Atoms:

Atomic number:

Each element is characterized by an atomic number.

Atomic number (Z) is defined as the number of protons in the nucleus of

an atom.

Ex. Helium atom has 2 protons

Atomic number of helium (Z) = number of protons = 2

Mass number:

Mass number (A) is the sum of the number of protons and the number of

neutrons or it is equal to the number of nucleons

Mass number (A) = Z + N

A = mass number

Z: atomic number

N: number of neutrons

Example: nitrogen atom has 7 protons and 7 neutrons

Mass number of Nitrogen = Z+N = 7 +7 = 14

Atomic mass:

The atomic mass of a single atom is simply its total mass and is typically

expressed in atomic mass units or amu

Atomic Mass = A amu

Example helium: Mass number = 4

Then atomic mass = A amu= 4 amu

Atomic representation:

Application 1:

Lithium (Li) contains 3 positively charged particles and 4 neutral

particles.

1. Give the symbol of lithium

2. Indicate the number of protons

3. Determine the atomic number

4. Calculate the mass number

5. Write the atomic symbol

Solution:

1) Li

6. 2) number of protons = number of positively charged particles = 3

7. 3) atomic number (z) = number of protons = 3

8. 4) Mass number (A) = Z+N = 3+ 4 = 7

9. 5) Atomic representation:

Application 2:

Potassium (K) contains 20 uncharged particles. Its mass number is 39.

1. Give the symbol of potassium.

K

2. Determine the number of protons.

A = Z + N

Z= A – N = 39 – 20 = 19

3. Write the atomic representation

Application 3:

Consider the following atoms:

126C - 19

9F

1. Identify the number of protons in each atom.

In an atom, the number of proton = atomic number (Z)

For carbon: nb of protons = 6

For Fluorine: nb. of protons = 9

2. Indicate the mass number in each atom.

For carbon A=12

For Fluorine A= 19

3. Calculate the number of neutrons in each atom.

A= Z + N

Then N = A – Z

For C: N = 12 -6 = 6

For F: N = 19 – 9= 10

Charge of an atom:

Q atom = Q nucleus + Q electron cloud

- Q nucleus = Q protons + Q neutrons

= number of protons X relative charge of 1 proton

- Q Electron cloud = Q electrons

= number of electrons X relative charge of 1 electron

In general, Q atom = 0 (atom is neutral)

So, Q nucleus = - Q electron cloud

Note: In a neutral atom, number of protons = number of electrons = Z

Application 4:

Consider the following atom:

1. Determine the composition of this atom.

2. Calculate the nuclear charge.

3. Deduce the charge of its electron cloud.

Given: Relative charge of 1 P+= 1+

1.Composition of an atom means the number of protons, neutrons and

electrons

In a neutral atom, number of protons = number of electrons = Z = 13

A= Z + N then N = A – Z = 27 - 13= 14

Number of neutron = 14.

2. Nuclear charge = Q p+ + Qn0

= number of p+ X relative charge of 1 p+

= 13 X 1 + = 13 +

3. Q atom = 0

Then Q electron cloud = - Q nucleus

= - (13 +) = 13 –

Application 5:

The following document shows the atomic structure of sodium atom:

1. referring to the document, indicate the

number of electrons.

2. Determine the atomic number.

3. Knowing that sodium atom has 12 uncharged

particles, calculate its mass number.

4. Write the atomic representation.

5. Calculate the nuclear charge of Na atom.

6. Deduce the charge of its electron cloud.

Given: Relative charge of 1 p+ = 1 +

Relative charge of 1 no = 0

Relative charge of 1 e- = 1

Solution of application 5:

1. Number of electron = 11

2. In a neutral atom, number of protons = number of electrons = Z = 11

3. 12 uncharged particles Neutron = 12

• Mass number is the sum of protons and neutrons in the nucleus (A)

• A = Z + N = 11 + 12 = 23

4. Chemical representation:

5. Nuclear charge = Q p+ + Qn0

= number of p+ X relative charge of 1 p+

= 11 X 1 + = 11 +

6. Q atom = 0

Then Q electron cloud = - Q nucleus

= - (11 +) = 11

Electron configuration:

Electrons are arranged in different energy shells (levels) around the

nucleus.

