Download If each orbital contains two electrons, the second energy level can

If each orbital contains two electrons, the second energy level can have four
orbitals: one s orbital and three individual p orbitals. These three p orbitals are
energetically equivalent to each other and are labeled 2px , 2 py and 2 pz to
indicate their orientation in space. The symbols 3s2, 3p6, and 3d10 illustrate
the sublevel breakdown of electrons in the third energy level. From this line of
reasoning, we can see that if there are sufficient electrons, f electrons first
appear in the fourth energy level. Table 4 shows the type of sublevel electrons
and maximum number of orbitals and electrons in each energy level.
Table 4 Sublevel electrons in each principal energy level and the maximum number of orbitals and
electrons in each energy level.
1
2
3
4
5
*
6
7
K
L
M
N
O
P
Q
Sublevel
Electrons
s
s.p
s.p.d
s.p.d.f
s.p.d.f
s.p.d
s
Maximum
Number of
Orbitals
1
4
9
16
Incomp
Incomp
Incomp
lete
lete
lete
Total Number
of Electrons
2
-
-
-
Principal
energy Level
*
8
18
32
*
*
Insufficient electrons to complete the shell.
Since the s p d f atomic orbitals have definite distribution in space, they
are represented by particular spatial shapes. We will consider only the s
and p orbilals, the d and f being much more complicated geometrical
patterns. The s orbitals are spherically symmetrical about the nucleus. A 2s
Page 16
Atomic Theory
orbital is a larger sphere than a 1s orbital. The p orbilals (px , py and pz) are
dumbbell-shaped and are oriented at right angles to each other along the x,
y and z axes in space. The electron has equal probability of being located
in either lobe of the p orbital. The boundaries of the orbitals enclose the
region of greatest probability (about 90 percent chance) of finding an
electron. In the ground state of a hydrogen atom this falls within a sphere
having radius of 0.53 Ao
The elements are numbered consecutively from 1 to 106, coinciding with
the number of protons in the nucleus. Hydrogen, element number I, has
one proton in the nucleus; helium, number 2, has two protons;
magnesium, number 12, has twelve protons in the nucleus. The atomic
number of an element is the same as the number of protons in the nucleus.
It tells us the amount of positive electrical charge in the nucleus and also
the number of electrons in the neutral atom.
The hydrogen atom, consisting of a nucleus containing one proton and an
electron, orbital containing one electron, is the simplest known atom.
(Some hydrogen atoms-are known to contain one or two neutrons in their
nucleus.
Page 17
Atomic Theory
The electron in hydrogen occupies an s orbital in the first energy level.
The electron does not move in any definite path but rather in a random
motion within its orbital, forming an electron cloud about the nucleus. The
diameter of the nucleus is believed to be about 10
-13
cm, and the diameter
of the electron orbital to be about 10-8 cm, The orbital occupied by the
electron is considerably larger than the nucleus.
What we have, then, is a positive nucleus surrounded by an electron
cloud formed by an electron in an s orbital.
The net electrical charge on the hydrogen atom is zero; it is called a neutral
atom. Figure 3 shows two methods of representing a hydrogen atom.
Figure 3 (a) shows a discrete electron moving around its nucleus; Figure 3
(b) shows the electron orbital surrounding the nucleus.
a)
b)
The hydrogen atom, (a) Represents the Bohr description, indicating a discrete
electron moving around its nucleus of one proton, (b) Represents the quantummechanical concept, showing the electron orbital as a cloud" surrounding the
nucleus.
Page 18
Atomic Theory
Experimental work done soon after the Bohr-Rutherford concept of the
atom was established showed that the masses of nearly all atoms were
greater than could be accounted for by simply adding up the masses of all
the protons and electrons that were known to be present. This fact led to
the concept of the neutron, a particle with no charge but with a mass
about the same as that of a proton. Since this particle has no charge, it
was very difficult to detect, and the existence. Of the neutron was not
proven experimentally until 1932. All atomic nuclei except that of the
simplest hydrogen atom are now believed to contain neutrons.
The atomic number corresponds to the number of protons in the nucleus
and identifies an atom as being a particular kind of element. Hence, all
atoms of a given element must have the same number of protons. But
experimental evidence showed that, in most cases, all the atoms of a given
clement did not have identical masses. These mass differences exist
because all of the nuclei of a given element do not contain the same number
of neutrons.
