Chemistry Form 2 Notes : ATOMIC STRUCTURE

A. ATOMIC STRUCTURE

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

• Protons
• Electrons
• Neutrons

(i) Protons

1. The proton is positively charged.
2. It is found in the centre of an atom called the nucleus.
3. It has a relative mass of 1.
4. The number of protons in an atom of an element is its Atomic number.

(ii) Electrons

1. The electron is negatively charged.
2. It is found in fixed regions surrounding the centre of an atom called energy levels or orbitals.
3. It has a relative mass of ¹/₁₈₄₀.
4. The number of protons and electrons in an atom of an element is always equal.

(iii) Neutrons

1. The neutron is neither positively nor negatively charged; it is neutral.
2. Like protons, it is found in the centre of an atom called the nucleus.
3. It has a relative mass of 1.
4. The number of protons and neutrons in an 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 shows the atomic structure of the first twenty elements.

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 ^m_n X (symbol of element)

Below is the conventional method of writing the first 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 ⁴⁰₂₀Ca

The table below shows some common natural isotopes of some elements.

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 the 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 the ¹²C isotope to give the relative atomic mass. The relative atomic mass (RAM) is therefore defined as “the mass of an average atom of an element compared to ¹/₁₂ of an atom of the 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 obtained from the mass spectrometer. A 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 examples below.

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

   Working

   100 atoms of chlorine contain 75 atoms of ³⁵₁₇Cl isotopes

   100 atoms of chlorine contain 25 atoms of ³⁷₁₇Cl isotopes

   Therefore;

   RAM of chlorine = (75/100 × 35) + (25/100 × 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.

2. Calculate the relative atomic mass of potassium given that it exists as:

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

   Working

   100 atoms of potassium contain 93.1 atoms of ³⁹₁₉K isotopes

   100 atoms of potassium contain 0.01 atoms of ⁴⁰₁₉K isotopes

   100 atoms of potassium contain 6.89 atoms of ⁴¹₁₉K isotopes

   Therefore;

   RAM of potassium = (93.1/100 × 39) + (0.01/100 × 40) + (6.89/100 × 41)

   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.

3. Calculate the relative atomic mass of Neon given that it exists as:

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

   Working

   100 atoms of Neon contain 90.92 atoms of ²⁰₁₀Ne isotopes

   100 atoms of Neon contain 0.26 atoms of ²¹₁₀Ne isotopes

   100 atoms of Neon contain 8.82 atoms of ²²₁₀Ne isotopes

   Therefore;

   RAM of Neon = (90.92/100 × 20) + (0.26/100 × 21) + (8.82/100 × 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.

4. Calculate the relative atomic mass of Argon given that it exists 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/orbitals.

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 the 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 or structure.

By convention, the electron configuration/structure of an atom of an element can be shown in the form of a diagram using either cross (x) or dot (●) to represent electrons.

Practice examples drawing electronic configurations

a) ¹₁H has – in nucleus 1 proton and 0 neutrons

– 1 electron in the 1^st energy level thus:

Nucleus

Energy levels

Electrons (represented by cross (x))

Electronic structure of Hydrogen is thus: 1:

b) ⁴₂He has – in nucleus 2 protons and 2 neutrons

– 2 electrons in the 1^st energy level thus:

Nucleus

Energy levels

Electrons (represented by cross (x))

Electronic structure of Helium is thus: 2:

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

– 2 electrons in the 1^st energy level

– 1 electron in the 2^nd energy level thus

Nucleus

Energy levels

Electrons (represented by cross (x))

Electronic structure of Lithium is thus: 2:1

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

– 2 electrons in the 1^st energy level

– 2 electrons in the 2^nd energy level thus

Nucleus

Energy levels

Electrons (represented by cross (x))

Electronic structure of Beryllium is thus: 2:2

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

– 2 electrons in the 1^st energy level

– 3 electrons in the 2^nd energy level thus

Nucleus

Energy levels

Electrons (represented by cross (x))

