Chemistry : Atoms And Molecules

atoms and molecules structure

Previous Topic :Formulae And Valency

Particulate Nature Of Matter

Concept Of Atoms And Molecules

Matter : is defined as anything that has weight and occupies space. Matter are made up of tiny molecules, ions, and atoms.


Molecule : A molecule is the smallest particle of a substance, either an element or a compound which exist in a free state.

A molecule is formed when two or more atoms join together chemically.

A molecule is known as a molecule of a compound when it contains different atoms as in the case of CaCO3, when the atoms are the same like in one atom of chlorine and another one atom of chlorine to form chlorine gas, it is called molecule of chlorine, hence, molecule of an↑

The number of atoms in each molecule of an element, is called the atomicity of the molecule, for example “let use the two common element” hydrogen gas and oxygen gas, H2 and O2 which they both consist of two atoms, the number present in the two atoms is 2 and this means they are Diatomic.

Other examples are sulphur and phosphorus which occur as polyatomic molecules and helium and neon as monoatomic because they exist independently as a single atom.


Atom : An atom is the smallest amount (particles) of an element which take part in a chemical↑

An atom is made up of tiny particles called electrons, neutrons and protons. These three particles are present in all atoms except Normal hydrogen with only one proton and one election.


Ions An ion is any atom or group of atoms which possesses an electric charge.

There are two types of ion, the positively charged ion which are also called cations e.g Na+, Fe2+, Fe3+, Ca2+, Mg2+ and NH4+, etc, positively charged ions are mostly element of metals and the negatively charged ion which are also called anions e.g Cl-, OH-, NO3- and CO3-, etc, negatively charged ions are mostly element of non-metals.

Dalton’s Atomic Theory Of Matter

The Greek philosophers stated that matter must be made up of small particles but in 1808 an England scientist John Dalton put forward his own famous atomic theory which some of his theory were later↑

John Dalton stated that;

a] All elements are made up of tiny indivisible particles called atoms.

b] Atoms can neither be created, subdivided nor destroyed.

c] Atoms of a particular element are alike in every aspect and different from the atoms of other elements.

d] Chemical reactions occur as a result of combination and separation of atoms.

e] Chemical compounds are made by the union of small whole numbers of atoms to give compound↑

Modification Of Dalton’s Atomic Theory

In the light of recent advancement in the science, events have overtaken Dalton’s atomic theory, which leads to modification of his theory to original theory.

a] Dalton stated that matter is made up of small indivisible particles called atoms.

This was prove wrong because some radioactive atoms undergo spontaneous disintegration or artificial disintegration to produce even smaller particles, for example, Uranium is an example of an atom, if uranium undergoes spontaneous disintegration, it splits into smaller atoms like thorium and helium.

Uranium = thorium + helium
Uranium with mass number of 238 and with atomic number of 92 will split to give thorium of 234 mass number and 90 atomic number with helium of 4 mass number and 2 atomic number.

Hence Dalton theory was prove wrong because some atom can still split↑

b] Dalton also stated that matter can neither be created nor destroyed. This postulate was also prove wrong.

During a chemical reaction, there are small changes in mass due to energy changes.
For example addition of water to sodium hydroxide pellets ( NaOH) , the reaction is exothermic, meaning that some energy have been released and therefore some matter must have been destroyed due to the heat loss. Hence, matter can be create and destroy.

c] Dalton also stated that, atoms of the same element are alike in every aspect and differ from atoms of all other elements.
This theory was also prove wrong, due to the existence of isotopes.

For example, Chlorine has two different isotopes,

Note : atoms are also isotopes.

Which differ in their atomic masses although they have the small atomic number and the same chemical↑

d] Dalton stated that, chemical compounds are made by the union of small whole numbers of atoms, this was also prove wrong because many molecules most especially the organic compounds are now known to be compounds which consists of very large number of atoms joined together especially in the formation of polymers.

It was only the 5th law of Dalton’s atomic theory that was accepted.
The 5th law which started that;
All chemical reactions occur as a result of combination and separation of atoms.

