Download Discovery of the atom and its components

Discovery of the atom and its
components
Dalton’s Atomic Theory
• In 1808, John Dalton developed an atomic
theory.
• Dalton believed that a few kinds of atoms made
up all matter.
• According to Dalton, elements are composed of
only one kind of atom and compounds are made
from two or more kinds of atoms.
• Compound – 2 or more atoms that are
ionically bonded
• Molecule – 2 or more atoms covalently
bonded. May be made of the same
element or different elements.
– Diatomic molecules – 2 atoms of the same
element that are covalently bonded to form a
more stable structure. Ex H2 O2 S2 and all
halogens – Br2 I2 Cl 2 F2
– If a molecule contains more than 1 element, it
is also considered a compound.
• Some molecules are not compounds. i.e. all monotomic
molecules are not compounds as only one element is
invovled. So molecules and compounds are neither
subsets of one another nor mutually exclusive.
• Scientists argue about the nuiances of the words for
hours…for our purposes a compound is bonded
ionically and a molecule is bonded
covalently…someone, no doubt, will tell you
different later in your life.
Dalton’s Atomic Theory , continued
Dalton’s Theory Contains Five Principles
1. All matter is composed of extremely small
particles called atoms, which cannot be
subdivided, created, or destroyed.
2. Atoms of a given element are identical in
their physical and chemical properties.
3. Atoms of different elements differ in their
physical and chemical properties.
Dalton’s Atomic Theory , continued
Dalton’s Theory Contains Five Principles, continued
4. Atoms of different elements combine in simple,
whole-number ratios to form compounds.
5. In chemical reactions, atoms are combined,
separated, or rearranged but never created,
destroyed, or changed.
•
Data gathered since Dalton’s time shows that
the first two principles are not true in all cases.
Subatomic Particles, continued
Electrons Were Discovered Using Cathode Rays
• To study current, J. J. Thomson pumped most of
the air out of a glass tube. He applied a voltage
to two metal plates, called electrodes, which
were placed at either end of the tube.
• One electrode, called the anode, was attached to
the positive terminal of the voltage source, so it
had a positive charge.
• The other electrode, called a cathode, had a
negative charge because it was attached to the
negative terminal of the voltage source.
• Thomson observed a glowing beam that came
out of the cathode and struck the anode and the
nearby glass walls of the tube.
– He called these rays cathode rays.
– The glass tube Thomson used is known as a cathoderay tube (CRT).
• CRTs are used in television sets, computer monitors, and
radar displays.
Subatomic Particles, continued
An Electron Has a Negative Charge
• Because the cathode ray came from the
negatively charged cathode, Thomson reasoned
that the ray was negatively charged.
– Thomson confirmed this prediction by seeing how
electric and magnetic fields affected the cathode ray.
• Thomson also observed that when a small
paddle wheel was placed in the path of the rays,
the wheel would turn.
– This suggested that the cathode rays consisted of tiny
particles that were hitting the paddles of the wheel.
Cathode The metal electrode
from which the electrons
originate. The cathode is the
negative electrode.
Anode The metal electrode
toward which the electrons travel.
The anode is the positive
electrode.
•A cathode is a terminal or electrode at which electrons enter a system,
such as an electrolytic cell or an electron tube.
•A cathode ray is a stream of electrons leaving the negative electrode, or
cathode, in a discharge tube.
• Thomson proposed that the
electrons of an atom were
embedded in a positively
charged ball of matter. His
model of an atom was named
the plum-pudding model.
Subatomic Particles, continued
Rutherford Discovers the Nucleus,
continued
• Ernest Rutherford performed the gold foil experiment,
which disproved the plum-pudding model of the atom.
– A beam of small, positively charged particles, called alpha
particles, was directed at a thin gold foil.
– Rutherford’s team measured the angles at which the particles
were deflected from their former straight-line paths as they came
out of the foil.
• Rutherford found that most of the alpha particles shot at
the foil passed straight through the foil. But very few were
deflected, in some cases backward.
• Rutherford reasoned that only a very
concentrated positive charge in a tiny space
within the gold atom could possibly repel the fastmoving, alpha particles enough to reverse the
alpha particles’ direction.
