Atomic number and mass number

FIRST TERM E-LEARNING NOTES

SUBJECT: CHEMISTRY

CLASS: SS1

 

WEEK 3

TOPICS: PARTICULATE NATURE OF MATTER

CONTENTS:

  1. Electronic configuration
  2. Atomic number and mass number (nucleon number)
  3. Relative atomic mass based on C-12 isotope
  4. Isotopy of elements

PERIOD 1: ELECTRONIC CONFIGURATION

Electrons are found revolving around the nucleus of an atom in circular paths known as rings, orbits, energy levels or shells. Each shell contains electrons with similar energy. Those with the lowest energies being nearest to the nucleus.

3 (M shell)

Nucleus

2(L shell)

1 (K shell)

Thus, the arrangement of electrons in the atom according to energy is called ELECTRONIC CONFIGURATION. Letters and figures are associated with these orbits or shells as shown above. The maximum possible number of electrons that can be accommodated in a shell is given by the formula:

Nmax= 2n2. Where Nmax = Maximum no of electron. n= no, of shell.

Thus K- shell can contain 2 × 12 = 2 electrons.

L- Shell can contain 2 × 22 = 8 electrons.

M- Shell can contain 2 × 32= 18 electrons etc

The electron structures of the atoms of the first twenty elements are given in the table below.

Element Symbol Number of protons(or atomic number) Number of electrons and their distribution in the shells. K L M N
Hydrogen H 1 1
Helium He 2 2
Lithium Li 3 2 1
Beryllium Be 4 2 2
Boron B 5 2 3
Carbon C 6 2 4
Nitrogen N 7 2 5
Oxygen O 8 2 6
Fluorine F 9 2 7
Neon Ne 10 2 8
Sodium Na 11 2 8 1
Magnesium Mg 12 2 8 2
Aluminium Al 13 2 8 3
Silicon Si 14 2 8 4
Phosphorus P 15 2 8 5
Sulphur S 16 2 8 6
Chlorine Cl 17 2 8 7
Argon Ar 18 2 8 8
Potassium K 19 2 8 8 1
Calcium Ca 20 2 8 8 2

The electronic configurations of some elements are shown below:

Hydrogen, H (atomic number 1)

Electronk1

Nucleus

Helium, He(atomic number 2)

electrons k2

nucleus

Lithium, Li (atomic number 3).

K2 L1

 

 

Neon, Ne (atomic number 10)

K2 L8

 

2,8

Sodium, Na (atomic number 11)

K L M

2 8 1

 

2, 8, 1

Argon, Ar (atomic number 18 )

K L M

2 8 8

 

2,8,8

Potassium, K (atomic number 19) K L M N

2 8 8 1

 

2, 8, 8, 1

Calcium, Ca (atomic number 20)

K L M N

2 8 8 2

 

2,8,8,2

EVALUATION: Draw the electronic configuration of the following elements.

  1. Carbon (b) Fluorine (c) Aluminium (d) Nitrogen

PERIOD 2: ATOMIC NUMBER AND MASS NUMBER (NUCLEON NUMBER)

ATOMIC NUMBER:

DEFINITION: Atomic number is the number of protons in an atom of an element.

The atomic number of an element is a whole number and is designated z. In a neutral atom the number of protons must be equal to the number of electrons (since protons are positively charged and electron are negatively charged).

All the atoms of a particular element have the same number of protons in their nuclei (i.e. they have the same atomic number). NO two elements have the same number of protons in their atoms.

DEFINITION:

MASS NUMBER (NUCLEON NUMBER): The mass number is the sum of the protons and neutrons in an atom of an element.

Mass number is represented by the letter A.

Mass number A = Number of protons + number of neutrons.

i.e. A =p + n. Where p=protons, n= neutrons.

Or number of neutrons n =A – Z.

An atom of an element can be described by writing its symbol together with its atomic number and mass number.

 

Examples: The atom of carbon, oxygen and sodium can be written as 126C, 168O and2311Na respectively.

EVALUATION:

(1) Define the following: (a) Atomic number (b) Mass number.

(2) Describe the atoms of the following elements using their symbol, atomic number and mass number: (a) Phosphorus (b) Silicon (c) Calcium

PERIODS 3 AND 4: ISOTOPY AND RELATIVE ATOMIC MASS

Definition: Isotopy is a phenomenon whereby atoms of an element exhibit different mass number but have the same atomic number.

Mass spectrometric studies show that the atoms of most elements exist in more than one form. This is due to the difference in number of neutrons present in these atoms. Such atoms are known as isotopes. Isotope of an element is represented by the original symbol of the element with the mass number and atomic numbers. For example 126C, 136C, 146C represent atoms of the isotopes of carbon. For each atom, the number of neutrons can be obtained by finding the difference between the mass number A and the atomic number Z i.e. A – Z. Each isotope of an element has its own mass known as isotopic mass.

Isotopes of an element have slightly different physical properties because neutrons contribute only to the mass of an atom and not its chemical behaviour. But isotopes of an element exhibit the same chemical properties because the number of valence electrons in an atom of an element determines its chemical behaviour (properties) and since isotopes have the same number of valence electrons they will be chemically alike.[mediator_tech]

NOTE:

(i) An analysis of the chlorine isotopes.

