1. CHEM 1305 Introductory Chemistry
“Introductory Chemistry: Concepts and Critical Thinking – 7th Edition,
Charles H. Corwin
Chapter 4. Models of Atoms
Modified by: Dr. Violeta F. Coarfa

2. Dalton Model of Atom
i. 5th century B.C.  Democritus: matter is made up from very small, indivisible particles = “atomos”; Plato and Aristotle didn’t accept this belief
ii  John Dalton: formulated precise definitions of “atomos” = atoms
-> Elements are composed of extremely small particles called
atoms = basic units of an element  they cannot be broken down by
chemical or physical processes.
-> All atoms of a given element are identical, having the same size, mass and
chemical properties. The atoms of one element are different from the atoms of
all other elements.
-> Atoms of different elements can form compounds, which can be broken down
into atoms by chemical processes.
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3. Chemistry Connection: John Dalton
Dalton began teaching at the age of 12 and developed an interest in science.
He was especially interested in meteorology and kept a lifelong daily journal of atmospheric conditions.
Although he was color blind, Dalton was able to make many great contributions to Chemistry, including his model of the atom and the law of definite proportions.
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4. Dalton Model of Atom – Summary
An element is composed of tiny, indivisible, indestructible particles called atoms.
All atoms of an element are identical and have the same properties.
Atoms of different elements combine to form compounds.
Compounds contain atoms in small whole number ratios.
Atoms can combine in more than one ratio for form different compounds.
The first two parts of atomic theory were later proven incorrect.
Proposals 3, 4, and 5 are still accepted today.
The Dalton theory was an important step in the further development
of atomic theory.
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5. Thomson Model of the Atom
1) 1808  John Dalton’s Atomic Theory: atoms are indivisible
2) Toward the end of the 1800s  evidence was seen that atoms were divisible.
Two subatomic particles were discovered.
Negatively charged electrons, e–, with relative charge of -1.
Positively charged protons, p+, with relative charge of +1
3) 1903: J. J. Thomson proposed a subatomic model of the atom
Thomson proposed that the electrons were distributed evenly throughout a homogeneous sphere of positive charge.
This was called the plum pudding model of the atom.
Originally, Thomson could only calculate the mass-to-charge
ratio of a proton and an electron.
4) 1911: Robert Millikan determined the charge of an electron
Thomson calculated the masses of a proton and electron:
An electron has a mass of 9.11 × 10–28 g and A proton has a mass of 1.67 × 10–24 g.

6. Rutherford Model of the Atom
Rutherford’s student fired alpha particles at thin gold foils. If the plum pudding model of the atom (Thomson’s model) was correct, α particles should pass through undeflected.
However, some of the alpha particles were deflected backward.
Most of the alpha particles passed through the foil because an atom is largely empty space.
At the center of an atom is the atomic nucleus, which contains the atom’s protons.
The alpha particles that bounced backward did so after striking the dense nucleus.
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7. Rutherford Model of the Atom
Rutherford proposed a new model of the atom:
The negatively charged electrons are distributed around a positively charged nucleus.
An atom has a diameter of about 1 × 10–8 cm and the nucleus has a diameter of about 1 × 10–13 cm.
If an atom were the size of the Superdome, the nucleus would be the size of a marble.
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8. Rutherford Model of the Atom *
Based on the heaviness of the nucleus, Rutherford predicted that it must contain neutral particles in addition to protons.
Neutrons, n0, were discovered by James Chadwick. A neutron is about the size of a proton without any charge.
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9. Atomic number, Mass number and Isotopes
Atomic number (Z) = number of protons in nucleus (= number of electrons too)
Mass number (A) = number of protons + number of neutrons
= atomic number (Z) + number of neutrons
Isotopes are atoms of the same element (X) with different numbers of
neutrons in their nuclei (same Z, same number of
protons and electrons, different number of neutrons)
Mass Number X A Z
Atomic Notation
Element Symbol
Atomic Number H 1
H (D) 2
H (T) 3 U
235 92
238
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10. The Isotopes of Hydrogen Element
How many protons, neutrons and electrons does
each isotope have?
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11. Learning Check How many protons, neutrons, and electrons are in C ?14 ?
Solution: 6 protons, 8 (14 - 6) neutrons, 6 electrons
How many protons, neutrons, and electrons are in C 11 ?
Solution: 6 protons, 5 (11 - 6) neutrons, 6 electrons
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12. Learning Check
Given the atomic notation for the following atoms, draw a diagram showing
the arrangement of protons, neutrons, and electrons.
a b.
Solution:
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13. Average Atomic Mass of an Element
Atomic Mass – mass of one atom; unit – amu (atomic mass unit)
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14. Average Atomic Masses of Elements
For an element with two isotopes, Y1 and Y2:
Example:
Atomic mass
Abundance
Cl-35
34.97 amu
75.53%
Cl-37
36.97 amu
24.47%
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15. Learning Check Mg-24 m = 23.985 78.70% atomic mass of Mg = 24.31amu
Determine the atomic mass of magnesium element.
Given:
isotope
Isotope atomic mass (amu)
abundance
Mg m = %
Mg m = %
Mg m = %
x 78.7% x 10.13% x 11.17%
atomic mass of Mg = 24.31amu
Solution:
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16. Learning Check ++++
Copper has two naturally occurring isotopes: Cu-63 with a mass of amu and a natural abundance of % and Cu-65 with a mass of amu and a natural abundance of %. Calculate the atomic mass of copper.
x 69.17% x 30.83% = amu
as the weighted average found in nature.
Solution:
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17. Bohr Model of the Atom
Niels Bohr speculated that electrons orbit about the nucleus in fixed energy levels.
Electrons are found only in specific energy levels,
and nowhere else.
The electron energy levels are quantized.
The first energy level has one sublevel designated 1s.
The second energy level has two sublevels designated 2s and 2p.
The third energy level has three sublevels designated 3s, 3p, and 3d.
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18. Energy levels, Sublevels and Orbitals
Energy levels occupied by electrons have assigned values:
n – principal quantum number: 1,2, 3… (integers)
n -> E electron
Each energy level has a maximum number of electrons: 2n2
Each energy level has one or more sublevels; electrons have identical
energy in the sublevels of the same level
-> sublevels: s < p < d < f
Increase of energy in sublevels
n = 1 -> 2 electrons max.; only s sublevel: 1s
n = 2 -> 8 electrons max.: s and p sublevels: 2s, 2p
n = 3 -> 18 electrons max.: s, p and d sublevels: 3s, 3p, 3d
n = 4 -> 32 electrons max.; s, p, d and f sublevels: 4s, 4p, 4d, 4f
Magnetic field
Pauli exclusion principle: no more than 2 electrons in an orbital

