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The atom is the smallest particle of an element that take part in a chemical reaction. The atom is made up of three subatomic particles:

          (i)Protons

          (ii)Electrons

          (iii)Neutrons

(i)Protons

1. The proton is positively charged

2. Is found in the centre of an atom called nucleus

3. It has a relative mass 1

4. The number of protons in a atom of an element is its Atomic number

 (ii)Electrons

1. The Electrons is negatively charged

2. Is found in fixed regions surrounding the centre of an atom called energy levels/orbitals.

3. It has a relative mass 1/1840

4. The number of protons and electrons in a atom of an element is always equal

(iii)Neutrons

1. The Neutron is neither positively nor negatively charged thus neutral.

2. Like protons it is found in the centre of an atom called nucleus

3. It has a relative mass 1

4. The number of protons and neutrons in a atom of an element is its Mass number

Diagram showing the relative positions of protons, electrons and neutrons in an atom of an element

 Diagram showing the relative positions of protons, electrons and neutrons in an atom of Carbon

The table below show atomic structure of the 1st twenty elements.


Element
SymbolProtonsElectronsNeutronsAtomic
number
Mass number    
HydrogenH11011
HeliumHe22224
LithiumLi33437
BerylliumBe44549
BoronB556511
CarbonC666612
NitrogenN777714
OxygenO888816
FluorineF9910919
NeonNe1010101020
SodiumNa1111121123
MagnesiumMg1212121224
AluminiumAl1313141327
SiliconSi1414141428
PhosphorusP1515161531
SulphurS1616161632
ChlorineCl1717181735
ArgonAr1818221840
PotassiumK1919201939
CalciumCa2020202040

Most atoms of elements exist as isotopes.

Isotopes are atoms of the same element, having the same number of protons/atomic number but different number of neutrons/mass number.

By convention, isotopes are written with the mass number as superscript and the atomic number as subscript to the left of the chemical symbol of the element. i.e.

                             mass number                          

                             atomic  number                m n  X            symbol of element

Below is the conventional method of writing the 1st twenty elements showing the mass numbers and atomic numbers:

11H                      42He            73Li              94Be            115B                126

 147N                         168O           199F             2010Ne                   2311Na                      2412Mg

2713Al                    2814Si           3115P            3216S            3517Cl              4018Ar

3919K                     4020C

The table below shows some common natural isotopes of some elements

ElementIsotopesProtonsElectronsNeutronsAtomic numberMass number
Hydrogen11H 21H(deuterium) 31H(Tritium)1 1 11 1 10 2 31 1 1  1 2 3
Chlorine3517Cl 3717Cl17 1717 1718 2017 1735 37
Potassium3919K 4019K 4119K19 19 1919 19 1920 21 2219 19 1939 40 41
Oxygen168O 188O8 88 88 108 816 18
Uranium23592U 23892U  92 9292 92143 14692 92235 238
Neon2210Ne 2010Ne 2110Ne10 10 1010 10 1012 10 1110 10 1022 20 21

The mass of an average atom is very small (10-22 g).Masses of atoms are therefore expressed in relation to a chosen element.

 The atom recommended is 12C isotope whose mass is arbitrarily assigned as 12.000 atomic mass units(a.m.u) .

All other atoms are compared to the mass of 12C isotope to give the relative at The relative atomic mass(RAM) is therefore defined as “the mass of average atom of an element  compared to  1/12  an atom of 12C isotope whose mass is arbitrarily fixed as 12.000 atomic mass units(a.m.u) ” i.e;

          RAM   =       mass of  atom of an element

                             1/12  of one atom of 12C isotope

Accurate relative atomic masses (RAM) are got from the mass spectrometer. Mass spectrometer determines the isotopes of the element and their relative abundance/availability.

 Using the relative abundances/availability of the isotopes, the relative atomic mass (RAM) can be determined /calculated as in the below examples.

  1. Chlorine occurs as 75% 3517Cl and 25% 3717Cl isotopes. Calculate the relative atomic mass of Chlorine.

Working

          100 atoms of chlorine contains 75 atoms of 3517Cl isotopes

          100 atoms of chlorine contains 75 atoms of 3717Cl isotopes

          Therefore;

          RAM  of chlorine = ( 75/100 x 35)   +  25/100  x 37 =  35.5

          Note that:

Relative atomic mass has no units

More atoms of chlorine exist as 3517Cl(75%) than as 3717Cl(25%)     therefore RAM is nearer to the more abundant isotope.

