In the previous section, we saw the mass of protons, neutrons and electrons. In this section, we will learn about Mass number and Atomic number. Also later in this section, we will learn about the arrangement of electrons in the atoms.
We have seen that the mass of a proton = 1u, mass of a neutron = 1u, and mass of an electron = 0. Consider an atom in which there are 2 protons, 2 neutrons and 2 electrons. We can calculate the mass of that atom as follows:
• Mass of 2 protons = 2 × 1 u = 2 u
• Mass of 2 neutrons = 2 × 1 u = 2 u
• Mass of 2 electrons = 2 × 0 = 0
∴ Total mass = 2 u + 2 u + 0 = 4 u
The same steps can be written in a shorter form:
• Total mass = 2 u + 2 u + 0
• ⇒ Total mass = (2 + 2) × u = 4 u
What we have inside the parenthesis is the total number of protons and neutrons. So, to calculate the mass of the atom, all we have to do, is to add the number of protons and neutrons, and multiply the sum by u.
The steps become that easy due to the following reasons:
• Both protons and neutrons have the same '1u' as mass. So it can be taken outside the paranthesis
• The mass of electrons do not come into the calculations because it is zero
Let us verify this by taking another example:
An atom has 12 protons, 13 neutrons and 12 electrons. Find the mass of the atom:
Solution:
• Total mass = 12 × 1 u + 13 × 1 u + 12 × 0
• same as Total mass = (12 u + 13 u + 0)
• Same as Total mass = (12 + 13) × u
■ We get the same final step: Add the number of protons and neutrons, and multiply the sum by u.
This can be further simplified:
• If we get the sum of the number of protons and number of neutrons, we can straight away calculate the mass of the atom
• So this sum has special place in chemistry. Because, this sum gives the mass of the atom
• Once we get the sum, all we have to do is to multiply it by 'u'.
As the sum gives the mass directly, it is called the Mass number. So:
■ No. of protons + No. of neutrons = Mass number
■ Mass number is denoted by the letter 'A'
Significance of particles
We have seen the three particles: Protons, neutrons and electrons. Protons and neutrons are inside the nucleus. While electrons revolve around the nucleus in fixed orbits. When the atoms collide with each other, one or more electrons, especially, those in the outer orbits may be knocked out. When this happens, the number of electrons will become less than the number of protons. The ‘equality in the number of protons and electrons’ can change under other conditions also. For example, during chemical reactions, some electrons may get exchanged between atoms. That is., some atoms may lose some electrons, and those electrons will be accepted by some other atoms. So the losing atom will have a decreased number of electrons, while the accepting atom will have an increased number of electrons.
So we see that the number of electrons can change. But such changes do not happen to the number of protons. Because, they are inside the nucleus. So we choose the number protons to identify an element.
■ The total number of protons in an atom is called the Atomic number of that atom. It is represented by the letter Z
Based on A and Z, we may get problems involving the calculations of the ‘number of various particles’. An example is given below:
Solved example 2.1
For an atom, Z = 17, and A = 35. Find the number of protons, neutrons and electrons in that atom.
Solution:
• Given Z = 17 and A = 35
• For any atom, the atomic number Z is the number of protons in it. So we get:
■ No. of protons = 17
• For any atoms (except ions), number of protons is equal to the number of electrons. So we get:
■ No. of electrons = 17
• For any atom, mass number A = No. of protons + No. of neutrons
• So 35 = 17 + no. of neutrons
■ ∴ No. of neutrons = 35 – 17 = 18
Solved example 2.2
The mass of an atom is 4u. It has 2 protons in it's nucleus. How many neutrons does it have?
Solution:
• Given mass of the atom = 4u and No. of protons = 2
• We have, Mass of atom = (No. protons + No. of neutrons) × u
• So we get 4u = (2 + No. of neutrons) × u
• ⇒ 4 = 2 + No. of neutrons
• ∴ No. of neutrons = 4 - 2 = 2
We have learnt about 'symbol of elements'. When we write the symbol, it also represent ‘one atom’ of the element. To this symbol, we can attach the atomic number Z and mass number A. Both are written on the left side. A on left side top and Z on left side bottom. For example, Sodium (Na) has Z = 11 and Z = 23. We can write the symbol as shown in the fig.2.5(a). Fig.2.5(b) shows the general case where X is any element:
Arrangement of electrons in an atom
We have seen that the electrons orbit around the nucleus in definite orbits or shells. We have also seen that these orbits are given specific names: K, L, M, N etc., as well as specific numbers: 1, 2, 3, 4 etc.,
The electrons cannot take any orbit they like. There are strict rules by which the electrons are arranged in the orbits. Let us learn those rules:
• Rule 1: The orbits of lower energy levels gets filled first. So the K-shell (No. =1) is filled first. Because it has the lowest energy level. Then the L-shell (No. = 2) is filled. Then M-shell, and so on..
