unit 2: the atom
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Textbook Chapter 5 & 13 Review Book Topic 1. Unit 2: The Atom. Atomic Fundamentals. All matter is composed of tiny fundamental particles called atoms Atom – smallest particle of an element that retains the properties of that element. Examples of atomic size: - PowerPoint PPT PresentationTRANSCRIPT
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UNIT 2: THE ATOM
Textbook Chapter 5 & 13Review Book Topic 1
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Atomic Fundamentals All matter is composed of tiny fundamental
particles called atoms
Atom – smallest particle of an element that retains the properties of that element
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Examples of atomic size:
A pure copper coin the size of a penny contains 2.4 x 1022 atoms
Compared to the Earth’s population (6 x 109 people), there are about 4 x 1012 as many atoms in the coin as there are people on Earth
If you could line up 100,000,000 copper atoms side by side, they would produce a line on 1 cm long
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Early Models of the Atom Democritus of Abdera
4th Century B.C. – Greece
First to suggest the existence of atoms as invisible and indestructible particles
Components: Fire, Earth, Wind and Water
The real nature of atoms was not established for more than 2000 years later
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Robert Boyle (1600s)Identified gold and silver as being
elemental
Not made of Earth, fire, wind or water
Considered the “Father of Modern Chemistry”
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John Dalton (1803)Studied the ratios in compounds
Set the groundwork for the current concept of the atom
Dalton’s Atomic Theory1. Atoms cannot be broken down
2. Atoms of the same element are identical
3. Each element’s atoms are different
4. Atoms of different elements can chemically or physically combine to form compounds
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Structure of the Atom Most of Dalton’s atomic theory is
accepted today but atoms can be broken down into even smaller particles:
ProtonsNeutronsElectrons
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Electrons – negatively charged particles
Discovered by J.J. Thomson in 1897
○ Experimented with a cathode tube
A cathode ray is composed of negatively charged particles
Negative electrical charges repels the rays, while positive charges are attracted
Refer to pages 109-110 in your textbook
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○ Called his atomic model the “plum pudding model”
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Robert A. Millikan (1868-1953)
○ An electron carries exactly one unit of negative charge
○ Mass of an electron is 1/1840 the mass of a hydrogen atom
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Protons and Neutrons
Atoms have no net electric charge
Entire atom is neutral so:(+) charges = (-) charges
So # protons = # electrons in a neutral atom!
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Protons – positively charged subatomic particles, each with a mass about 1840 times that of an electron
Neutron – subatomic particles with no charge but with a mass nearly equal to that of a proton
So if an atom is overall neutral….
# protons (+) = # electrons (-)
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Properties of Subatomic Particles
Particle Symbol Relative electrical charge
Relative mass (mass of
proton = 1)
Actual mass
(g)
Location
Electron e- 1 - 1/1840 amu 9.11 x 10-28
Outside
Proton p+ 1 + 1 amu 1.67 x 10-24
Nucleus
Neutron n0 0 1 amu 1.67 x 10-24
Nucleus
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Structure of the Atomic Nucleus Ernest Rutherford (1909)
Proposed an atom model where theelectrons surround a dense nucleus and all remaining areas are empty space
Directed a narrow beam of alpha (α) particles (with a + charge) at a very thin sheet of gold foil
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A majority of alpha particles passed straight through the gold atoms, without deflecting while some particles bounced off the gold foil at very large angles or back toward the source
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He found that:
○ An atom is mostly empty space (99.9% of the alpha particles went straight through the gold foil)
○ (+) charges and the mass of the atom are concentrated in the nucleus (0.1% of the alpha particles deflected)
See animation on his experiment
Nucleus – the central core of an atom and is composed of protons and neutrons (tiny when compared to the size of the atom overall - remember the Bill Nye video!)