Atomic Structure

Atoms are found to contain three types of particles, which are, proton, neutron, and↑

An atom consists of centrally placed nucleus which is surrounded by a cloud of electrons and neutrons.

The proton and the neutron each has a mass of one, proton carries a positive charge while neutron has no charge. The electrons are very light and are negatively charged, and electron revolves round the nucleus in an orbital manner in order to Neutralize the positive charge in the nucleus, hence an atom is electrically neutral.

An atom is electrically neutral because the number of  electrons revolving round the nucleus is equal to the number of protons in the nucleus.

Rutherford Postulate About Atom

Rutherford is the scientist that stated the structure of an atom and he stated that;a] The diameter of an atom is about 10^↑
b] The diameter of the nucleus of an atom is 10^-14.

c] The region around the nucleus is large compared to the space occupied by the nucleus.

4] The electrons are located round the nucleus.

5] The mass of an atom is concentrated at the nucleus.
Those are the Rutherford postulate, other scientists that also work on atom are;
Milikan : milikan is the scientist that discover electron and the charge on the electron as -6 × 10^19 C


Also J.J Thompson stated that, atom are spherical and contain positively charge which are embedded with electron. And he give the specific charge ratio of atom as; top↑


Electronic Configuration

Electron Distribution

Electron distribution : is how electrons are arranged outside and around the nucleus which is also called Electronic Configuration
The electrons are arranged in the orbit known as Shells, The electrons revolve round the centrally placed nucleus in these shells.


There are seven shells in all, and they are denoted by symbols K, L, M, N, O, P, and Q The nearest shell to the nucleus is K shell, it has the lowest energy and contains the smallest number of electrons which is two. K shell cannot hold more than two elections.

Q shell is the farthest shell to the nucleus, hence Q shell has the highest energy and contains many electrons as many as 98 electrons.

K shell

The maximum number of electron that can be accommodated by each shell is given as 2n²
Where n = is the number of shell top↑

In K shell n = 1
So therefore, 2n² = 2(1)²
= 2
Hence, K shell can only occupy 2 electrons.
In L shell n = 2
Hence, 2n² = 2(2)²
= 8
Hence, L shell can only occupy 8 or less electron.
In M shell n = 3
So therefore, 2n² = 2(3)²
= 18
Hence, M shell can only occupy 18 or less electron.

In N shell n = 4
Therefore, 2n² = 2(4)²
= 32
Hence, N shell can only occupy 32 or less electron.

In O shell n = 5
Therefore, 2n² = 2(5)²
= 50
Hence, O shell can only occupy 50 or less electron.


Nehl Bohl’s Model About An Electron

a] Nehl bohl’s model show that the angular momentum (mvr) is an integral multiple of ;

mvr = h
b] The chemical properties of an element depends on the number of electrons in the outer shell.

c] The chemical and physical properties of an element depend on the arrangement of the electrons revolving round the nucleus.

d] The electrons move round the nucleus along imaginary circular path with certain amount of energy at a particular level.

e] Light energy is always emitted when electrons move from a higher energy level to a lower energy↑

Atomic Number And Atomic Mass Number

Atomic Number

Atomic number of an atom is the number of protons in the nucleus of an atom.
Atomic number increases by one in going from one element to the next heavier element on the periodic table.
For example , sodium (Na), sodium has atomic number of 11.

Note Atomic number of an element is always written below such element while its mass number is always written ahead of such↑
For better explanation, view the image below

Atomic mass

Atomic Mass Number

Atomic mass number is also called mass number, and it is the sum of the number of protons and the number of neutrons in an nucleus of an atom.