  • Calculate the relative atomic mass of potassium given that it exist as;

93.1%  3919K ,  0.01%  4019K  ,  6.89%  4119K ,

Working

          100 atoms of potassium contains 93.1 atoms of 3919K isotopes

          100 atoms of potassium contains 0.01 atoms of 4019K isotopes

          100 atoms of potassium contains 6.89 atoms of 4119K isotopes

          Therefore;

          RAM  of potassium = (93.1/100 x39) + (0.01/100 x 40) +(6.89 /100 x 39)          Note that:

Relative atomic mass has no units

More atoms of potassium exist as 3919K (93.1%) therefore RAM is nearer to the more abundant 3919K  isotope.

  • Calculate the relative atomic mass of  Neon given that it exist as;

90.92%  2010Ne ,  0.26%  2110Ne  ,  8.82%  2210Ne,

Working

          100 atoms of Neon contains 90.92 atoms of 2010Ne  isotopes

          100 atoms of Neon contains 0.26 atoms of 2110Ne  isotopes

          100 atoms of Neon contains 8.82 atoms of 2210 Ne  isotopes Therefore;

          RAM  of  Neon = (90.92/100 x20) + (0.26/100 x 21) +(8.82 /100 x 22)

          Note that:

Relative atomic mass has no units

More atoms of Neon exist as 2010Ne (90.92%) therefore RAM is nearer to the more abundant 2010Ne  isotope.

  • Calculate the relative atomic mass of  Argon given that it exist as;

90.92%  2010Ne ,  0.26%  2110Ne  ,  8.82%  2210Ne,

NB

The relative atomic mass is a measure of the masses of atoms. The higher the relative atomic mass, the heavier the atom.

Electrons are found in energy levels/orbital.

An energy level is a fixed region around/surrounding the nucleus of an atom occupied by electrons of the same (potential) energy.

By convention energy levels are named 1,2,3… outwards  from the region nearest to nucleus.

Each energy level is occupied by a fixed number of electrons:

          The 1st energy level is occupied by a maximum of two electrons

          The 2nd  energy level is occupied by a maximum of eight electrons

The  3rd  energy level is occupied by a maximum of eight electrons( or   eighteen electrons if available)

The  4th   energy level is occupied by a maximum of eight electrons( or eighteen or thirty two electrons if available)

This arrangement of electrons in an atom is called electron configuration / structure.

By convention theelectron configuration / structure of an atom of an element can be shown in form of a diagram using either cross(x) or dot() to  

Practice examples drawing electronic configurations

a)11H has – in nucleus1proton and 0 neutrons

                -1 electron in the 1st energy levels thus:

Nucleus

Energy levels

Electrons (represented by cross(x)

Electronic structure of Hydrogen is thus: 1:

b) 42He has – in nucleus 2 proton and 2 neutrons[s1] 

                  -2 electron in the 1st energy levels thus:

Nucleus

Energy levels

Electrons (represented by cross(x)

Electronic structure of Helium is thus: 2:

c) 73Li has – in nucleus 3 proton and 4 neutrons

                 – 2 electron in the 1st energy levels

                 –1 electron in the 2nd energy levels thus

Nucleus

Energy levels

Electrons (represented by cross(x)

Electronic structure of Lithium is thus: 2:1

d) 94Be has – in nucleus 4 proton and 5 neutrons

                    – 2 electron in the 1st energy levels

                   –2 electron in the 2nd energy levels thus

Nucleus

Energy levels

Electrons (represented by cross(x)

Electronic structure of Beryllium is thus: 2:2

          e) 115B has – in nucleus 5 proton and 6 neutrons

                  – 2 electron in the 1st energy levels

                   –3 electron in the 2nd energy levels thus

Nucleus

Energy levels

Electrons (represented by cross(x)

Electronic structure of Boron is thus: 2:3

 f)    126C  has      – in nucleus 6 proton and 6 neutrons

                            – 2 electron in the 1st energy levels

                            –4 electron in the 2nd energy levels thus

Nucleus

Energy levels

Electrons (represented by cross(x)

Electronic structure of Carbon is thus: 2:4

g) 147N has      – in nucleus 7 proton and 7 neutrons

                            – 2 electron in the 1st energy levels

                            –5 electron in the 2nd energy levels thus

Nucleus

Energy levels

Electrons (represented by cross(x)

