive
   charge.
4. Electrons are embedded in this
   sphere, so that positive and negative
   charges balance and overall the
   atoms is neutral or uncharged.

Question 7

1911 – Ernest Rutherford

Answer 7

• Rutherford’s Gold Foil Experiment
  – wanted to see how big atoms were
  – shot positively charged alpha particles at gold foil

Question 8

1911 – Rutherford’s prediction

Answer 8

• prediction: most (+)
  particles pass through
  foil in a straight line
  through spaces between particles in the foil

• results: most particles
did pass through but
some were repelled and
deflected backwards

Question 9

Rutherford’s results from experiment

Answer 9

• his experiment showed him
  that…
  – atom is mostly empty space
  – atom has a small, dense,
  positive (+) center called the
  nucleus (that repels and deflects
  alpha particles it if they get too
  close)
  – electrons are separate from
  the nucleus
• electrons revolve around the
  nucleus (like a planet around the
  Sun)

question: If positive and negative charges attract, then
why doesn’t the atom collapse on itself?!

His question: If positive and negative charges attract, then

Question 10

Nelis Bohr

Answer 10

atomic spectra

Question 11

Nelis Bohr model of the atom

Answer 11

• Bohr’s model of the atom:
  – proposed that electrons
  could only move around
  the nucleus at fixed
  regions, or energy levels
  – to move from one level to a
  higher one, an electron has
  to absorb a quantum of
  energy

Question 12

Bohr-Rutherford model of the atom

Answer 12

Adding a specific amount of energy causes an electron to move to a higher energy level

Question 13

1932 Chadwick

Answer 13

• discovered the neutron
• neutron: an uncharged
  (neutral) particle that is part of
  almost every atomic
  nucleus

Question 14

What is the Modern View of the ATOM

Answer 14

• The atom is mostly
  empty space
• It has 2 regions:
  1. nucleus (protons + neutrons)
  2. electron cloud (region where
  you may find an electron)

Question 15

Subatomic Particles(protons)

Answer 15

Protons
Symbol:p+
Charge:positive(+1)
Relative Mass:2000
location:nucleus

Question 16

Subatomic particles :a particle that is smaller than the atom(electrons)

Answer 16

Electrons
symbol:e-
Charge:negative(-1)
Relative Mass:1
location: orbits around nucleus

Question 17

Subatomic particles (neutrons)

Answer 17

neutrons
Symbol:n0
Charge: neutral(zero)
Relative Mass:2000
Location:nucleus