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The Atom
Vocabulary
Element
Compound
Mixture
Chemical Symbol
Chemical Reaction (RXN)
Law of Conservation of
Mass
Atom
Proton
Electron
Neutron
Atomic Mass
Atomic Mass Unit
Isotope
Ion
Weighted average
Objectives
1. Differentiate between elements, compounds and
mixtures.
2. Understand elements are represented by chemical
symbols.
3. Define atom.
4. Distinguish between the subatomic particles in terms
of relative charge and mass.
5. Draw a Bohr Model of an atom. Describe the
structure of the atom, including the location of
subatomic particles.
6. Define atom, isotope and ion.
7. Calculate the number of electrons, protons and
neutrons in an atom.
8. Calculate the mass of an atom.
9. Explain and Calculate a weighted average.
Classifying Substances
Elements cannot be broken down or changed
into smaller substances by chemical means.
–Elements are the simplest forms of matter that can
exists in normal laboratory conditions.
Chemical Symbols
Chemical symbols are a shorthand for the elements.
A Chemical Symbol…
-begins with a capitalized letter. The first letter
MUST be capitalized to denote an element!
-any additional letters are lowercase.
-Some symbols do not make sense. This is
because the symbol is derived from the Greek or
Latin name for the element such as: aurum or gold
has the symbol Au
History, Theory and Structure of the ATOM
The smallest particle of an element is an atom.
-Atoms contain 3 subatomic particles.
-The development of our atomic model occurred
throughout most of human history.
Before learning history, we must first understand:
•A model is a verbal, visual, or mathematical
explanation of experimental data.
•A theory is an explanation that has been repeatedly
supported by many experiments
–A theory states a broad principle of nature that has been
supported over time by repeated testing.
–Theories are successful if they can be used to make
predictions that are true.
History, Theory and Structure of the ATOM
A scientific law is a relationship in nature that is
supported by many experiments, and no exceptions to
these relationships are found.
The law of conservation of mass states that mass is
neither created nor destroyed in a chemical reaction, it is
conserved.
History, Theory and Structure of the ATOM
Greek Philosophers postulated what matter was
composed of and took one of 2 positions:
- Some believed matter was composed of such
things as earth, water, air, and fire.
- Others believed matter
could be endlessly divided
into smaller and smaller
pieces.
History, Theory and Structure of the ATOM
Two philosophers that had opposing views were:
• Democritus (460–370 B.C.)
– He was the first person to propose the idea that matter
was not infinitely divisible, but made up of individual
particles called atomos.
• Aristotle (484–322 B.C.)
– He disagreed with Democritus because he did not
believe empty space could exist.
Aristotle’s views went unchallenged for 2,000 years
until science developed methods to test the validity of
his ideas.
History, Theory and Structure of the ATOM
A man named John Dalton returned to Democritus’s
belief of atoms.
Dalton developed the “Atomic Theory” which
explained conservation of mass in a reaction as the
result of the combination, separation, or
rearrangement of atoms.
History, Theory and Structure of the ATOM
Dalton’s theory stated the following beliefs:
– Matter is composed of extremely small particles: atoms
– Atoms are indivisible and indestructible.
– Atoms of a given element are identical in size, mass and
chemical properties.
– Atoms of a specific element are different from those of
another element.
– Different atoms combine in simple whole-number ratios
to form compounds.
– In a chemical reaction, atoms are separated, combined
or rearranged.
History, Theory and Structure of the ATOM
The cathode ray tube:
−Within a ray tube, an electric charged is applied. A
ray of radiation travels from the cathode to the anode.
The ray is negative.
−In 1886, Eugen Goldstein discovered that there was
an opposing ray that contained a positive charge.
History, Theory and Structure of the ATOM
While studying negative cathode rays, J.J.
Thomson found:
− the charge-mass ratio of the negative particles making
up the rays.
− the mass of this particle was much less than a
hydrogen atom.
− Thomson received the Nobel Prize in 1906 for
identifying the first subatomic particle—the electron
History, Theory and Structure of the ATOM
Using the information that matter is neutral, Thomson
developed the “plum pudding model”
-a uniform, positively charged sphere containing
electrons.
History, Theory and Structure of the ATOM
In 1911, while studying how positively charged alpha
particles interact with solid matter, Ernest Rutherford
discovered the nucleus.
The Gold-foil Experiment:
Hypothesis: By aiming the particles
at a thin sheet of gold foil the paths
of the alpha particles should be only
slightly altered by a collision with
an electron.