For example, Sodium (Na), sodium has proton number of 11 and neutron number of 12, adding both the proton number which is 11 and neutron number which is 12 together will give us 23 which is the mass or atomic mass number of sodium


Each shell is subdivided into orbitals. These orbitals are called s, p, d, f orbitals.
Each shell contains its orbitals.
For example, K shell contains only s orbital, while L shell contains s and p orbitals, M contains s, p and d orbitals and N shell contains s, p, d, f↑

Each orbital has a maximum number of electrons it can hold.
The sub-level S orbital has a maximum of 2 electrons, the sub-level P orbital has a maximum of 6 electrons, the sub-level D has a maximum of 10 electrons and the sub-level F orbital has a maximum of 14 electrons.

Shape Of Orbitals

a] S orbital has a linear shape.

b] P orbital has a dumbbell shape.
There are three types of P orbital, which are Px, Py and pz and they all have a dumbbell shape but facing different direction.

c] D orbital has a double dumbbell↑

d] F orbital has no definite shape.
In distributing the electrons In the atom so as to find the electronic configuration of the atom, the principal quantum shell K is first filled, then the 2nd principal quantum shell L and then the 3rd quantum shell M, then the 4th principal quantum shell N and so on….

Note : please pay attention to this part because it’s kinda tricky.
The sequence of filling up the orbitals with electrons is as follow;top↑

Please master the table below;

Aufbau Principle

2s² 2p6
3s² 3p6 3d10
4s² 4p6 4d10 4f14
5s² 5p6 5d10 5f14
6s² 6p6 6d10 6f14
7s² 7p6 7d10 7f14

1s 2s 2p 3s 3p 4s 3d 4d 5s

4s is filled before 3d because 4s energy level is lower than 3d energy level,
When you want to write an electronic configuration for an element, you start from 1s², 2s², 2p6 3s²…. you will start from the back to the front, as in, when you write 1s², 2s², 2p6 not 1s², 2s², 3s²top↑

Note : you use the atomic number of an element to determine its electronic configuration.

For example Sodium (Na), sodium has atomic number of 11
Sodium (Na) = 1s², 2s², 2p6, 3s¹

Calcium (Ca) which has atomic number of 20 will be written as follow;
Calcium (Ca) = 1s², 2s², 2p6, 3s², 3p6, 4s²

Aluminum (Al) which has atomic number of 13 will be written as follow;

Aluminum (Al) = 1s², 2s², 2p6, 3s², 3p¹

Note : if you add up all that figure together it will be equal to the atomic number of the element, this is also use to test a student during an↑

If you are given an element in this form;
1s², 2s², 2p6, 3s², 3p¹ and you are asked to calculate its atomic number, just add the spdf variables together

E.g 1s², 2s², 2p6, 3s², 3p¹ = 2+2+6+2+1 = 13
Hence the element is Aluminum, try the same thing for other elements.

In a situation where you have an element in ionic state, such as Al³+, O²- Ca²+, Mg²+ , F-, etc the method to calculate their electronic configuration is a little bit↑

For example Al³+, seeing Al³+ means aluminum has lose 3 electrons, so in order to draw it electronic configuration, you have to subtract the 3 electrons that it has lose from its atomic number;
Atomic number of Al – its lost charges

Hence, the atomic number of aluminum becomes 10 instead of 13

Al³+ = 1s², 2s², 2p6

Example ii
O²-, seeing this, is telling you that oxygen has gain 2 electrons, hence you add 2 to its atomic number which is 8, 8 + 2 = 10
hence, the electronic configuration of O²- is;
O²- = 1s², 2s², 2p6

Example iii
F-, seeing this, is telling you that fluorine has gain 1 electron, hence you add 1 to its atomic number which is 9, 9 +1 = 10
Hence, the electronic configuration of F- is;
F- = 1s², 2s², 2p6

Note : at the end of each shell, there must be a comma, whenever an element gain electrons, you add and whenever an element lose electrons you subtract. Metal reacts by losing electrons while non-metal reacts by gaining↑

Let stop here till next class, don’t forget to share the post, thank you.
Always feel free to ask me any question via the comment box or ask question page and I will be happy to answer your question and take the session again if necessary, your comment notifications is like a bank alert to me so don’t forget to write↑

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