19. Orbital – region with the highest probability to find an electron
max. 6 electrons
max. 2 electrons
max. 10 electrons
max. 14 electrons
s sublevel – 1 s orbital, which is like a sphere: 1x2=2 electrons
p sublevel – 3 p orbitals, each has 2 lobes and is oriented
in three different directions: 3x2=6 electrons
d sublevel – 5 d orbitals: 5x2=10 electrons
f sublevel – 7 f orbitals: 7x2=14 electrons
The size of an orbital increases
with the energy level
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20. Orbital – region with the highest probability to find an electron
s-orbitals
p-orbitals
d-orbitals
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21. Orbitals Center = nucleus Example: Neon has 10 electrons 1s22s22p6
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22. Electron configurations
Sublevels are filled with electrons in
order of increasing energy
(Z = 30)
Homework:
Write a complete electron
configuration for all elements
with Z=1 to Z=24
In red – electronic configuration
In green – orbital diagram
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23. Electronic configurations
1. Pauli’s exclusion principle: no more than two electrons are found in an orbital. If two electrons occupy an orbital, they must have different spins (1/2 and -1/2)
2. Hund’s rule: electrons will be placed on each orbital, then they will be paired.
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24. Electron Configurations
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25. Sublevel blocks in the periodic table
Electronic configuration of elements is related to their positions in the periodic table
The s-block elements:
- H, He, Gr1A and Gr2A
- the last 1 or 2 electrons are in a s sublevel
- number of the period, x, indicates the xs sublevel
The p-block elements:
- Gr13A – Gr18A
- the last 1,2, electrons are in a p sublevel
- number of the period, x, indicates the xp sublevel
The d-block of elements:
- transition groups
- the last 1-10 electrons are in a d sublevel
- number of the period, x, indicates the (x-1)d sublevel
The f-block elements:
- the lanthanides and actinides
- the last 1-14 electrons are in a f sublevel
- the number of period, x, indicates the (x-2)f sublevel

26. Sublevel blocks in the periodic table
HW – draw the electronic configuration for the following elements:
Z = 37; Z = 65; Z = 77; Z = 85
(both long and abbreviated configurations)
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27. Group 1 Abbreviated Electron Configurations
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28. Review Chapter 4 Dalton Model of Atom. Thompson Model of Atom
Rutherford Model of the Atom
Atomic number. Mass Number. Isotopes.
Bohr Model of the Atom. Energy levels. Energy sublevels. Orbitals.
Electron configurations. Orbital diagrams. Pauli’s exclusion principle. Hund’s rule.
Blocks of elements: s, p, d and f
Dr. Coarfa - Chem1305