Objective 2.01: Understand the history of global agriculture.
Objective To understand what atoms are and how their ...
Transcript of Objective To understand what atoms are and how their ...
Objective – To understand what atoms
are and how their characteristics
determine the periodic table.
Throughout history, scientists have tried to
explain what made up matter
Democritus – Greek philosopher
First to propose “atoms”
Invisible, indestructible, fundamental units of matter
Formulated the first “atomic theory” Elements are made of tiny
particles called atoms.
All atoms of a given element are identical, but different from atoms of any other element.
Compounds are formed when atoms of different elements combine in fixed proportions.
A chemical reaction involves the rearrangement of atoms, not a change in the atoms themselves.
Found that atoms are made of smaller particles
Used cathode ray tube to shoot a beam of electrons that travel in a straight line Put magnets on sides of tube and the
ray bent towards the positive side.
Since opposites attract, electrons must be negative
His model was that electrons floated in a soup of positive particles “plum pudding” or “chocolate chip
cookie” model
Millikan – first measured the electrical charge of an electron Oil drop method – put
a charge on a drop of oil and dropped it between two charged plates. He would adjust the power of the plates to suspend the drop in mid-air, defying gravity
Goldstein – found the proton Used a cathode ray
tube to observe canal rays (protons) traveling in opposite directions of cathode rays and were attracted to the negative end of the magnet.
Found the nucleus of the atom Gold foil experiment – shot
positively charged Helium at gold foil to see if atom was same all the way through
Most particles when straight through, some were deflected
Because H+2 is positively charged and some were deflected, he concluded there must be a positively charged mass in the atom.
Atom is mostly empty space.
Shot alpha particles, He+2, at an atomic nucleus
Found that mass changed, but not the charge.
Had to be a particle – the same mass as a proton with no charge Essential discovery for
the fission of uranium Necessary for nuclear
energy
Determined what keeps electrons in orbit around nucleus
Proposed that electrons have a set amount of energy putting them in different energy levels, orbits, around the nucleus
Electrons can change energy levels; higher levels are further from the nucleus
Atom -a basic unit of matter that consists of
a dense, central nucleus surrounded by a
cloud of negatively charged electrons.
Element - a pure chemical substance
consisting of one type of atom distinguished
by its atomic number
Found on the periodic table
Atomic Number – the number of protons in the
nucleus of an atom
Isotope – atoms of the same element with
different numbers of neutrons
Protons – positively charged particles Found in the nucleus
Neutrons – neutral particles same average mass as protons
Found in the nucleus
Electrons – negatively charged particles Found in orbits around the
nucleus
Very small mass
Mass Number – the total number of protons and neutrons Protons and neutrons have the same mass and are found in the
nucleus of an atom
Electrons are approximately 2000 times smaller than protons and neutrons
The nucleus of an atom is relatively heavy since it holds most of the atom‟s mass Protons and neutrons
Different isotopes have different mass numbers
Atomic Mass = the average mass of all of the isotopes of an element Because electrons are so small, rounding the atomic mass will
give you the average mass number.
Atomic number = protons = electrons
Protons + neutrons = mass number
Mass number – atomic number = neutrons
Mass number – protons = neutrons
A way to organize the 118 known elements
based on increasing atomic number
Also organized based on other trends
To be discussed later
Developed by Dmitri Ivanovich Mendeleev
(with historical help from many others)
Late 1800‟s
First to develop a table that predicted
undiscovered elements based on gaps in size
Also first to recognize other trends in the table.
Shorthand for the
box on the
periodic table
56 26Fe
Using your periodic
table…
How many protons
does helium (He)
have?
How many neutrons
are in an atom of
carbon (C)?
How many electrons
are in an atom of
lithium (Li)?
Symbol Mass Number
Atomic Number
When atoms gain or lose electrons Cation – positively
charged, lost one or more electron
Anion – negatively charged, gained one or more electron
Charge can be found on the nuclide
2713Al+3
If charge is positive, the atom lost electrons. If it‟s negative, it gained them.