Brain Pop
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Niels Bohr (1913)
Proposed that electrons are arranged in circular paths, or orbits, around the nucleus with a fixed energy (no energy can be lost by the electron)
○ Model is patterned after the motions of planets around the sun (“planetary model”)
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Erwin Schrödinger & Albert Einstein (1926)
Used a mathematical equation to describe the location and energy of an electron in a hydrogen atom
Known as the “quantum mechanical model” or “wave
mechanical model”
○ Previous models were mostly physical based on the motion of large objects
Restricts the energy of electrons to certain
values
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Does not define an exact path an electron around the nucleus
Estimates the probability of finding an electron in a certain volume of space surrounding the nucleus (in a “fuzzy cloud”)
○ Cloud is more dense where there is high probability
○ Regions of probability are called “orbitals”
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○ Energy level (of an electron) – region around the nucleus where the electron is likely to be moving
An electron can jump from one energy level to another
To move from one energy level to another, an electron must gain or lose just the right amount of energy
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○ Quantum (of energy) – amount of energy required to move an electron from its present energy level to the next higher one
The higher the energy, the farther away from the nucleus the electron is located
Energy levels become more closely spaced the farther they are from the nucleus (stairs example)
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History of Atomic Models
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Subatomic Particles in Detail
Protons(+) charged particle found in the nucleus of an atom
# of protons in an atom determines which element it is
○ # protons = atomic number
○ Found on the periodic table of elements
If the # protons change in an atom, it becomes a different element
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Neutrons
Subatomic particles with no charge
Number of neutrons can change within the atom without changing the element involved
Isotope – two atoms with the same number of protons but different numbers of neutrons
○ Ex. C-12 & C-14
○ Both are carbon so they have 6 protons...but C-14 has 2 more neutrons than C-12
○ Brain Pop
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Neutrons Continued….
Mass Number = # protons + # neutrons
To find the # of neutrons:
○ Mass # - Atomic # = # Neutrons
○ (p+ + n0) - (p+) = (n0)
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Symbol used to show atomic number and mass number of an element:
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Can also be written as:
Element – (mass #)
Ex. Carbon-12 versus Carbon-14 (shows the change in mass number in these two isotopes)
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Electrons(-) charged particles of an atom
To simplify the wave-mechanical model, we will draw “rings” around the nucleus to show electron configuration
Atoms overall are NEUTRAL (so p+ # = e- #) but….
○ Ions – atoms that have gained or lost electrons
Charge is determined by the different between # of protons and # of electrons
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To Do: Complete the yellow table by using your
periodic table in your reference tables
You may work with a partner
The first row is done for you….
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Isotopes Isotopes of an element have different #s of
neutrons thus also a different mass #
Yet….isotopes of the same element have identical chemical behaviors
Ex. Hydrogen isotopes:H-1H-2 (deuterium)H-3 (tritium)
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Examples:Find the number of neutrons in an atom of Se-79
A neutral atom with 6 electrons and 8 neutrons is an isotope of….
Note: a few isotopes are listed on Table N in your reference tables…..
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Atomic Mass The weighted average of the naturally
occurring isotopes of an element
Atomic mass unit (amu) – 1/12th the mass of a carbon-12 atom
In nature, most elements occur as a mixture of isotopes but one is more abundant than the others (which is the mass we estimate as a whole # from the periodic table)
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To calculate atomic mass, you need:# of stable isotopes of the element
Mass of each isotope (in amu units)
Natural percent abundance of each isotope
Multiply atomic mass of each isotope by its abundance, expressed as a decimal, then add the results
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Note:
Mass number is the whole number which is found by rounding the atomic mass on the periodic table for the element
Atomic mass is the average mass of all of the isotopes of the element
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Example #1:Chlorine has two isotopes: chlorine-35 and
chlorine-37 (75% and 25%)
Which should the weighted average be closer to?.....Cl-35
To calculate:
Relative abundance MassCl-35 .75 x 35 = 26.25Cl-37 .25 x 37 = + 9.25
35.50
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Example #2:Calculate the atomic mass of the two isotopes
of Boron: B-10 (19.78%) and B-11 (80.22%)
Relative abundance MassB-10 .1978 x 10 = 1.978B-11 .8022 x 11 = + 8.8242
10.8022
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Example #3:
Atomic mass can be calculated from the mass and abundance of naturally occurring isotopes. Carbon has two naturally occurring stable isotopes. Most carbon atoms – 98.89% - are C-12, while the remaining 1.108% are C-13. What is the atomic mass of carbon?
