CHEMISTRY FORM 2 REVISION QUESTIONS AND ANSWERS-ATOMIC STRUCTURE

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 1^st twenty elements.

Element	Symbol	Protons	Electrons	Neutrons	Atomic number	Mass number
Hydrogen	H	1	1	0	1	1
Helium	He	2	2	2	2	4
Lithium	Li	3	3	4	3	7
Beryllium	Be	4	4	5	4	9
Boron	B	5	5	6	5	11
Carbon	C	6	6	6	6	12
Nitrogen	N	7	7	7	7	14
Oxygen	O	8	8	8	8	16
Fluorine	F	9	9	10	9	19
Neon	Ne	10	10	10	10	20
Sodium	Na	11	11	12	11	23
Magnesium	Mg	12	12	12	12	24
Aluminium	Al	13	13	14	13	27
Silicon	Si	14	14	14	14	28
Phosphorus	P	15	15	16	15	31
Sulphur	S	16	16	16	16	32
Chlorine	Cl	17	17	18	17	35
Argon	Ar	18	18	22	18	40
Potassium	K	19	19	20	19	39
Calcium	Ca	20	20	20	20	40

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 1^st twenty elements showing the mass numbers and atomic numbers:

¹₁H                      ⁴₂He            ⁷₃Li              ⁹₄Be            ¹¹₅B                ¹²₆C 

 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

Element	Isotopes	Protons	Electrons	Neutrons	Atomic number	Mass number
Hydrogen	11H 21H(deuterium) 31H(Tritium)	1 1 1	1 1 1	0 2 3	1 1 1	1 2 3
Chlorine	3517Cl 3717Cl	17 17	17 17	18 20	17 17	35 37
Potassium	3919K 4019K 4119K	19 19 19	19 19 19	20 21 22	19 19 19	39 40 41
Oxygen	168O 188O	8 8	8 8	8 10	8 8	16 18
Uranium	23592U 23892U	92 92	92 92	143 146	92 92	235 238
Neon	2210Ne 2010Ne 2110Ne	10 10 10	10 10 10	12 10 11	10 10 10	22 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 1^st energy level is occupied by a maximum of two electrons

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

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

The  4^th   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 1^st 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 1^st 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 1^st energy levels

                 –1 electron in the 2^nd 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 1^st energy levels

                   –2 electron in the 2^nd 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 1^st energy levels

                   –3 electron in the 2^nd 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 1^st energy levels

                            –4 electron in the 2^nd 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 1^st energy levels

                            –5 electron in the 2^nd 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 1^st energy levels

                        –6 electron in the 2^nd 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 1^st energy levels

                        –7 electron in the 2^nd 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 1^st energy levels

                       –8 electron in the 2^nd 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 1^st energy levels

                       –8 electron in the 2^nd energy levels

                      –1 electron in the 3^rd   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 1^st energy levels

                       –8 electron in the 2^nd energy levels

                      –2 electron in the 3^rd   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 1^st energy levels

                       –8 electron in the 2^nd energy levels

                      –3 electron in the 3^rd   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 1^st energy levels

                       –8 electron in the 2^nd energy levels

                      –4 electron in the 3^rd   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 1^st energy levels

                       –8 electron in the 2^nd energy levels

                      –5 electron in the 3^rd   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 1^st energy levels

                       –8 electron in the 2^nd energy levels

                      –6 electron in the 3^rd   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 1^st energy levels

                       –8 electron in the 2^nd energy levels

                      –7 electron in the 3^rd   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 1^st energy levels

                       –8 electron in the 2^nd energy levels

                      –8 electron in the 3^rd   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 1^st energy levels

                       –8 electron in the 2^nd energy levels

                      –8 electron in the 3^rd   energy levels

                      –1 electron in the 4^th   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 1^st energy levels

                       –8 electron in the 2^nd energy levels

                      –8 electron in the 3^rd   energy levels

                      –2 electron in the 4^th   energy levels thus

Nucleus

Energy levels

Electrons (represented by dot (.)

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

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