• Rule 2: The maximum number of electrons that can be accommodated in a shell is given by 2n2. Where n is the shell number.
♦ Based on this rule, the maximum number of electrons that can be accommodated in the K-shell = 2 × 12 = 2 × 1 = 2
♦ Maximum number of electrons that can be accommodated in the L-shell = 2 × 22 = 2 × 4 = 8
♦ Maximum number of electrons that can be accommodated in the M-shell = 2 × 32 = 2 × 9 = 18
♦ Maximum number of electrons that can be accommodated in the N-shell = 2 × 42 = 2 × 16 = 32
• Rule 3: The outer most shell in any atom can accommodate upto a maximum of 8 electrons only
We will now arrange the electrons in various atoms. While doing the arrangement, we will see the application of the three rules also. First we take the element hydrogen with atomic number 1. It has only one electron. That electron must occupy the shell with the lowest energy level, which is the K-shell. This is shown in the fig.2.6 below: A tabular form of the arrangement is also shown at the side.
In the tabular form, the electron is indicated by a small red square. It comes under the K-shell.
Next consider Helium, which has an atomic number 2. The fig. 2.7 below shows the arrangement:
The K-shell can accommodate 2 electrons. So the 2 electrons of Helium will occupy the K-shell.
Next consider Lithium, which has an atomic number 3. The fig. 2.8 below shows the arrangement:
The K-shell can accommodate a maximum of only 2 electrons. So the third electron of Lithium must go to the L-shell.
In this way, we can write the configuration for various elements. What happens when the L-shell gets filled up ? The next electron will go to the M-shell. To demonstrate this, we look at the configuration of Neon and Sodium together. The fig. 2.9 below shows the details:
Neon has 10 electrons. 2 electrons go to the K-shell. There are 8 electrons remaining. They can occupy the L-shell. But then, the L-shell is filled up. Because 8 is the maximum no. of electrons that the L-shell can accommodate. It does not cause a problem for Neon because, it does not have more than 10 electrons. But when we come to the next element Sodium, the number of electrons is 11. So the eleventh electron must occupy the M-shell.
The configuration of elements up to Argon can be seen here.
In the next section we will see some solved examples.
We have seen that the mass of a proton = 1u, mass of a neutron = 1u, and mass of an electron = 0. Consider an atom in which there are 2 protons, 2 neutrons and 2 electrons. We can calculate the mass of that atom as follows:
• Mass of 2 protons = 2 × 1 u = 2 u
• Mass of 2 neutrons = 2 × 1 u = 2 u
• Mass of 2 electrons = 2 × 0 = 0
∴ Total mass = 2 u + 2 u + 0 = 4 u
The same steps can be written in a shorter form:
• Total mass = 2 u + 2 u + 0
• ⇒ Total mass = (2 + 2) × u = 4 u
What we have inside the parenthesis is the total number of protons and neutrons. So, to calculate the mass of the atom, all we have to do, is to add the number of protons and neutrons, and multiply the sum by u.
The steps become that easy due to the following reasons:
• Both protons and neutrons have the same '1u' as mass. So it can be taken outside the paranthesis
• The mass of electrons do not come into the calculations because it is zero
Let us verify this by taking another example:
An atom has 12 protons, 13 neutrons and 12 electrons. Find the mass of the atom:
Solution:
• Total mass = 12 × 1 u + 13 × 1 u + 12 × 0
• same as Total mass = (12 u + 13 u + 0)
• Same as Total mass = (12 + 13) × u
■ We get the same final step: Add the number of protons and neutrons, and multiply the sum by u.
This can be further simplified:
• If we get the sum of the number of protons and number of neutrons, we can straight away calculate the mass of the atom
• So this sum has special place in chemistry. Because, this sum gives the mass of the atom
• Once we get the sum, all we have to do is to multiply it by 'u'.
