Download 4.1 Studying Atoms

Studying Atoms
Lecture 4.1
Objectives:
• Describe ancient Greek models of
matter
• List the main points of Dalton’s atomic
theory and describe his evidence for the
existence of atoms
• Explain how Thomson and Rutherford
used data from experiments to produce
their atomic models
Daily Question
• Do you know what makes
up the parts of an atom?
Overview
•
•
Most natural materials are
mixtures of pure
substances
“Pure” substances are
either elements or
compounds
Ancient Greek Models
• Compounds always contain the same
elements in the same proportions
- Law of Constant Composition
• Democritus (460-370 B.C.): was the first to
use the word “atomos”( which meant
atoms); stated that matter is composed of
atoms; atoms are indestructible and
indivisible
Ancient Greek Models
• Aristotle (384-322 B. C.): said that
empty space could not exist; matter is
made of earth, fire, air, and water
Dalton’s Theory
• Dalton proposed that all matter is made up
of individual particles called atoms, which
cannot be divided.
Dalton’s Atomic Theory (1808)
1) Elements are made of tiny particles
called atoms
2) Atoms of any element are identical
3) Atoms of an element are different from
atoms of any other element
4) Atoms of elements combine to form
compounds
–Always the same numbers and
types of atoms
Dalton’s Atomic Theory (1808)
5) Atoms
are indivisible
– Cannot be created or destroyed
– Atoms rearrange during chemical
reactions
Importance of Dalton’s Theory
• It explained many important
observations
–Conservation of mass
–Constant composition
Atoms and Conservation of Mass
A +
1 mass unit
A
B
4 mass units
B
3 mass units
+ A C
5 mass units
A
B
4 mass units
A
+
A
B
C
2 mass units 7 mass units
Thomson
• Thomson took Dalton’s model, and
expanded on it.
• His experiments provided the first
evidence that atoms are made of even
smaller particles.
Importance of Thomson’s
Experiments
• Confirmed existence
of electrons
• Implied existence of
protons
Schematic of a cathode ray tube.
Cathode Ray Experiments
• Proved the existence of electrons
• Sir Joseph John Thomson
– Mass of the electron (9.109 x 10-28 g)
• Approximately 1/1800 the mass of the
simplest hydrogen atom
– The electron is the smallest possible
negative charge
• All other negative charges are wholenumber multiples of the electron
Plum Pudding model of Thompson pg
114
Models of the Atom
THOMSON
“Plum Pudding”
DALTON
Indivisible
-
-
The Nucleus
• A dense, positively charged mass
located at the center of the atom
• Central portion of the atom
– Contains most of the atom’s mass,
but . . .
– Occupies only a tiny fraction of its
volume
– The plural form of nucleus is nuclei
3min
Earnest Rutherford (1899)
• Discovered that uranium emits fast-moving
particles with positive charges
• He wanted to test Thompson’s Plum pudding
model
• He predicted that most of the particles would go
straight through
• According to Rutherford’s model, all of the
atom’s positive charge is concentrated in its
nucleus
Rutherford’s Gold Foil
experiment.
Results of foil
experiment…
If the Plum
Pudding
model had
been correct.
Actual results
Rutherford’s Model of an Atom
Niels Bohr
• (1885-1962)
• Worked with Rutherford and agreed with
Rutherford’s model
• But his model focused more on the
electrons which surrounded the nucleus
Bohr’s Model
• Electrons move with constant speed in
fixed orbits around the nucleus
– Like planets around a sun
• Each electron in an atom has a specific
amount of energy
– Electron can gain or lose energy
Bohr Model Picture
Evidence of Energy Levels
• Movement of electrons between energy
levels explains the light you see from
fireworks
– Light is form of energy
– No two elements have same set of energy
levels , so different elements emit different
colors of light
Color
Compound
Red
strontium salts, lithium salts
lithium carbonate, Li2CO3 = red
strontium carbonate, SrCO3 = bright red
Orange
calcium salts
calcium chloride, CaCl2
calcium sulfate, CaSO4·xH2O, where x = 0,2,3,5
Firework Colorants
Gold
incandescence of iron (with carbon), charcoal, or lampblack
Yellow
sodium compounds
sodium nitrate, NaNO3
cryolite, Na3AlF6
Electric White
white-hot metal, such as magnesium or aluminum
barium oxide, BaO
Green
barium compounds + chlorine producer
+
barium chloride, BaCl = bright green
Blue
copper compounds + chlorine producer
copper acetoarsenite (Paris Green),
Cu3As2O3Cu(C2H3O2)2 = blue
copper (I) chloride, CuCl = turquoise blue
Purple
mixture of strontium (red) and copper (blue) compounds
Silver
burning aluminum, titanium, or magnesium powder or flakes
Electron Cloud
• Scientists use the electron cloud model to
describe the possible locations of
electrons around the nucleus
– Electrons move in a less predictable way
Orbital
• Is a region of space around the nucleus
where an electron is likely to be found
– The electron cloud represents all of the
orbitals in an atom
Figure 15 pg 117
Energy Level
# of orbitals
1
1
Max. # of
electrons
2
2
4
8
3
9
18
4
16
32
Electron Configuration
• arrangement of electrons in the orbitals
• The most stable electrons in orbitals with
lowest possible energies
• Electron moving up an higher energy orbital=
configuration is in excited state
4.2 Structure of the Atom
The Nucleus is:
• Positively charged
• Very Dense
• the center portion of the atom
– Contains most of the atom’s mass,
but . . .
– Occupies only a tiny fraction of its
volume
Composition of the Nucleus:
• Proton
– found in the nucleus of an atom
– Positive charge
– Mass of 1.673 x 10-24 grams or 1amu
• Neutron
– found in the nucleus of an atom.
– No electrical charge (neutral)
– Mass approximately equal to the proton
Composition of the Nucleus:
• Electron
– moves around the nucleus
– Negative charge
Atom
– Very small compared to proton and neutron
1/1800
– relatively far away from the nucleus
• Most of the atom is empty space
Electron
Nucleus
Protons, Neutrons, and Electrons
Relative Mass
Charge
proton
1
+1
neutron
1
0
electron
1/1800
–1
Atomic number
The atomic number (“Z”) = number of
protons in an atom
The atomic number identifies element
– elements have different numbers of
protons
Mass number
• Mass number (“A”) =protons + neutrons.
• Mass number identifies the particular
isotope
• In an atom, electrons equal the number of
protons makes atoms electrically neutral
Nuclear Symbol
Mass Number
65
29
Cu
Atomic Number (Z)
29
Electrons
Protons + Neutrons
36 Neutrons
65
29
Cu
Protons
Remember!! Atoms are neutral!!
• https://phet.colorado.edu/sims/html/buildan-atom/latest/build-an-atom_en.html
Isotopes
Isotopes are atoms of the same element
with different mass numbers
– Same atomic number
– Different mass numbers because
they have different numbers of
neutrons
Two isotopes of sodium.
How to calculate the number of
neutrons in an atom?
• Number of neutrons= mass number –
atomic number
Practice
• Write the nuclear symbol for bromine-80
– How many protons, electrons, and neutrons
are in this atom?
• Write the hyphen notation for the
element that has 6 protons and 7 neutrons
– How many electrons does this atom have?
– Write the nuclear symbol for this atom