Question

“The atomic number and mass number of sodium are 11 and 23 respectively.” What information does this statement convey?

Answer 1

Hint: Elements in the modern periodic table are arranged according to their increasing atomic number, which generally denotes its number of electrons. Atomic mass, on the other hand, denotes the mass of the nucleus of the atom (which is composed of neutrons and protons).

Complete step by step answer:
Let us understand the meaning of atomic number and mass number first.
Atomic number –
It is denoted by the letter ‘Z’.
Atomic number is also known as a proton number.
It denotes the number of protons found in the nucleus of every atom of that element.
All elements have different atomic numbers. It, therefore, uniquely identifies a chemical element.
It is identical to the charge number of the nucleus.
The elements in the modern periodic table are arranged according to their atomic elements.
Atomic mass –
It is denoted by the letter ‘A’.
It is equal to the sum of protons and neutrons in an atom.
Elements in Mendeleev’s periodic table were arranged according to their atomic weights.
It is expressed as ‘atomic mass units’ or ‘a.m.u.’ or ‘u’.
Therefore, we can conclude the following from the discussion above –
Z = p = e
A = p + n
Where,
Z – atomic number
A – Atomic mass
p – number of protons
n – number of neutrons
e – number of electrons.
According to the question, atomic number and mass number of sodium are 11 and 23 respectively.
Therefore,
Z = 11, A = 23
Since, Z = p = e,
The number of protons and electrons in sodium are 11.
Since A = p + n,
So, n = A – p,
The number of neutrons in Sodium are 23 – 11 = 12.

Additional Information:
Sodium is a s-block element of the periodic table, which is placed in group 1 and period 3 of the modern periodic table.

Note: The elements having the same atomic number but different atomic numbers are called isotopes of each other. Isotopes occur because they contain different numbers of neutrons. There are many elements in the periodic table that have isotopes. For example – Hydrogen. The three main isotopes are:
Protium – 0 neutron
Deuterium – 1 neutron
Tritium – 2 neutrons