Atoms of an element having the same atomic number but different atomic
masses are called isotopes of that clement. Atoms of the isotopes of an
element, therefore, have the same number of protons and electrons but
different numbers of neutrons.
There are three known isotopes of hydrogen. Each is atomic number 1
and has one proton in the nucleus and one electron in the first energy
Page 19
Atomic Theory
level. The first isotope (protium) does not have any neutrons in the
nucleus, giving it a mass of 1; the second (deuterium) has one neutron in
the nucleus, giving it a mass of 2; the third (tritium) has two neutrons,
giving it a mass of 3. The atomic structures of the isotopes of hydrogen are
shown in the next Figure. These isotopes may be represented by the
symbols 1H, 2H and 3H, indicating an atomic number of 1 and a mass of 1, 2,
and 3 mass units, respectively. This method of representing atoms is called
isotopic notation. The subscript number to the left of the symbol indicates the
atomic number of the atom (number of protons in the nucleus). The
superscript number to the left of the symbol is the mass number (the total
number of protons and neutrons).
Diagram of the isotopes of hydrogen
A = Mass number (sum of the protons and
neutrons in the nucleus)
Z = Atomic number (number of protons in
the nucleus)
Using nuclear symbols to determine the number of p, n, e, and total charge
Mass Number = 16
Atomic Number = 8
Page 20
Atomic Theory
# protons = atomic number = 8
# neutrons = Mass # - Atomic # = 16 - 8 = 8
# electrons = # protons = 8
Mass Number = 16
Atomic Number = 8
# protons = atomic number = 8
# neutrons = Mass # - Atomic # = 16 - 8 = 8
# electrons = # protons - charge = 8 - (-2) = 10
Mass Number = 137
Atomic Number = 56
# protons = atomic number = 56
# neutrons = Mass # - Atomic # = 137 - 56 = 81
# electrons = # protons - charge = 56 - (+2) = 54
1. Example: Write the nuclear symbol for the following atoms:
1)
50 p, 70 n
2)
17 e-, 20 n
2. Example: Write the nuclear symbol for the following ions:
1)
53 p, 74 n, 54 e-
2)
23 e-, 30 n, net charge = +3
All the elements occur as two or more isotopes. However, not all
isotopes are stable. Some isotopes have unstable nuclei and are continually
decomposing into other element. For example, of the seven known
isotopes of carbon, only two, carbon 12 and carbon 13 are stable. Of the
seven known isotopes of oxygen, three are stable 86O 87O and 8I8O. Of the 15
known isotopes of arsenic, 3375 As is the only stable one.
Page 21
Atomic Theory
The structure of the atoms of the first 20 elements, arranged in order of
increasing atomic number (number of protons). We start with hydrogen,
and as we progress to helium, lithium, beryllium, etc., the atoms of each
succeeding element contain one more proton and one more electron than
the atoms of the preceding element. This sequence, without exception,
continues throughout the entire list of known elements. (See periodic Table
of element). This sequence is an impressive example of the orderly
arrangement of nature- If you recognize at this time that the elemental
building blocks of matter are arranged in a systematic fashion, and then
you may begin to appreciate the orderly arrangement of the universe.
The number of neutrons in an atom also increases as we progress from
the simpler elements to the more complex ones, but not in as uniform a
manner as do the protons and electrons. For example, the predominant
isotope of helium has two protons, two electrons, and also two neutrons. In
the helium atom the protons and the neutrons are found in the nucleus;
the electrons occupy the 1s orbital in the first energy level, which is now filled
to capacity. The electron structure of helium is written as 1s2.
Page 22
Atomic Theory
The most predominant isotope of lithium, atomic number 3, has three
protons, four neutrons, and three electrons. .Since the first energy level can
contain no more than two electrons, the third electron is located in the 2s
sublevel of the second energy level. The electron structure of lithium is
1s22s1. In succession, the atoms of beryllium (4), boron (5), carbon (6),
nitrogen (7), oxygen (8), fluorine (9), and neon (10) have one more proton
and one more electron than the preceding element. Both the first and
second energy levels are filled to capacity by the ten electrons of neon,
which has two electrons in the first and eight electrons in the second energy
level.
