E-27
ATOMIC STRUCTURE I
Prerequisite to NGSS: HS: PS1-1 Use the periodic table as a model to predict the relative properties of
elements based on the patterns of electrons in the outermost energy level of atoms. CC Patterns. SEP
Developing and Using Models.
Our version of the atom owes its name to a Greek, Democritus, and its conceptual model to early 20th
century physicists. In the early 1800's, Dalton proposed the first modern theory of the atom. He suggested
that the atom is an indivisible, solid sphere. This meant that the atom itself was the smallest particle of
matter! At the beginning of the 20th century Thomson proposed a new model, called the plum pudding
model, which described a positively charged sphere with electrons embedded in it. Soon, thanks to the work
of Rutherford, this model gave way to the planetary system model, which proposed a dense, positive
center (the nucleus), with electrons orbiting around it. The Bohr model represented a more sophisticated
understanding of the three major elementary subatomic particles (protons, electrons, and neutrons) and how
they function.
In E-25 - "Electrostatics", you observed the interactions between neutral & charged particles (aluminum
balls). These electrostatic forces operate in the world of the atom and govern the way in which the three
elementary particles relate to each other and among themselves.
People have difficulty with the size of an atom. It's so infinitesimally small that we neglect it. But if we
could expand an atom's size to fit our visible world, we would find it impossible to ignore. If the nucleus of
an atom were the size of a grape, the electrons would be about one mile away! Or put another way, if an
atom were the size of the Astrodome, the nucleus would be about as big as a fly at the center. It's pretty
obvious that most of an atom is empty space. That was Rutherford's conclusion too, after he conducted his
gold foil experiments. Rutherford's view of the atom included a dense, positively charged center (nucleus)
which balanced the negatively charged electrons that moved about outside the nucleus. He proposed that the
nucleus contained protons, a positively charged particle having an equal but opposite charge to the electron.
A short time later, Rutherford and Chadwick showed that the atomic nucleus also contained neutrons, a
particle with no charge but having the same mass as a proton.
There are over 112 elements, all made of unique atoms. The numbers of each kind of particle in an atom of
one element make it different than atoms of different elements. The Periodic Table describes the number of
subatomic particles that make up each element! In the E-14 – “Molar Mass” study sheet, you were
introduced to the Periodic Table of the Elements. On the Periodic Table each element has its own square
containing a one, two, or three letter symbol for that element and two important pieces of quantitative data
about each element - its atomic number and its mass number.
The atomic number tells us how many protons all atoms of an element contain. In a free atom, the positive
and negative charges are equal so that the net charge is zero. Therefore, in an atom the number of protons
and electrons are equal. When an atom loses or gains electrons it is called an ion. In this case, the number
of electrons differs from the number of protons, and must be calculated.
The mass number represents the sum of the protons and neutrons, those particles that contribute weight
to an atom. Electrons have so little mass (1 / 1837 times the mass of the proton) that we neglect it. Thus, if
we subtract the atomic number from the mass number, we can determine the number of neutrons. The mass
of one proton or one neutron is equal to one a.m.u. (atomic mass unit). Isotopes are atoms of an element
that have a different number of neutrons.
All elements are described in terms of their symbol, atomic number and mass number. They are written in
the following manner, called a nuclide symbol:
A
Z
Where
Example 1:
X
A
Z
X
=
=
=
40
20
the symbol for the element
the mass number (P + N), and
the atomic number (P)
Ca
This nuclide symbol tells us that calcium (Ca) has an atomic number of 20 & a mass number of 40.
Therefore,
# of protons
= 20
(Z)
# of electrons = 20
(Z)
# of neutrons = 40 - 20 = 20
(A-Z)
Next we will look at an ion. Remember that an ion has a non-zero charge and may be positive or negative
depending on whether it has gained or lost an electron. Positive ions, atoms that have lost electrons, are
called cations, and have fewer electrons than protons. Negative ions, atoms that have gained electrons, are
called anions, and have more electrons than protons.
Metals (those elements to the left of the zigzag line on the Periodic Table) tend to give up electrons and form
cations to become stable. Non-metals (those elements to the right of the zigzag line on the Periodic Table)
tend to collect electrons and form cations to become stable
Example 2:
40
20
Ca+2
This nuclide symbol represents the calcium cation. The ion has a +2 charge because it has lost 2 electrons.
Therefore,
# of protons
= 20
(Z)
# of electrons = 20 - 2 = 18
(Z-2)
# of neutrons = 40 - 20 = 20
(A-Z)
Next we will look at isotopes. As mentioned above, these are atoms the have a different number of
neutrons, but have the same number of protons.
Example 3:
39
20
Ca
This calcium (Ca) atom has a mass number of 39 instead of 40.
Therefore,
# of protons
= 20
(Z)
# of electrons = 20
(Z)
# of neutrons = 39 - 20 = 19
(A-Z)
Notice that the number of protons is the same as in the previous two examples! Remember that the atomic
number is the subatomic particle that defines the element - the electrons or neutrons may be different, but the
number or protons is ALWAYS the same!
PRACTICE PROBLEMS
1.
Name 8 scientists who worked towards developing the modern model of the atom.
2.
Complete the following table:
Particle
Electron
Proton
Neutron
3.
Location
Sodium
(Na)
Nitrogen
(N)
Barium
(Ba)
Boron
(B)
5.
Charge
Constant or variable?
Complete the following table:
Name
4.
Mass
Nuclide
symbol
Atomic
number
11
Mass
number
23
5
11
Protons
Electrons
Neutrons
15
7
136
56
N
Ba
Complete the following table:
Name
Neutrons
Protons
Electrons
Chlorine (Cl)
18
Atomic #
(Z)
17
Neon (Ne)
10
10
Helium (He)
2
2
Sodium (Na)
12
11
Calcium (Ca)
21
20
Carbon (C)
8
6
Mass #
(A)
Nuclide
Symbol
Complete the following table
Name
Sodium (Na)
ion
Oxide (O) ion
Nuclide symbol
23
+1
11
Protons
Electrons
Neutrons
8
10
8
Nitride (N) ion
7
10
7
Magnesium
(Mg ) ion
12
10
12
Na
6.
Write the atomic symbol (X) for each of the isotopes described below:
a.
Z = 8, number of neutrons = 7
b.
the isotope of chlorine in which A = 37
c.
Z = 27, A = 60
d.
the number of protons = 26, the number of neutrons = 31
e.
the isotope of iodine with a mass number of 131
f.
Z = 16, the number of neutrons = 18
g.
the isotope of aluminum with 14 neutrons in its nucleus
h.
the isotope of sulfur with a mass number of 32
Multiple choice: Choose one correct answer.
7.
For an atom to have a net charge of zero
a.
the number of neutrons and protons must be the same
b.
the number of neutrons and electrons must be the same
c.
the number of protons and electrons must be the same
d.
the number of all three particles must be the same
8.
Isotopes of a given element have identical
a.
atomic numbers
b.
mass numbers
c.
number of neutrons
d.
atomic weights
9.
A particle having 11 protons, 12 neutrons, and 10 electrons has
a.
no charge
b.
a charge of +1
c.
a charge of -1
d.
a charge of +2