• Rutherford also hypothesized that the mass of
this positive-charge containing region, called the
nucleus, must be larger than the mass of the
alpha particle.
• Rutherford argued that the reason most of the
alpha particles were undeflected, was that most
parts of the atoms in the gold foil were empty
space.
The nucleus of the atom would be the size of a marble on the “F” in the
middle of the the SWAMP. That is how much empty space that
comprises the volume of an atom.
Charges in the atom
neutrons
protons
electrons
Charges in the atom
Atomic Number and Mass Number
Atomic Number Is the Number of Protons of the
Nucleus
• The number of protons that an atom has is known as the
atom’s atomic number.
– The atomic number is the same for all atoms of an element.
– Because each element has a unique number of protons in its
atoms, no two elements have the same atomic number.
• Example: the atomic number of hydrogen is 1 because the nucleus
of each hydrogen atom has one proton.
• Atomic numbers are always whole numbers.
• The atomic number also reveals the number of
electrons in an atom of an element.
• For atoms to be neutral, the number of negatively charged
electrons must equal the number of positively charged
protons.
Atomic Number and Mass Number, continued
Mass Number Is the Number of Particles of the
Nucleus, continued
• The mass number is the sum of the number of
protons and neutrons in the nucleus of an atom.
• You can calculate the number of neutrons in an
atom by subtracting the atomic number (the
number of protons) from the mass number (the
number of protons and neutrons).
• mass number – atomic number = number of
neutrons
• Unlike the atomic number, the mass number can
vary among atoms of a single element.
• Sample Problem A
• How many protons, electrons, and neutrons are
present in an atom of copper whose atomic
number is 29 and whose mass number is 64?
• The atomic number indicates the number of protons in
the nucleus of a copper atom.
atomic number (29) = number of protons = 29
• A copper atom must be electrically neutral, so the
number of electrons equals the number of protons.
number of protons = number of electrons = 29
• The mass number indicates the total number of protons
and neutrons
mass number (64) - atomic number (29) =
number of neutrons = 35
• The atomic number always appears on the lower left
side of the symbol.
H
He
1
2
Li
3
Be
B
4
5
• Mass numbers are written on the upper left side of the
symbol.
1
H
2
H
3
He
4
6
He
Li
7
Li
9
10
Be
B
• Both numbers may be written with the symbol.
1
1
H
4
2
He
7
3
Li
9
4
Be
11
5
B
11
B
neutrons
protons
electrons
What happens when you change the number of protons?
6 protons in
carbon
7 protons in
nitrogen
You obtain a different element!
The number of protons is also called
the atomic number for that element.
8 protons in
oxygen
neutrons
protons
electrons
What happens when you change the number of electrons?
11 protons
A neutral sodium atom
Na
The protons and
electrons cancel
each other out
Balanced charges
A positive sodium ion
1+
Na
One proton is not neutralized
by an electron, making this a
+1 charged atom
One electron short
8 protons
A negative oxygen ion
2O
Two electrons are not neutralized
by protons, making this a –2
charged atom
Two extra electrons
neutrons
protons
electrons
What happens when you change the number of neutrons?
Isotopes!!
Atomic Number and Mass Number, continued Isotopes
of an Element Have the Same Atomic Number
• All atoms of an element have the same atomic
number and the same number of protons. Atoms
do not necessarily have the same number of
neutrons.
• Atoms of the same element that have different
numbers of neutrons are called isotopes.
• One standard method of identifying isotopes is to
write the mass number with a hyphen after the
name of an element.
• helium-3 or helium-4
Two lithium isotopes in nature
Atomic mass
How can lithium
have 0.941
neutrons?
Number of neutrons for each of 100 lithium atoms randomly
sampled from nature
It’s an AVERAGE mass!
Isotope periodic table (first 4 rows)
Radioactivity
Lithium-8 is unstable and decays:
radioactivity: a process by which the nucleus of an atom
spontaneously changes itself by emitting particles or
energy.
decay: the process during which a nucleus undergoes
spontaneous change.
Carbon dating…cool!!