UNIT 3 MODULE 2 ATOMS: INSIDE OUT  Atoms were infinite in number, always moving and capable of joining together

 The atomic model has changed throughout the centuries, starting in 400 BC,
The History of Atomic Model when it looked like a billiard ball.

*******This theory was ignored and forgotten for more than 2000 years!*********
 WHY??
 >The eminent philosophers of the time, Aristotle and Plato, had a more
respected, (and ultimately wrong) theory.
 He believed all substances were made of small amounts of these four elements of
matter.
 >Aristotle and Plato favored the earth, fire, air and water approach to the
nature of matter. Their ideas held sway because of their eminence as
philosophers. The atomos idea was buried for approximately 2000 years.

Element Symbol
Fire triangle
Air ellipse
Water circle
Earth square

Dalton’s Model
 In the early 1800s, the English Chemist John Dalton performed a number of
DEMOCRITUS experiments that eventually led to the acceptance of the idea of atoms.

 LEUCIPPUS, originally came up with the atomic theory, but it was then Dalton’s Theory
adopted by Democritus o Elements are composed of atoms.
 This is the Greek philosopher Democritus who began the search for a description o Atoms are indivisible and indestructible particles.
of matter more than 2400 years ago. He asked: Could matter be divided into o This theory became one of the foundations of modern chemistry
smaller and smaller pieces forever, or was there a limit to the number of times a
piece of matter could be divided?
 His theory: Matter could not be divided into smaller and smaller pieces forever,
eventually the smallest possible piece would be obtained.
 This piece would be indivisible.
 He named the smallest piece of matter “atomos,” meaning “not to be cut.”
 To Democritus, atoms were small, hard particles that were all made of the same
material but were different shapes and sizes.
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Thomson’s Plum Pudding Model Rutherford’s experiment
In 1897, the English scientist J.J. Thomson provided the first hint that an
atom is made of even smaller particles. Most of the positively
charged “bullets” passed
Thomson Model right through the gold
 He proposed a model of the atom that is sometimes called the atoms in the sheet of gold
foil without changing
“PlumPudding” model. Atoms were made from a positively
course at all.
chargedsubstance with negatively charged electrons scattered about, like
raisins in a pudding. Some of the positively
 Thomson studied the passage of an electric current through a gas. charged “bullets,”
 As the current passed through the gas, it gave off rays of negatively however, did bounce away
charged particles. from the gold sheet as if
they had hit something
solid.

He knew that positive

charges repel positive
charges.

• This could only mean that the gold atoms in the sheet were mostly open
space.
• Atoms were not a pudding filled with a positively charged material.
• Rutherford concluded that an atom had a small, dense, positively charged
• Thomson concluded that the negative charges came
center that repelled his positively charged “bullets.”
from within the atom. • He called the center of the atom the “nucleus”
• A particle smaller than an atom had to exist. • The nucleus is tiny compared to the atom as a whole.
• The atom was divisible!
 Rutherford reasoned that all of an atom’s positively charged particles were
contained in the nucleus
Thomson called the negatively charged “corpuscles,” today
known as electrons. o Atoms are building blocks of matter
o Atoms are too small to study easily
Since the gas was known to be neutral, having no charge, he
reasoned that there must be positively charged particles in o Size of earth : matter = soda can : atom
the atom.

Rutherford’s Gold Foil Experiment

• In 1908, the English physicist Ernest Rutherford was hard at work on an
experiment that seemed to have little to do with unraveling the mysteries of the
atomic structure.

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 c) Electron
What is the structure of an atom? o has a negative (-) charge
NUCLEUS – center of the atom and it is the Home of Protons and Neutrons o has a relative mass of 0 (zero)
PARTICLE CHARGE MASS LOCATION
o are very small compared to a proton.
o determines the ion
Proton o found outside the nucleus
Neutron o Bohr model – electrons are in specific energy levels
o electron cloud model – electrons are in a random cloud
Electron
How are p+, e-, no related?
Number of protons=number of electrons
Number of Neutrons= atomic mass - atomic number

1. Determine the number of protons; neutrons and electron of Helium atom

 Eugen Goldstein No. of p+= _____ No. of no = ______ No. of e-= _______
o A German chemist was the first to observe the existence of protons 2. Determine the number of protons; neutrons and electron of Sodium
o He conducted an experiment using a modified cathode-ray tube and
observes not only the streams of electrons emitted but also the positive atom
rays (or canal rays) which are called the protons. No. of p+= _____ No. of no = ______ No. of e-= _______
a) Proton
o Has a positive (+) charge
o Has a relative mass of 1
o Determines the atomic number
o Found inside the nucleus
b) Neutron
 In 1932, James Chadwick discovered neutron
o has no charge
o has a relative mass of 1
o found inside the nucleus
o This particle bears neither a positive nor a negative charge, and its actual
mass of 1.675 x 10-24g is slightly greater than that of a proton.

