UNIT 3
WHAT IS THE BASIC STRUCTURE OF THE ATOM? (pp 52 - 53)
Atomic Theory
1. The idea that matter is made from atoms can be traced to the early Greek philosophers.
2. The first theories were explanations based on their own life experiences so they concluded that matter
was composed of air, earth, fire, and water.
3. One philosopher, ____________________, believed otherwise and proposed that matter was composed
of tiny particles called atomos (atoms). His ideas were:
- Matter is composed of empty space through which atoms move.
- Atoms are solid, homogeneous, indestructible and indivisible.
- Different kinds of atoms have different sizes and shapes.
- The differing properties of matter are due to the size, shape and movement of atoms.
- Apparent changes in matter result from changes in the groupings of atoms and not from changes in the
atoms themselves.
4. Unfortunately, Aristotle did not agree. Since he was more influential the ideas of Democritus were
discarded and the air, earth, fire and water theory lasted much longer.
Evidence Supporting Atomic Theory
1. Observations of compounds and chemical reactions led to three laws that describe how compounds are
formed.
2. _________________________ states that a compound contains the same elements in exactly the same
proportions by mass regardless of the size of the sample or source of the compound.
3. _________________________ states that when two or more elements react to produce a compound, the
total mass of the compound is the same as the sum of the masses of the individual elements.
4. _________________________ applies to different compounds made from the same elements. The mass
ratio for one of the elements that combines with a fixed mass of the other element can be expressed in small
whole numbers.
OVER
John Dalton
1. Showed that, collectively, the laws demonstrate the existence of atoms. He argued that these laws could
not hold true if atoms did not exist.
2. This led to the development of Dalton’s Atomic Theory:
- all matter is made up of indivisible and indestructible atoms.
- atoms of a given element are identical in their physical and chemical properties.
- atoms of different elements have different physical and chemical properties.
- atoms of different elements combine in simple, whole number ratios to form chemical compounds.
- atoms cannot be subdivided, created or destroyed when they are combined, separated or rearranged in
chemical reactions.
THE STRUCTURE OF THE ATOM (pp. 55 - 57)
Electrons
1. Scientists studying electrical current discovered that current would flow from the negative cathode to the
positive cathode in a tube that was a near vacuum.
2. The remaining gas would glow and the beam was capable of moving a small paddle wheel.
3. This suggested the cathode ray consists of small individual particles.
4. J.J. Thomson studied the cathode ray. He discovered magnetic and electrical fields could deflect the
rays. By carefully measuring the effect of the fields, Thomson calculated the ____________________ of
the charged particles.
5. Thomson compared his results with other known information and found that his particles were smaller
than an atom. He had identified the first subatomic particle: the ____________________.
6. In 1909, Robert Millikan conducted an experiment, known as the _________________________. This
experiment determined the actual charge on the electron. This was set at __________.
7. He then used this information and Thomson’s work to calculate the actual mass of an electron.
8. Since matter is neutrally charged, atoms must be neutral. But if all atoms contain electrons, how could
this be?
9. Thomson theorized that the atom was a ball of positive charge with the negatively charged electrons
embedded in the sides. This model became known as the _________________________.
Nucleus
1. In 1911, Ernest Rutherford and a group of scientists set out to study how alpha would interact with solid
matter. Alpha particles are radioactive and consist of the nucleus of the helium atom. Therefore, the
particle has a positive charge.
2. They decided to fire a beam of alpha particles at a thin sheet of gold foil. A screen coated with zinc
sulfide surrounded the foil. The thought was that the particles would cause a flash of light on the screen
when they struck it.
3. Rutherford, knowing about Thomson’s model, thought the particles would be deflected slightly, if at all.
4. He discovered that some of the particles had very large deflections. Some even came straight back
toward the source of the alpha particles.
5. Because of this he concluded the plum pudding model was incorrect and set out to develop a new
model.
6. Rutherford’s model states the atom is mostly empty space through which the electrons travel. He
concluded there is a tiny, dense region of positive charge, which he called the _________________. The
electrons travel around the nucleus.
7. Rutherford also concluded that most of the atom’s mass resides in the nucleus.
Protons and Neutrons
1. Rutherford refined his model and concluded the nucleus contained positively charged particles called
_______________.
2. The proton’s charge is equal to, but opposite that of an electron.
3. Scientists still had a dilemma because the mass of the known elements, except hydrogen, had been
found to be heavier than that predicted by the protons and electrons.
4. In 1932, James Chadwick showed that the nucleus contained another particle. This particle had the
same mass as the proton, but had no charge. It was called a _______________.
PROPERTIES OF SUBATOMIC PARTICLES
PARTICLE
SYMBOL
LOCATION
RELATIVE
CHARGE
RELATIVE
MASS
ACTUAL
MASS (g)
electron
proton
neutron
HOW ATOMS DIFFER
Atomic Number
1. Not long after Rutherford’s gold foil experiment, one of his fellow scientists beamed X rays at samples
of the elements and discovered that the wavelength of the resultant X rays decreased with increasing atomic
weight.
2. This scientist, Henry Moseley, concluded that the number of electrons around the nucleus must have
been increasing. If this were so, then the number of protons in the nucleus must also increase in order to
maintain a neutral charge.
3. Atoms of different elements contain a unique positive charge and therefore a unique number of protons
in the nucleus.
OVER
4. The number of protons in the nucleus is referred to as the _________________________.
5. If one knows the atomic number, one also knows the number of electrons present since the atom is
neutral.
Isotopes and Mass Number (pp. 60 – 63)
1. While the number of protons in an atom is fixed, the number of neutrons in the nucleus can vary. Even
for the same element.
2. Atoms of the same element that have the same umber of protons, but different neutrons are called
____________________.
3. Despite this, isotopes still have very similar chemical properties.
4. To make it easier to identify each of the various isotopes of an element, chemists add a number after
each’s name. The number is called the _________________________.
***** Determine the mass number for the isotope of potassium containing 19 protons and 20 neutrons.
Write its name.
***** Determine the mass number of the isotope of mercury containing 80 protons and 124 neutrons.
Write its name.
5. An isotope can also be represented by a symbol. This symbol is known as an ____________________.
It looks like:
***** Write the isotopic symbol for each of the following:
uranium-238
iron-57
calcium-46
***** One of the most harmful components of nuclear waste is a radioactive isotope of strontium-90. It
can be deposited in your bones where it replaces the calcium. Write the isotopic symbol for strontium-90.
How many protons are in the nucleus? How many neutrons?
Mass of an Atom
1. Because the actual mass of a proton, neutron and electron are so small, the actual mass of an atom is a
very small number.
2. These number are hard to work with so chemists developed a method of measuring the mass of an atom
relative to the mass of a specifically chosen atomic standard.
3. The standard is the ____________________ atom. Scientists assigned the carbon-12 atom a mass of
exactly 12 atomic mass units.
4. One _________________________ is equal to _______ the mass of the carbon-12 atom.
5. The atomic masses of the elements are listed on the periodic table, but they are not whole numbers.
6. This is because the _______________ of an element is the weighted average mass of that element.
7. Weighted averages depend on two factors: the __________ and the ____________________ of each
isotope:
***** Calculate the average atomic mass for silicon. Silicon has three naturally occurring isotopes:
silicon-28
silicon-29
silicon-30
27.98 amu
28/98 amu
29.97 amu
92.21%
4.700%
3.090%
***** Calculate the atomic mass of oxygen. Oxygen has three naturally occurring isotopes:
oxygen-16
oxygen-17
oxygen-18
15.99 amu
17.00 amu
18.00 amu
99.76%
0.03800%
0.2000%
OVER