For the following, how many electrons can be found in the atom?
12
6C+2
35
17Cl-
79
34Se-2
Protons and Neutrons in an atom
are found clustered together in
the nucleus
Electrons are found in orbits, or
energy levels, around the nucleus
If electrons move between energy
levels they absorb or emit energy
Moving away from the nucleus requires
energy, moving toward the nucleus
releases energy
Each energy level can only hold a certain number of electrons
Octet rule – each orbital (energy level) is full once 8 electrons are found in it Except the first, it only has two electrons
Example:
Oxygen has an atomic number of 8.
It has 8 protons and 8 electrons.
Two electrons in its first orbital, and 6 in the second
The number of electrons in the outermost orbital are considered valence electrons
Helps determine the reactivity of the element The closer to a full or empty orbital the more reactive the atom
Ie Na only has one valence electron, it is very close to empty. It is highly reactive.
Ions only gain or lose valence electrons
Atoms want full valence orbitals
They want to be like the closest Nobel Gas to them
The last column of the periodic table
This makes them the least reactive.
The number of electrons they are likely to gain or lose is based on emptying or filling an orbital
This is the Oxidation Number of the element
The elements on the left side of the periodic table are more likely to lose electrons, elements on the right side are more likely to gain electrons
The ones in the middle can become either anions or cations
Example: Na will likely be a ____ ion with a charge of ___
Use the Bohr‟s model (planetary model) to
draw atoms
Protons, neutrons are found in the nucleus
Electrons are found the in energy levels
surrounding the nucleus
Don‟t go to the next energy level until the one before
it is full
Example: Be
Atomic #?
Mass #?
Protons?
Neutrons?
Electrons?
4p
5N
-
-
-
-
Determine the number of subatomic particles
for the following ions and atoms, and draw
the Bohr‟s model of the atom
Atom/
Ion
Name Nuclide Atomic
#
Mass # Protons Neutrons Electrons Valence
Electrons
Li
S
Ne
Cl-1
Mg+2
The Bohr‟s model is a very simple way to
draw atoms
Through technology we‟ve determined where
electrons are arranged within at atom or
molecule
Electron configuration is the arrangement of
electrons in an atom or molecule. They tell
you how many electrons are in each energy
level
Use the periodic table as a map Divide it into four parts
There are three parts of the electron configuration to indicate where the electrons are around the atom The big number: stands for the energy level
The letter (s, p, d, & f): the shape of the orbital s – spherical etc
The exponent: the number of electrons in that orbital
Each orbital gets filled before you move on to the next one.
Example: O 1s2, 2s2, 2p4
Elements are organized by increasing atomic number
Also organized into periods (rows)
Read from left to right
Groups/families (columns) All elements in a family
have similar trends All have the same
number of valence electrons
Metals
Alkali Metals
Alkaline Earth metals
Transition Metals
Inner Transition Metals
Metalloids
Non-Metals
Metalloids
Gasses
The organization of the periodic table is not
just based on atomic number
There are other trends that show up on the
periodic table
Trend #1 – Atomic Radius
Definition – the average distance from the
nucleus to the outermost electron
As you travel to the left of the periodic table,
the elements have a larger radius
As you travel down the periodic table, the
elements have a larger radius
Trend #2 – Ionization energy
Definition: The amount of energy required to remove one electron
As you travel to the right across a period, the ionization energy increases
As you travel up a group, the ionization energy increases
Trend #3 – Electron Affinity
Definition – the amount of energy gained when an electron is added to it
As you travel to the right across a period, the electron affinity increases
As you travel up a group, the electron affinity increases
Trend #4 – Electronegativity Definition – the ability of an element to
attract pairs of electrons in a covalent bond
As you travel to the right across a period, electronegativity increases.
