How has the Model of the Atom Changed Over the Years?

How has the Model of the Atom Changed Over the Years?

The Atomic Structure

The historical development of atomic models:
Over the last 100 years, scientists have done investigations which show that atoms are made up of even smaller particles.

	The atom was imagined as a small indivisible ball similar to a very tiny ball.
	J. J. Thomson discovered the electron, a negatively-charged particle. The atom was described as a positively-charged sphere embedded with electrons.
	Ernest Rutherford discovered the proton, a positively-charged particle in an atom. The central region of an atom has a very small positively-charged nucleus which contains almost all the mass of the atom. This nucleus is tiny and the rest of the atom is mostly empty space. Sufficient electrons surround the nucleus. This atomic model was discovered through the bombardment experiment of alpha particles on gold foil.
	The electrons in an atom move in shells around the nucleus which contains protons.
	James Chadwick proved the existence of neutrons, the neutral particles in the nucleus. The nucleus of an atom contains protons and neutrons and the nucleus is surrounded by electrons. Neutrons contribute approximately to half the mass of an atom.

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Subatomic Particles Examples

Subatomic particles of an atom:

1. Atoms are made up of three types of smaller particles, namely protons, neutrons and electrons.
2. These particles are known as subatomic particles.
3. The relative masses and charges of these three subatomic particles are shown in Table 2.4. The masses and charges are measured relative to a proton because their actual values are incredibly small.
   Relative masses and charges of subatomic particles

   Subatomic particle	Symbol	Relative mass	Relative electric charge
   Proton	P	1	+ 1
   Neutron	n	1	0
   Electron	e–	1/1840	-1

4. Protons and neutrons are found in the nucleus of an atom while electrons surround the nucleus.
   Nucleus contains protons and neutrons
   (a) As the masses of protons and neutrons are greater than electrons, most of the mass of an atom is concentrated in the nucleus.
   
   (b) The nucleus has an overall positive charge due to the positively-charged protons in it.
   (c) An atom consists of an equal number of electrons and protons. Hence, an atom is electrically neutral.

How do you determine the number of nucleons?

Proton number and nucleon number:

1. Scientists use the proton number and nucleon number to describe an atom.
2. Proton number of an element is the number of protons in its atom.
3. Since atoms are neutral, the proton number is also the number of electrons in the atom.
4. Each element has its own proton number. For example, sodium has a proton number of 11. Hence, all atoms of sodium have 11 protons. Oxygen has a proton number of 8, so all oxygen atoms have 8 protons.
5. Nucleon number of an element is the total number of protons and neutrons in its atom. From definition,
   nucleon number = number of protons + number of neutrons
   However, number of protons = proton number
   Therefore,
   nucleon number = proton number + number of neutrons
   or
   number of neutrons = nucleon number – Proton number
6. The nucleon number is also known as the mass number.
7. The relative mass of an atom is almost the same as its nucleon number. The nucleon number is sometimes used as the approximate relative mass in calculations.
8. Protons and neutrons are collectively called nucleons because protons and neutrons occupy the nucleus.
9. In a neutral atom, number of electrons = number of protons.

Proton number and Nucleon number Example Problems with Solutions

1. A chlorine atom has 17 protons and 18 neutrons.What are the proton number and nucleon number of the atom?
Solution:
Proton number = number of protons = 17
Nucleon number= number of protons + number of neutrons
= 17 + 18
= 35

2. Lithium has a proton number of 3 and a nucleon number of 7. How many protons, electrons and neutrons are present in an atom of lithium?
Solution:
Number of protons = proton number = 3
An atom is neutral.
Therefore,
number of electrons = number of protons = 3
Number of neutrons
= nucleon number – proton number
= 7 – 3
= 4

What element is represented by the symbol N?

Symbols of elements:

1. Each element is given a name and a symbol. Some examples of elements and their symbols are shown in Table.
   

   Element	Symbol	Element	Symbol
   Hydrogen	H	Sodium	Na
   Helium	He	Magnesium	Mg
   Lithium	Li	Aluminium	Al
   Beryllium	Be	Silicon	Si
   Boron	B	Phosphorus	P
   Carbon	C	Sulphur	S
   Nitrogen	N	Chlorine	Cl
   Oxygen	O	Argon	Ar
   Fluorine	F	Potassium	K
   Neon	Ne	Calcium	Ca

2. Notice that:
   (a) each symbol consists of one or two letters. For elements with two-letter symbols, the first letter is always a capital letter while the second letter is always a small letter.
   (b) for most elements, the letters used in their symbols take either the first letter or the first and another letter of their names. Some examples include the symbols of hydrogen, H; neon, Ne and magnesium, Mg.
   (c) for some elements, the symbols come from Latin names such as natrium (Na) for sodium and kalium (K) for potassium.
3. The standard representation for an atom of any element shows the proton number and the nucleon number of the element. It can be written as follows:
   
   For example, a sodium atom is represented as \(_{ 11 }^{ 23 }{ Na }\). This means that sodium has a proton number of 11 and a nucleon number of 23.
4. Sometimes an element is represented by using only the nucleon number. For example, \(_{ 11 }^{ 23 }{ Na }\) is represented by sodium-23.

Representation of Atom Example Problems with Solutions

1. A carbon atom has 6 protons and 7 neutrons. Represent the atom in the form of \(_{ Z }^{ A }{ X }\).
Solution:
The symbol of carbon is C.
A denotes the nucleon number.
Nucleon number = number of protons + number of neutrons
= 6 + 7 = 13
Z denotes the proton number.
Proton number = number of protons = 6
Hence, the carbon atom is represented as \(_{ 6 }^{ 13 }{ C }\).

2. Phosphorus-32 has 17 neutrons. What are the proton number and nucleon number of the atom? Represent the atom in the form of \(_{ Z }^{ A }{ X }\).
Solution:
The symbol of phosphorus is P.
Proton number = nucleon number – number of neutrons
= 32-17 = 15
Nucleon number = 32
Hence, the phosphorus atom is represented as \(_{ 15 }^{ 32 }{ P }\).