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Using your periodic table, identify the no. of p+, no, e-of the following The maximum number of electrons that can occupy a principal energy level is given by the
elements: formula 2n2, where n is the principal quantum number.
ELEMENT SYMBOL ATOMIC ATOMIC p+ n0 e-
NUMBER MASS Electrons are arranged in Energy Levels or Shells around the nucleus of an
FLOURINE F 9 19 atom.
CALCIUM Ca 20 40 • first shell = a maximum of 2 electrons
• second shell = a maximum of 8 electrons
SULFUR S 16 32
• third shell = a maximum of 18 electrons
POTASSIUM K 19 39
• fourth shell = a maximum of 32 electrons
SILVER Ag 47 108
Remember
Bohr Model • An atom usually has a neutral charge. That means it has the same
number of protons as electrons
- In 1913, the Danish scientist Niels Bohr proposed an improvement.
• A proton has a positive charge and an electron has a negative charge
- In his model, he placed each electron in a specific energy level.
- According to Bohr’s atomic model, electrons move in definite orbits ION – an atom that has lost or gained one or more electrons and has become
around the nucleus, much like planets circle the sun. charged either positively or negatively
- These orbits, or energy levels, are located at certain distances from the  Atoms may gain or loose electrons.
nucleus. o When an atom LOSES electrons, it becomes more POSITIVE. Why?
o If you are getting rid of negative particles (electrons) but your number
 This model of the atom may look of positive particles (protons) is staying the same.
familiar to you.
o In other words, you are subtracting negative numbers
 This is the Bohr model.
 In this model, the nucleus is orbited by  Types of Ion
electrons, which are in different energy 1. Cations – positive ions; loses electrons (gives/donate)
levels. o When an atom LOSES electrons, it becomes more POSITIVE. Why?
 A model uses familiar ideas to o If you are getting rid of negative particles (electrons) but your number
explain unfamiliar facts observed in of positive particles (protons) is staying the same.
nature. o In other words, you are subtracting negative numbers
 A model can be changed as new 2. Anions – negative ions; gains electrons (receive/take)
information is collected. o When an atom GAINS electrons it becomes more NEGATIVE. Why?
o Electrons have a negative charge, so the more you have, the more
negative you become

Arrange the electrons of the following atoms:

3
Li –
8
O–
12
Ca –
4
Before losing an electron After losing an electron Determine the number of protons, Neutrons and electrons of isotopes of chlorine
11 11
Na – 2)e- 8)e- 1)e- Na – 2)e- 8)e-
No. of p+ = _____ No. of p+ = _____
No. of p+= _____ No. of p+= _____
No. of e- = _____ No. of e- = _____
Charge= _______ No. of no = ______ No. of no = ______
No. of e-= _______ No. of e-= ______

Identify the following as either Ions or isotopes of each other

1. B p+ = 5 ________________ B p+ = 5
n =5 n =6
e- = 5 e- = 5

EXAMPLES
2. N p+ = 7 ________________ N p+ = 7
n =7 n =7
1. What would be the charge be if: e- = 8 e- = 7
The neutral form of Gold(Au) lost 4 of its 79 electrons. It now has 79 protons and 75
electrons. 3. Ca p+ = 20 ________________ Ca p+ = 20
Before losing 4 electrons: no. of p+ = 79 no. of e- = 79 valence e = __ n = 22 n = 20
+
After losing 4 electrons: no. of p = 79 no. of e- =__ charge = __ e- = 20 e- = 20

2. What would be the charge be if:

The neutral form of Sulfur(S) gained 2 electrons. It now has 16 protons and 18 electrons.
Before gaining 4 electrons: no. of p+ = 16 no. of e- = 16 valence e = __
+
After gaining 4 electrons: no. of p = 16 no. of e- = charge = __ Complete the table below
3. The neutral form of Phosphorus gained 3 electrons. It now has 15 protons and
18 electrons. What is the charge of phosphorus ion? _______

Isotopes
 Atoms with the same number of protons, but different numbers of
neutrons.
 Atoms of the same element (same atomic number) with different mass
numbers.
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