As you travel up a group, the electronegativity increases
Metallic Characteristics Increase with lower valence electrons
and larger atomic radius
Non-metallic characteristics Increase with higher valence electrons
and smaller atomic radius
Place the following elements in increasing
order based on each criteria
C, Na, Sr, Al, Ne
Atomic Radius
Ne, C, Al, Na, Sr
Ionization Energy
Sr, Na, Al, C, Ne
Electron Affinity
Sr, Na, Al, C, Ne
Electronegativity
Ne, Sr, Na, Al, C
Results from a loss of the forces of the nucleus
Strong nuclear force: a super strong force that acts
between protons and neutrons in the nucleus, binding
them together
This attractive force is stronger than the force that repels
„like charges‟ and that attracts „opposite charges‟
Only acts at extremely small distances (10-15m)
When the nucleus becomes unstable, this
nuclear force becomes unbalanced, radioactive
decay occurs
Tends to occur in atoms with large proton to neutron
ratios
When they break down they emit radiation
Types
Alpha α
Beta β
Gamma γ
Alpha α
Most common form of radiation
Alpha radiation consists of fast flying positively charged particles
Combination of protons and neutrons Aka the nucleus of a Helium atom, atomic number 2
Beta β
Medium strength of radiation
Beta radiation consists of fast flying negatively charged particles
Each beta particle is an electron that is ejected by an atomic nucleus
Gamma γ
strongest form of radiation
Occurs when an atom in an excited state releases energy
Extremely short wavelength, much more energetic than visible light
Gamma radiation carries lots of electric charge and no mass
When atoms/elements break down, they
become another element
This process emits radiation
Types
Alpha particle emission
Beta particle emission
Alpha particle emission When an atom breaks down and emits an alpha
particle A 4He nucleus (2 neutrons and 2 protons)
Occurs with massive nuclei that have too large of a neutron to proton ratio
To determine the products of alpha particle emission, you subtract a He nucleus
Example:
23592U 231
90Th + 42He
Just like a math equation: the top numbers have to be equal and the bottom numbers have to be equal. They symbol goes with the atomic number found on the bottom
Beta Particle Emission
When a atom breaks down and emits a beta
particle
Energy converts a proton into a neutron (β+) and emits
a positive charge
OR energy can convert a neutron into a proton (β-) and
emits a negative charge
Example
231
90Th 23191Pa + 0-1e
The time required for half of the atoms in a sample of a radioactive isotope to decay Different isotopes decay at different rates
The longer the half life, the greater the stability
Example Radium-226 has a half life of 1620years
This DOES NOT mean that in 3240 years it will be gone!
This does mean that after another 1620 years, half of the remaining half will be gone, leaving ¼ of the original sample
The half life of the sample continues like this: ½ will remain, ¼ will remain, 1/8 will remain, 1/16 will remain, and so on Each half life cycle leaves 1/(2n) of the original
Half lives are VERY consistent and not affected by environmental conditions
Half-lives can be measured by a radioactive detector and by measuring how much decay occurs per year.
A 100 gram sample of 13C decays to 25 grams in 20.6 seconds. What is its half-life? Original = 100g
Left = 25g
25/100 = ¼
This means it when through 2 half life cycles. 20.6 seconds / 2 = 10.3s
The half life of 258Md is 2,800 years. If there are 33g of the sample left after 1,400 years, how many grams were in the original? Half life = 2,800 years
Time passed = 1,400 years
½ of a half life has passed, so Only ¼ of a sample has decayed, so ¾ is left
33/(3/4) = 44
There are 5.0g of 210Bi left after 30.45 days.
How many grams were in the original sample
if its half-life is 6.09days?
Original = ?
Left = 5.0g
Half life = 6.09days
Time passed = 30.45 days
How many half life cycles?
30.45/6.09 = 5
5 cycles means 1/(25) of the sample is left
1/32 left
5.0g /(1/32) = 160g
Nuclear fusion Taking two atoms and making a new
one plus neutrons
Occurs mostly in lighter atoms
Releases radiant energy as gamma radiation
Found in stars and the hydrogen bomb
Nuclear Fission Splitting atoms into two new ones
Creates two new smaller nuclei and releases neutrons
Generally only occurs in heavier atoms
Releases gamma radiation
Method behind nuclear power and nuclear weapons