Isotope 3517Cl Isotope 3717Cl

Mass number, A 35 37

Atomic number,Z 17 17

Number of protons 17 17

Number of electrons 17 17

Number of neutrons (A-Z) 35 – 17=18 37 – 17=20

Abundance in nature (%) 75 25

ISOTOPES OF THE SAME ELEMENTS.

Element Carbon Oxygen
ISOTOPES 126C 136C 168O 178O 188O
ABUNDANCE IN NATURE (%) 98.9 1.1 99.76 0.04 0.20

 

(ii) The names of the isotopic forms of hydrogen

11H — Protium (or hydrogen)

21H— Deuterium (or heavy hydrogen or D)

31H— Tritium or T

(iii) The relative atomic mass, RAM of an element which exhibits isotopy is the average mass of its various isotopes as they occur naturally in any quantity of the element and they are not usually in whole numbers.

CALCULATION INVOLVING ISOTOPY.

WORKED EXAMPLE:

  1. Determine the relative atomic mass of element X from the data below

ISOTOPE MASS % ABUNDANCE

24X 24 78.70

25x 25 10.13

26x 26 11.7

  1. An element X has two isotopes of 2010X and 2210X in the ratio 1:3. What is the relative atomic mass?

Add ratio of occurrence together.

1 + 3= 4

X = 21.5

  1. Isotopes of an element X have isotopic masses 65 and 63 respectively. If the relative atomic mass of X is 63.60. Find the relative abundance of each isotope of the element.

Let the relative abundance of element X be y and Z respectively.

Z +Y =100

Z = 100 – y……. (i)

65y + 63z = 63.60 × 100

65y + 63z = 6360 ……(ii)

65y +63(100 – y) = 6360

65y – 63y + 6300 = 6360

2y = 60

Y = = 30

Y= 30

Z = 100 – y= 100 – 30 = 70

Y = 30, Z = 70

The relative abundance of X = 30% 0f 65X and 70% of 63X

EVALUATION:

(a) How many neutrons are present on the isotopes of 4119X (45%) and 4019x (55%)

(b)Calculate the relative atomic mass of X.

RELATIVE ATOMIC MASSES BASED ON C-12 ISOTOPE

Definition: The Relative Atomic Mass of an element is the number of times the average mass of an atom of the element is heavier than one-twelfth of the mass of one atom of carbon -12

RAM of an element=

Thus the atom of carbon -12 is adopted as the standard for defining the relative atomic mass of the other elements and is given a basic mass value of 12units.

The relative atomic mass of each element has been determined accurately with the aid of the mass spectrometer. This instrument measures the masses of the isotopes of the elements and their abundance and the relative atomic mass is calculated from the data.

Relative atomic masses of the first twenty elements in the periodic table

Element Atomic number Relative atomic mass
Hydrogen 1 1.008
Helium 2 4.0026
Lithium 3 6.939
Beryllium 4 9.0122
Boron 5 10.81
Carbon 6 12.011
Nitrogen 7 14.0067
Oxygen 8 15.9994
Fluorine 9 18.9884
Neon 10 20.183
Sodium 11 22.9898
Magnesium 12 24.312
Aluminum 13 26.9812
Silicon 14 28.086
phosphorus 15 30.9738
Sulphur 16 32.06
Chlorine 17 35.453
Argon 18 39.948
Potassium 19 39.102
Calcium 20 40.08

The relative atomic masses of the first twenty elements in the periodic table are given in the table below.

EVALUATION: From the complete periodic table of elements write out the relative atomic masses of (A) Magnesium (b) Oxygen (c) Chlorine (d) Carbon

GENERAL EVALAUATION

OBJECTIVES TEST :

  1. The maximum number of electrons that can be accommodated in the M-shell is. (a) 18 (b) 8 (c) 2 (d) 32
  2. The atomic number of chlorine is (a) 17 (b) 18 (c) 20 (d) 7
  3. Which of the following is an isotope of hydrogen?

(a)41H (b) 51H (c) 31H (d) 01H

ESSAY QUESTIONS

(1) Chlorine exists in two isotopic mixtures. The first has 17protons and 18 neutrons while the second isotope has 17 protons and 20 neutrons. If the two isotopes are present in ratio 3:1 respectively, calculate the relative atomic mass of chlorine.[mediator_tech]

(2) Show the electron structure of the following.

(a) Calcium (b) Magnesium (c) Sodium (d) Oxygen

(3) If the numbers of charged and unchanged particles in the centre of an atom are 6 and 7 respectively, what is the mass number of the atom?

(4) Calculate the number of neutrons in

(a) 2311Na (b) 3717Cl

(5) (a)What is an ‘isotopy’?

(b) Explain briefly why the chemical properties of isotopes of an element are similar.

WEEKEND ASSIGNMENT: Read about the topic ‘’mass spectrometer’’

WEEK ACTIVITY:

(a) Draw a labelled structure of a mass spectrometer showing its basic features. (b) Write two features of a mass spectrometer.

PRE-READING ASSIGNMENT:

Read the topic ‘’symbols of elements’’ page 26 of New school Chemistry. By Osei Yaw Ababio.

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