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Homework
Page 7 in REVIEW BOOK, questions #13-26- or-
Isotope Practice Worksheet
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Ions Overall, atoms are neutral UNLESS they are
considered ions…
An ion is an atom with a (+) or (-) charge
(+) ions contain more protons (e- is lost)
(-) ions contain more electrons (e- is gained)
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Examples:K+
Cl-
Mg2+
I-
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Atomic Orbitals In the Bohr and quantum mechanical models, energy
levels of electron are designated by (n) – the principal quantum number
Each principal quantum number refers to a major energy level or orbital, represented by rings around the atom
Assigned in order of increasing energy (n = 1, 2, 3, 4) as distance from the nucleus increases
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Atomic orbital – a region of space around the nucleus of an atom where there is a high probability of finding an electron
Letters are used to denote the shape:○ s orbital – spherical ○ p orbital – dumbbell-shaped○ d orbital – clover-leaf shapes○ f orbital – too complex to visualize
Nodes – in p and d orbitals, there are regions close to the nucleus where the probability of finding the electron is very low
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Summary of Principal Energy Levels, Sublevels, and Orbitals
PrincipalEnergy Level
Number ofSublevels
Type ofSublevel
Shape ofSublevel
n = 1 1 S
n = 2 2 s, p
n = 3 3 s, p, d
n = 4 4 s, p, d, f
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The maximum number of electrons that can occupy a principal energy level is given by the formula 2n2, where n is the principal quantum number
The number of electrons allowed in each of the first four energy levels are as follows:
Energy level n 1 2 3 4
Maximum number of electrons allowed
2 8 18 32
Increasing energy(Increasing distance from nucleus)
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Homework Draw orbital diagrams for each element
with an atomic number between 1-20 on a blank sheet of paper
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As (n) increases, all previous sublevels of lower orbitals must be included
Ground State – occurs when e- occupy the lowest available orbital
Electrons can gain or lose a specific amount of energy to move among atomic sublevels
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Excited State – occurs when e- absorb energy and temporarily move to a higher energy level
Heat, light, electricity allow e- to move into increasing energy levels
The excited e- quickly returns to a lower energy level, emitting the same amount of energy it absorbed in the form of light (Infrared, ultraviolet or visible)
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The light emitted is of multiple wavelengths (colors) and can be collected by a spectrometer to form a bright line spectra
○ Each bright line spectra is unique to a specific type of atom (can be used to identify elements!)
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Examples of e- in excited states
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Electron Configuration Distribution of electrons in an atom around
the nucleus
A complete electron configuration of an atom is shown by writing symbols for all the occupied sublevels in sequence
Shown on the periodic table for each element
Ex. Oxygen – 1s22s22p4 or 2-6
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Can be shown in shorthand notation:Coefficient represents the principal energy level
(n = 1, 2, 3, etc.)
Sublevel shape is designed by s, p, d, or f
Superscript represents the # of e- in that sublevel
The sum of the superscripts equals the number of electrons total in the atom
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Rules:
Electrons enter the orbitals of the lowest energies first
No more than two electrons can be placed into any orbital box
A single e- must be placed into each orbital box of a given sublevel before pairing takes place
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Order of sublevels
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10…
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Additional Info:s orbital can hold a max of 2 e-
p orbital can hold a max of 6 e-
d orbital can hold a max of 10 e-
f orbital can hold a max of 14 e-
So…… n =1 can hold just 2 e- since it contains an s orbital; n = 2 can hold 8 e- since is contains both an s and p orbital, n = 3 holds 18 e- because it has s, p, and d orbital…..etc.
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Example: Helium
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Example: Sulfur
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Example: Nickel
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Example: Calcium
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Ex. Try these!
Carbon
Sodium
Silver
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Ex. Try these!
Carbon - 1s2 2s2 2p2
Sodium - 1s2 2s2 2p6 3s1
Silver - 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d9
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Using the electron configuration, you can determine whether an atom is in its ground or excited states
Indicated if any lower energy sublevel is not completely full yet the next sublevel contains that e-
Ex. Which of the following is in an excited state?1s22s22p2
1s22s22p1
1s22s22p53s2
1s22s22p63s1
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Using the electron configuration, you can determine whether an atom is in its ground or excited states
Indicated if any lower energy sublevel is not completely full yet the next sublevel contains that e-
Ex. Which of the following is in an excited state?1s22s22p2
1s22s22p1
1s22s22p53s2
1s22s22p63s1
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Homework
Review book topic 1, read pages 12-15
Complete electron configuration worksheet
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Lewis Dot Structures Diagrams the show the valence
electrons of an element
Element symbol is used to represent the atom’s nucleus and all inner orbital electrons
Valance electrons are shown using small dots around the element’s symbol (to a max of 8 dots)
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Valence electrons - # of e- on the last ring (highest energy level)
Can be found using the element’s electron configuration
Within a group (vertical columns) on the periodic table, each element has the same # of valence electrons
Determines the element’s chemical properties
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Octet rule:
Atoms tend to achieve the electron configuration of a noble gas (8 valence e-)
Ions are formed to when an atom gains or loses electrons in order to reach this “octet”
○ Plays a role in ionic bonding (which we will learn about later)
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Examples:
K Br Al P
F Si Sn At
Xe In Ca Bi
H He