Page 16 Atomic Theory

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.

Principal

energy Level

1

K

2

L

3

M

4

N

5*

O

6*

P

7*

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

lete

Incomp

lete

Incomp

lete

Total Number

of Electrons

2 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 17 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 18 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 quantum-mechanical concept, showing the electron orbital as a cloud" surrounding the nucleus.

Page 19 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 20 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 21 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 8

7O and 8I8O. Of the 15

known isotopes of arsenic, 3375 As is the only stable one.

Page 22 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 23 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 24 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 1s1

He 2 1s2

Li 3 1s2 2s

1

Be 4 1s2 2s

2

B 5 1s2 2s

2 2p

1

C 6 1s2 2s

2 2p

2

N 7 1s2 2s

2 2p

3

O 8 1s2 2s

2 2p

4

F 9 1s2 2s

2 2p

5

Ne 10 1s2 2s

2 2p

6

Na 11 1s2 2s

2 2p

6 3s

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 25 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 26 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 27 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 . When the orbital contains one electron,

an arrow 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 28 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.

atom orbital box diagram

B 1s

2s

2p

C 1s

2s

2p

N 1s

2s

2p

O 1s

2s

2p

F 1s

2s

2p

Cl 1s

2s

2p

3s

3p

Mn 1s

2s

2p

3s

3p

… 3d

4s