Electronic structure of Nitrogen is thus: 2:5

h) 168O has      – in nucleus 8 proton and 8 neutrons

                       – 2 electron in the 1st energy levels

                        –6 electron in the 2nd energy levels thus

Nucleus

Energy levels

Electrons (represented by cross(x)

Electronic structure of Oxygen is thus: 2:6

i) 199F has      – in nucleus 9 proton and 10 neutrons

                       – 2 electron in the 1st energy levels

                        –7 electron in the 2nd energy levels thus

Nucleus

Energy levels

Electrons (represented by cross(x)

Electronic structure of Fluorine is thus: 2:7

i) 2010Ne has   – in nucleus 10 proton and 10 neutrons

                      – 2 electron in the 1st energy levels

                       –8 electron in the 2nd energy levels thus

Nucleus

Energy levels

Electrons (represented by cross(x)

Electronic structure of Neon is thus: 2:8

j) 2311Na has   – in nucleus 11 proton and 12 neutrons

                      – 2 electron in the 1st energy levels

                       –8 electron in the 2nd energy levels

                      –1 electron in the 3rd   energy levels thus

Nucleus

Energy levels

Electrons (represented by dot (.)

Electronic structure of Sodium is thus: 2:8:1

k) 2412Mg has   – in nucleus 12 proton and 12 neutrons

                      – 2 electron in the 1st energy levels

                       –8 electron in the 2nd energy levels

                      –2 electron in the 3rd   energy levels thus

Nucleus

Energy levels

Electrons (represented by dot (.)

Electronic structure of Magnesium is thus: 2:8:2

l) 2713Al has   – in nucleus 13 proton and 14 neutrons

                      – 2 electron in the 1st energy levels

                       –8 electron in the 2nd energy levels

                      –3 electron in the 3rd   energy levels thus

Nucleus

Energy levels

Electrons (represented by dot (.)

Electronic structure of  Aluminium is thus: 2:8:3

m) 2814Si has   – in nucleus 14 proton and 14 neutrons

                      – 2 electron in the 1st energy levels

                       –8 electron in the 2nd energy levels

                      –4 electron in the 3rd   energy levels thus

Nucleus

Energy levels

Electrons (represented by dot (.)

Electronic structure of Silicon is thus: 2:8:4

n) 3115P has   – in nucleus 14 proton and 15 neutrons

                      – 2 electron in the 1st energy levels

                       –8 electron in the 2nd energy levels

                      –5 electron in the 3rd   energy levels thus

Nucleus

Energy levels

Electrons (represented by dot (.)

Electronic structure of Phosphorus is thus: 2:8:5

o) 3216S has   – in nucleus 16 proton and 16 neutrons

                      – 2 electron in the 1st energy levels

                       –8 electron in the 2nd energy levels

                      –6 electron in the 3rd   energy levels thus

Nucleus

Energy levels

Electrons (represented by dot (.)

Electronic structure of Sulphur is thus: 2:8:6

p) 3517Cl has   – in nucleus 18 proton and 17 neutrons

                      – 2 electron in the 1st energy levels

                       –8 electron in the 2nd energy levels

                      –7 electron in the 3rd   energy levels thus

Nucleus

Energy levels

Electrons (represented by dot (.)

Electronic structure of Chlorine is thus: 2:8:7

p) 4018Ar has   – in nucleus 22 proton and 18 neutrons

                      – 2 electron in the 1st energy levels

                       –8 electron in the 2nd energy levels

                      –8 electron in the 3rd   energy levels thus

Nucleus

Energy levels

Electrons (represented by dot (.)

Electronic structure of Argon is thus: 2:8:8

q) 3919K has   – in nucleus 20 proton and 19 neutrons

                      – 2 electron in the 1st energy levels

                       –8 electron in the 2nd energy levels

                      –8 electron in the 3rd   energy levels

                      –1 electron in the 4th   energy levels thus

Nucleus

Energy levels

Electrons (represented by dot (.)

Electronic structure of Potassium is thus: 2:8:8:1

r) 4020Ca has   – in nucleus 20 proton and 20 neutrons

                      – 2 electron in the 1st energy levels

                       –8 electron in the 2nd energy levels

                      –8 electron in the 3rd   energy levels

                      –2 electron in the 4th   energy levels thus

Nucleus

Energy levels

Electrons (represented by dot (.)

Electronic structure of Calcium is thus: 2:8:8:2


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