History, Theory and Structure of the ATOM
The Gold-foil Experiment:
Procedure: A lead block was used to emit alpha-particles
towards a piece of gold-foil. A zinc-sulfide coated screen
around the foil was used as a detector.
Results: Most of the alpha particles went through the foil,
HOWEVER a few of them bounced back at large angles!
Conclusion:
-atoms are mostly empty space
-almost all of the atom’s positive
charge and mass are in the
center of an atom, the nucleus
-Electrons are held in the atom
by the positively charged
nucleus
History, Theory and Structure of the ATOM
The Gold-foil Experiment:
Conclusion:
-atoms are mostly empty space
-almost all of the atom’s positive charge and mass are
in the center of an atom, the nucleus
-Electrons are held in the atom by the positively
charged nucleus
This led to the development of Rutherford’s Model:
Rutherford refined the model to include
positively charged particles in the
nucleus called protons.
History, Theory and Structure of the ATOM
In 1913, Niels Bohr introduced a new model, the
Rutherford-Bohr model (Bohr Model). He added a
quantum physical property. The electrons were now
organized in energy levels.
The new model:
-contains a small positively
charged nucleus
-electrons are organized into
energy levels
History, Theory and Structure of the ATOM
James Chadwick received the Nobel Prize in 1935 for
discovering the existence of neutrons, neutral particles
in the nucleus which accounts for the remainder of an
atom’s mass.
This led to the development of the next model:
− All atoms are made of three fundamental subatomic
particles: the electron, the proton, and the neutron.
− Atoms are spherically shaped.
− Atoms are mostly empty space, and
electrons travel around the nucleus
held by an attraction to the positively
charged nucleus.
History, Theory and Structure of the ATOM
After more research in quantum mechanics, our current
model, the quantum mechanical model, was developed.
This can also be called a valence shell atom.
In this model:
-Electrons still exist in an e- cloud
-the cloud is defined by electron probability shells
-the electrons do not travel in a fixed path
Quantum
Mechanical
Model
Subatomic Particles in an Atom
The atomic number is the number of protons (p+) in the
nucleus and identifies the element.
−Each element contains a unique positive charge in their
nucleus.
−All atoms of a particular element have the same
number of protons and electrons (e-).
−The number of neutrons (n0) in the nucleus can differ.
−The mass number is the sum of the protons and
neutrons in the nucleus.
Subatomic Particles in an Atom
Practice Problems: For each of the following calculate
the number of protons, electrons and neutrons.
1. Lithium (Mass: 7)
2. Sodium (Mass: 23)
3. Phosphorus (Mass: 31)
4. Iron (Mass: 56)
Subatomic Particles in an Atom
Isotopes are atoms with the same number of protons
(p+) but different numbers of neutrons (n0).
− Isotopes containing more neutrons will have a greater mass
− Isotopes of an element will share chemical behavior.
Practice problem: Calculate the number of p+ and n0 for
Carbon (Mass 14)
Carbon (Mass 12)
When noting that the element used is an isotope, two
different notations can be used: Carbon (Mass 13)
Hyphen notation: Carbon-13 or Nuclear Symbol:
Subatomic Particles in an Atom
History, Theory and Structure of the ATOM
How to draw a modified Bohr’s model:
1. Draw the nucleus with both the protons and
neutrons.
2. Around the nucleus, draw electron orbits.
3. Fill the orbits according to the number of
electrons allowed in each.
1. The number of electrons in each energy level can
be determined by counting how many elements
are in each row of the periodic table.
2. It turns our that the first 4 energy levels hold the
respective numbers of electrons: 2, 8, 8, 18
Subatomic Particles in an Atom
An ion is an atom that is charged.
− This occurs by the addition or loss of an electron.
− Ions will only gain or lose electrons…
LEAVE THE PROTONS ALONE!!!!!!!!!!!!!!!!!!
A cation or positive ion is achieved by the loss of
electrons
– smaller and usually metals
An anion or negative ion is achieved by gaining
electrons
– Larger and usually non-metals
Practice Problems: Count the # of p+ & e− in each ion.
Mg+2 p+ = _____
F−1 p+ = _____
e− = ______
e− = ______
Mass of Atoms
One atomic mass unit (amu) is defined as 1/12th the
mass of a carbon-12 atom.
− One amu is nearly, but not exactly, equal to one
proton or one neutron.
− The atomic mass of an element is the weighted
average mass of the isotopes of that element.