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Atomic Number

Atom consists of subatomic particles like electrons, the negative particle, proton, the positive particles and neutron, the neutral particle.

Depending upon the number of these sub atomic particles present in an element, there are two properties which distinguish one atom from another.

The two important identity of an atom is:

  1. Atomic number
  2. Mass Number

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Atomic Mass Number

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We know that atom is made up of neutrons, protons and electrons. The sum of the mass of these constituent particles of an element is called its atomic mass. Thus,
Atomic mass of an element = Mass of neutrons + Mass of protons + Mass of electronsSince mass of an electron is almost nil (0.0005449), the atomic mass of an element is taken as:
Atomic mass of an element = Mass of neutrons + Mass of protons.Thus atomic mass number is defined as the sum of the number of protons and neutrons present in the nucleus of an atom.

For example,

1. The atomic mass of carbon = 12
The total mass of protons + neutrons = 12

2. The atomic mass of oxygen = 16
The total mass of protons + neutrons = 16

Element Symbol


The symbol of the element is taken from the name of element. And it also includes the mass number as superscript and atomic number as subscript.

For example,

1. Element symbol of carbon is given as 24Cr52

2. Element symbol of oxygen is given as 8O1

Atomic Number vs Atomic Mass

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According to the Rutherford atomic model, an atom has a nucleus, which carries a positive charge on it. The positive charge carried by the nucleus is the nuclear charge which is equal to the total number of positive charge on the nucleus of the atom of the element.

“The total number of unit positive charge on the nucleus is called the atomic number.”Since each proton has a +1 charge and neutron has no charge, the total number of unit positive charge on the nucleus is due to only the charge of the protons or is equal to the number of protons present in the nucleus of an atom.

Thus, Atomic number of an element is defined as: "The number of protons present in the nucleus.”
Thus, Atomic number of an element is defined as the number of unit positive charges on the nucleus (nuclear charge) of the atom of that element or as the number of protons present in the nucleus.Now, since the atom as a whole is electrically neutral, the number of protons or the positively charged particles in the nucleus is equal to the number of electrons or the negatively charged particles revolving around the nucleus. Consequently, atomic number of an element is also equal to the number of electrons revolving around the nucleus. Thus

Atomic number, Z = Number of unit positive charge on the nucleus
= Total number of unit positive charges carried by all protons present in the nucleus.
= Number of protons in the nucleus (p)
= Number of electrons revolving in the orbits (e)

So, z = p = e
Atomic number of an element is also defined as the serial number of its position in the periodic table, starting from hydrogen as the first element.

Comparing atomic mass Vs atomic number
Atomic mass involves the mass of particles inside the nucleus, while atomic number involves the number of positively charged or negatively charged particles present in an atom.

Atomic Mass Table

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Atomic number, atomic mass and table of elements

Element
Atomic number
Mass number
Protons
Neutrons
Electrons
H 1 1 1 0 1
He
2
4
2
4 – 2 = 2 2
Li
3
7
3
7 – 3 = 4 3
Be
4
9
4
9 – 4 = 5 4
B
5
11
5
11 – 5 = 6 5

Three different atomic mass scales are used to calculate the relative atomic mass:
  1. Hydrogen mass scale: The standard element selected here is Hydrogen. Its mass is unity and the atomic masses of other elements are compared with this.
  2. Oxygen atomic scale: Earlier, it was seen that if oxygen is taken as a standard and other elements are compared with it by taking atomic mass of oxygen as 16, most elements had atomic masses as whole numbers. On this scale, hydrogen had an atomic mass of 1.008. This scale was adopted before the discovery of isotopes.
  3. Carbon atomic mass scale: It was decided to select carbon – 12 as the standard element to take care of all discrepancies.
So, Atomic mass unit, amu, according to the carbon scale is defined as:

It is one twelfth $\frac{1}{12}$ of the mass of a single atom of C-12 isotope of carbon. → Read More

Atomic Weight

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Atomic mass is also expressed as atomic weight and is a dimensionless quantity.

It is given by the ratio of masses of all the atoms of an element to that of one twelfth of mass of carbon atom.

1 atom of 12C weighs 12.000 amu
1 gram-atomic weight of 12C weighs 12.000 g

Drawback of Using Hydrogen as Atomic Mass Unit

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Hydrogen gas in its natural state has 3 isotopes of atomic mass 1, 2 and 3 respectively. Thus average mass works out to be 1.008 a.m.u rather than 1 a.m.u. This in turn complicates the atomic masses of all other elements.

Later on, an atom of oxygen was preferred as standard by taking its mass as 16 units. In 1961, the international union of chemists selected the most stable isotope of carbon (C-12 isotope) as standard for comparison.

For example, if 1 atom of Na weights as much as 23 parts of $\frac{1}{12}$ of 12C isotopes, then the atomic mass of sodium is 23 a.m.u.

Atomic Mass Calculation

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Atomic mass or the atomic weight of an element is a useful tool in finding out the number of neutrons in an element.

Solved Examples

Question 1: How many neutrons are present in an element, which has 24 protons and has a mass number of 59?
Solution:
 
Since number of protons = number of electrons = atomic number.
So, atomic number = 24.
Mass number = 59.
Number of neutrons = 59 – 24 = 35
 

Question 2: An element has 12 electrons and 12 protons. Calculate the mass number and the atomic number of the element. Identify the element.
Solution:
 
Atomic number = number of protons = number of electrons.
Since number of electrons = 12, atomic number of the element is 12.
Mass number = number of protons + number of neutrons.
                           = number of electrons + number of neutrons
                           = Atomic number + number of neutrons
                          = 12 + 12 = 24.
Atomic number = 12. Element is Magnesium, Mg.
 

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