As the sum gives the mass directly, it is called the Mass number. So:
■ No. of protons + No. of neutrons = Mass number
■ Mass number is denoted by the letter 'A'
Significance of particles
We have seen the three particles: Protons, neutrons and electrons. Protons and neutrons are inside the nucleus. While electrons revolve around the nucleus in fixed orbits. When the atoms collide with each other, one or more electrons, especially, those in the outer orbits may be knocked out. When this happens, the number of electrons will become less than the number of protons. The ‘equality in the number of protons and electrons’ can change under other conditions also. For example, during chemical reactions, some electrons may get exchanged between atoms. That is., some atoms may lose some electrons, and those electrons will be accepted by some other atoms. So the losing atom will have a decreased number of electrons, while the accepting atom will have an increased number of electrons.
So we see that the number of electrons can change. But such changes do not happen to the number of protons. Because, they are inside the nucleus. So we choose the number protons to identify an element.
■ The total number of protons in an atom is called the Atomic number of that atom. It is represented by the letter Z
Based on A and Z, we may get problems involving the calculations of the ‘number of various particles’. An example is given below:
Solved example 2.1
For an atom, Z = 17, and A = 35. Find the number of protons, neutrons and electrons in that atom.
Solution:
• Given Z = 17 and A = 35
• For any atom, the atomic number Z is the number of protons in it. So we get:
■ No. of protons = 17
• For any atoms (except ions), number of protons is equal to the number of electrons. So we get:
■ No. of electrons = 17
• For any atom, mass number A = No. of protons + No. of neutrons
• So 35 = 17 + no. of neutrons
■ ∴ No. of neutrons = 35 – 17 = 18
Solved example 2.2
The mass of an atom is 4u. It has 2 protons in it's nucleus. How many neutrons does it have?
Solution:
• Given mass of the atom = 4u and No. of protons = 2
• We have, Mass of atom = (No. protons + No. of neutrons) × u
• So we get 4u = (2 + No. of neutrons) × u
• ⇒ 4 = 2 + No. of neutrons
• ∴ No. of neutrons = 4 - 2 = 2
We have learnt about 'symbol of elements'. When we write the symbol, it also represent ‘one atom’ of the element. To this symbol, we can attach the atomic number Z and mass number A. Both are written on the left side. A on left side top and Z on left side bottom. For example, Sodium (Na) has Z = 11 and Z = 23. We can write the symbol as shown in the fig.2.5(a). Fig.2.5(b) shows the general case where X is any element:
 |
| Fig.2.5 |
We have seen that the electrons orbit around the nucleus in definite orbits or shells. We have also seen that these orbits are given specific names: K, L, M, N etc., as well as specific numbers: 1, 2, 3, 4 etc.,
The electrons cannot take any orbit they like. There are strict rules by which the electrons are arranged in the orbits. Let us learn those rules:
• Rule 1: The orbits of lower energy levels gets filled first. So the K-shell (No. =1) is filled first. Because it has the lowest energy level. Then the L-shell (No. = 2) is filled. Then M-shell, and so on..
• Rule 2: The maximum number of electrons that can be accommodated in a shell is given by 2n2. Where n is the shell number.
♦ Based on this rule, the maximum number of electrons that can be accommodated in the K-shell = 2 × 12 = 2 × 1 = 2
♦ Maximum number of electrons that can be accommodated in the L-shell = 2 × 22 = 2 × 4 = 8
♦ Maximum number of electrons that can be accommodated in the M-shell = 2 × 32 = 2 × 9 = 18
♦ Maximum number of electrons that can be accommodated in the N-shell = 2 × 42 = 2 × 16 = 32
• Rule 3: The outer most shell in any atom can accommodate upto a maximum of 8 electrons only
We will now arrange the electrons in various atoms. While doing the arrangement, we will see the application of the three rules also. First we take the element hydrogen with atomic number 1. It has only one electron. That electron must occupy the shell with the lowest energy level, which is the K-shell. This is shown in the fig.2.6 below: A tabular form of the arrangement is also shown at the side.
 |
| Fig.2.6 |
Next consider Helium, which has an atomic number 2. The fig. 2.7 below shows the arrangement:
 |
| Fig.2.7 |
Next consider Lithium, which has an atomic number 3. The fig. 2.8 below shows the arrangement:
 |
| Fig.2.8 |
In this way, we can write the configuration for various elements. What happens when the L-shell gets filled up ? The next electron will go to the M-shell. To demonstrate this, we look at the configuration of Neon and Sodium together. The fig. 2.9 below shows the details:
 |
| Fig.2.9 |
The configuration of elements up to Argon can be seen here.
In the next section we will see some solved examples.
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