Number 1 2 3 4

Energy level (shell) K L M N

Max number of

electrons

2 8 8 2

K

L

M N

Correction of homework:

Write the electron configuration of the following atoms:

11Na - 10Ne - 15S - 6C - 9F - 17Cl

In a neutral atom, the number of protons = nb of electrons = Z

For Na: number of e- = 11 Electron configuration: 11Na: K2 L8 M1

For Ne: number of e- = 10 Electron configuration: 10Ne: K2 L8

For S: number of e- = 15 Electron configuration: 15S : K2 L8 M5

For C: number of e- = 6 Electron configuration: 6C : K2 L4

For F: number of e- = 9 Electron configuration: 9F : K2 L7

For Cl: number of e- = 17 Electron configuration: 17Cl: K2 L8 M7

Periodic table:

In the modern periodic table, the elements are arranged in their

increasing atomic number (Z).

The periodic table has 18 columns (vertical lines) and 7 rows (horizontal

rows)

How to determine the location in the periodic table?

Column = Number of valence electrons (In case number of valence

electron is more than 2 then we add 10)

Valence electron = the number of electrons in the outer shell

Row = number of occupied shells or energy levels

Example:

Consider the following atoms:

1. Write the electron configuration of each atom.

In a neutral atom, the number of protons = number of electrons = z

For Li: number of electrons = 3 then the electron configuration is: K2 L1

For P: number of electrons= 15 then P: K2 L8 M5

2. Find the valence electrons of each atom.

Valence electrons = number of electrons on the outer shell: For li: v.e. =

1 and for P: v.e. = 5

3. Determine the location of each atom in the periodic table.

Li is located in column 1 since it has 1 v.e and in row 2 since it has 2

energy shells

P is located in column 15 since it has 5 v.e. and in row 3 since it has 3

energy shells.

Lewis dot symbol:

Chemical symbol of the element surrounded by dots (valence electrons)

Application 7:

Chlorine (Cl) is used as a disinfectant, it is used also to treat drinking

water and swimming pools. A study of chlorine element gave the

following information:

- The relative charge of the electron cloud of chlorine is 17-

- The number of neutrons in the nucleus of the chlorine atom is

greater than the number of protons by 1 (N=Z+1)

1. Determine the number of electrons in chlorine atom, knowing that

the relative charge of an electron is (1-).

2. Deduce the atomic number of chlorine.

3. Calculate the mass number of chlorine.

4. Write its atomic representation.

5. Write its electron configuration.

6. Determine the location of chlorine in the periodic table.

7. Write the Lewis dot structure of chlorine.

Solution:

1. Q e- cloud = Q e-

17 - = number of electrons x relative charge of 1electron

17 - = number of electrons x 1-

Number of electrons = 𝟏𝟕 −

𝟏− = 𝟏𝟕

2. In a neutral atom, number of protons = number of electrons = Z =

17

3. A = Z + N

Given N= Z + 1 = 17 +1= 18

Then A = 17 + 18 = 35

4.

5. Number of electrons = 17

17Cl: K 2 L8 M7

6. Since Cl has 3 energy shells then it is located in row 3

Since Cl has 5 valence electrons then it is located in column 17

7.

Application 8:

Silicon of symbol Si is a metalloid used in fabrication of solar cells and

transistors. It is prepared from pure sand.

Silicon has 4 valence electrons and is located in row 3 in the periodic

table.

The number of nucleons in silicon atom is 28

1. Write its Lewis dot symbol.

2. Write the electron configuration of Si. Justify.

3. Determine its atomic number.

4. Calculate the number of uncharged particles in Si.

5. Determine the atomic number of the element just above Si in the

periodic table.

Solution:

1.

2. Since Si has 4 v.e. then the number of electrons in the last shell = 4

Since Si is located in row 3 then it has 3 occupied energy shells

Then the electron configuration of Si: K 2 L8 M4

3. Number of electrons = 2 + 8+ 4=14

In a neutral atom the number of electrons =number of protons = Z=14

4. Number of nucleons = Mass number = 28

A = Z + N then

N = A – Z = 28 -14

= 14

5. Element just above Si then it is located in column 14, row 2

Its electron configuration is K 2 L4

Then number of electron= 2 + 4 = 6,

In a neutral atom, nb of protons = nb of electrons = Z = 6