Chemistry

Structure of the Atom

Atomic Number

We know that protons are present in the nucleus of an atom. It is the number of protons present in an atom which determines its atomic number. We can define atomic number as follows : The number of protons in one atom of an element is known as atomic number of that element.That is :
Atomic number Number of protons
of an element in one atom of element
For example, one atom of sodium element has 11 protons in it, so the atomic number of sodium is 11.Similarly, one atom of carbon element has 6 protons in it, so the atomic number of carbon is 6. And a hydrogen atom has just 1 proton in its nucleus,therefore the atomic number of hydrogen is 1.The atomic number of an element is denoted by the letter Z.The atomic number of sodium is 11, so we can say that for sodium, Z = 11. The atomic number of carbon is 6, so for carbon, Z = 6. And the atomic number of hydrogen is 1, therefore, for hydrogen, Z = 1.
All the atoms of the same element have the same number of protons in their nuclei,and hence they have the same atomic number.Atoms of different elements have different number of protons in their nuclei, so they have different atomic numbers.No two elements can have the same atomic number. Since each element has its own fixed atomic number, therefore, atomic number can be used to identify an element. For example, atomic number 6 tells us that it is carbon element. No other element can have atomic number of 6. Thus,it is the number of protons (or atomic number) which distinguishes the atoms of one element from the atoms of another element.The atoms of various elements are different because they have different atomic numbers (because of different number of protons in them).
In a normal atom (or neutral atom), the number of protons is equal to the number of electrons in it. So,we can also say that the atomic number of an element is equal to the number of electrons in a neutral atom of that element.That is,
Atomic number Number of electrons
of an element in one neutral atom
For example, one neutral atom of sodium contains 11 electrons, so the atomic number of sodium is 11.It is very important to note here that the atomic number of an element is equal to the number of electrons only in a neutral atom, and not in an ion because only a neutral atom contains an equal number of protons and electrons. On the other hand, an ion is formed by the removal of electrons from a normal atom or by the addition of electrons to a normal atom and, therefore, contains either less or more electrons than protons.
Only the electrons of an atom take part in chemical reactions, the protons do not take part in a chemical reaction. So, during a chemical reaction, the number of electrons in the atoms may change, but the number of protons remains the same. Thus,the atomic number of an element does not change during a chemical reaction, it remains the same.
It is clear from the above discussion that the atomic number of an element tells us two things :
1.It tells us the number of protons in one atom of the element.
2.It tells us the number of electrons in one normal atom of the element.
For example, the atomic number of sodium is 11. It tells us that a sodium atom contains 11 protons. It also tells us that a normal sodium atom contains 11 electrons. We will now discuss the mass number of an element.

Notes

Structure of the Atom - Notes

1. Drawback of Rutherford’s Model of the Atom