Electronic structure of Boron is thus: 2:3

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

– 2 electrons in the 1^st energy level

– 4 electrons in the 2^nd energy level thus

Nucleus

Energy levels

Electrons (represented by cross (x))

Electronic structure of Carbon is thus: 2:4

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

– 2 electrons in the 1^st energy level

– 5 electrons in the 2^nd energy level thus

Nucleus

Energy levels

Electrons (represented by cross (x))

Electronic structure of Nitrogen is thus: 2:5

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

– 2 electrons in the 1^st energy level

– 6 electrons in the 2^nd energy level thus

Nucleus

Energy levels

Electrons (represented by cross (x))

Electronic structure of Oxygen is thus: 2:6

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

– 2 electrons in the 1^st energy level

– 7 electrons in the 2^nd energy level thus

Nucleus

Energy levels

Electrons (represented by cross (x))

Electronic structure of Fluorine is thus: 2:7

j) ²⁰₁₀Ne has – in nucleus 10 protons and 10 neutrons

– 2 electrons in the 1^st energy level

– 8 electrons in the 2^nd energy level thus

Nucleus

Energy levels

Electrons (represented by cross (x))

Electronic structure of Neon is thus: 2:8

k) ²³₁₁Na has – in nucleus 11 protons and 12 neutrons

– 2 electrons in the 1^st energy level

– 8 electrons in the 2^nd energy level

– 1 electron in the 3^rd energy level thus

Nucleus

Energy levels

Electrons (represented by dot (●))

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

l) ²⁴₁₂Mg has – in nucleus 12 protons and 12 neutrons

– 2 electrons in the 1^st energy level

– 8 electrons in the 2^nd energy level

– 2 electrons in the 3^rd energy level thus

Nucleus

Energy levels

Electrons (represented by dot (●))

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

m) ²⁷₁₃Al has – in nucleus 13 protons and 14 neutrons

– 2 electrons in the 1^st energy level

– 8 electrons in the 2^nd energy level

– 3 electrons in the 3^rd energy level thus

Nucleus

Energy levels

Electrons (represented by dot (●))

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

n) ²⁸₁₄Si has – in nucleus 14 protons and 14 neutrons

– 2 electrons in the 1^st energy level

– 8 electrons in the 2^nd energy level

– 4 electrons in the 3^rd energy level thus

Nucleus

Energy levels

Electrons (represented by dot (●))

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

o) ³¹₁₅P has – in nucleus 15 protons and 16 neutrons

– 2 electrons in the 1^st energy level

– 8 electrons in the 2^nd energy level

– 5 electrons in the 3^rd energy level thus

Nucleus

Energy levels

Electrons (represented by dot (●))

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

p) ³²₁₆S has – in nucleus 16 protons and 16 neutrons

– 2 electrons in the 1^st energy level

– 8 electrons in the 2^nd energy level

– 6 electrons in the 3^rd energy level thus

Nucleus

Energy levels

Electrons (represented by dot (●))

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

q) ³⁵₁₇Cl has – in nucleus 17 protons and 18 neutrons

– 2 electrons in the 1^st energy level

– 8 electrons in the 2^nd energy level

– 7 electrons in the 3^rd energy level thus

Nucleus

Energy levels

Electrons (represented by dot (●))

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

r) ⁴⁰₁₈Ar has – in nucleus 18 protons and 22 neutrons

– 2 electrons in the 1^st energy level

– 8 electrons in the 2^nd energy level

– 8 electrons in the 3^rd energy level

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

s) ³⁹₁₉K has – in nucleus 19 protons and 20 neutrons

– 2 electrons in the 1^st energy level

– 8 electrons in the 2^nd energy level

– 8 electrons in the 3^rd energy level

– 1 electron in the 4^th energy level thus

Nucleus

Energy levels

Electrons (represented by dot (●))

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

t) ⁴⁰₂₀Ca has – in nucleus 20 protons and 20 neutrons

– 2 electrons in the 1^st energy level

– 8 electrons in the 2^nd energy level

– 8 electrons in the 3^rd energy level

– 2 electrons in the 4^th energy level thus

Nucleus

Energy levels

Electrons (represented by dot (●))

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