Element 11, sodium (Na), has two electrons in the first energy level and
eight electrons in the second energy level, with the remaining electron
occupying the 3s orbital in the third energy level. The electron structure of
sodium is 1s22s22p63s1. Magnesium (12), aluminum (13), silicon (14),
phosphorus (15), sulfur (16), chlorine (17), and argon (18) follow in order,
Page 23
Atomic Theory
each adding one electron to the third energy level up to argon, which has
eight electrons in the M shell
Table 5 Electron configurations of the first 11 elements, in subshell notation. Notice how
configurations can be built by adding one electron at a time.
atom
Z
ground state electronic configuration
H
1
1s
1
He
2
1s
2
Li
3
1s 2s
2
1
Be
4
1s 2s
2
2
B
5
1s 2s 2p
2
2
1
C
6
1s 2s 2p
2
2
2
N
7
1s 2s 2p
2
2
3
O
8
1s 2s 2p
2
2
4
F
9
1s 2s 2p
2
2
5
Ne
10
1s 2s 2p
2
2
6
Na
11
1s 2s 2p 3s
2
2
6
1
The placement of the last electron in potassium and calcium, elements
number 19 and 20, defects somewhat from the expected order. One might
expect that if the third energy level can contain a maximum of 18
electrons, electrons would continue to fill this shell until the maximum
capacity was reached. However, this is not the case. The 4S sublevel is
at a lower energy. Hence, in elements 19 and 20 the last element is found
in the 4s level. The electron structure for potassium is Is22s2 2p6 3s2 3p64S1
Calcium has an electron structure similar to potassium, except that it has
two 4s electrons. This break in sequence does not invalidate the formula
2n2 which merely prescribes the maximum number of electrons that each
shell may contain, but does not state the order in which the shells are filled.
Page 24
Atomic Theory
The elements following calcium have a less regular pattern of adding
electrons. The lowest energy level available for the twenty-first electron is
the 3d level, thus, scandium (21) has the following electron arrangement:
first energy level, two electrons; second energy level, eight electrons; third
energy level, nine electrons; fourth energy level, two electrons. The last
electron is located in the 3d level. The structure for scandium is
Is22s22p63s23p63d14s2 The elements following scandium, titanium (22) to
copper (29), continue to add d electrons until the third energy level has its
maximum of 18. Two exceptions in the orderly electron addition are
chromium (24) and copper (29. The third energy level of the electrons is
first completed in the element copper. The next table shows the order of
filling of the electron orbitals and the electron configuration of all the known
elements.
Example: Determine the electron structure of chlorine atom?
chlorine atom has 17 electrons we begin by placing two electrons in the
1s orbital, then two electrons in the 2s orbital, and six electrons in the 2p
orbitals. We now have used ten electrons. Finally, we place the next two
electrons in the 3s orbital and the remaining five electrons in the 3p
orbitals, which use all 17 electrons. The electron structure for a chlorine
atom is ls22s22p63s23p5. The sum of the superscripts equals 17, the
number of electrons in the atom.
Page 25
Atomic Theory
Table 6 Electron configurations of most elements, in subshell notation. Notice how
configurations can be built by adding one electron at a time.
Page 26
Atomic Theory
Several methods can be used to diagram atomic structures of atoms,
depending on what we are trying to illustrate. When we want to show both
the nuclear makeup and the total electron structure of each energy level,
we can use a diagram such as
Atomic structure diagrams of
sodium atom. The number of protons
(p) and neutrons (n) in the nucleus
are shown in the shaded circle;
outside the nucleus is shown the
number of electrons (e) in each
principal energy level
A method of diagramming energy sublevels is shown in next table. Each
orbital is represented by a circle
an arrow
. When the orbital contains one electron,
is placed in the circle. A second arrow, pointing downward
indicates the second electron in that orbital. The diagram for hydrogen is
Helium with two electrons is drawn as
both electrons are 1s
electrons. Lithium has three electrons in two energy levels, ls22s1. All four
electrons of beryllium are s electrons ls22s2. Boron has the first p electron,
which is located in the 3p, orbital. Since it is energetically more difficult for
the next p electron to pair up with the electron in the p orbital than to occupy
an empty p orbital, the second p electron in carbon, is located in the 2p. orbital.
The third p electron in nitrogen is still unpaired and is found in the 2p. orbital.
Page 27
Atomic Theory
The next three electrons pair with each of the 2p electrons up to the element
neon.
Table 7 Examples of ground state electron configurations in the orbital box notation that
shows electron spins.
orbital box diagram
atom
B
C
N
O
F
Cl
Mn
1s
2s
2p
1s
2s
2p
1s
2s
2p
1s
2s
2p
1s
2s
2p
1s
2s
2p
3s
3p
1s
2s
2p
3s
3p
3d
4s
…
Page 28
Atomic Theory