It was once believed that every matter in the world was made up of minute indivisible substances. The Greek philosopher Aristotle objected to this statement and belief. It was later found that every matter is made up of atoms that are further divisible. Then came the protons, neutrons, and electrons. Researchers have gone on to discover positrons and the god particle. The mass number of electrons, protons, and neutrons characterise any metal or material’s chemical and physical nature. The basic properties of atoms include the count of such subatomic particles.

The subatomic particles: What is an atom made of?

The subatomic particles are the particles that exist within the atom. Breaking an atom yields its subatomic particles. The subatomic particles of an atom decide the characteristics of the molecules. The subatomic particles present within an atom are;

Protons 

  • Protons are the positive particles present within an atom. 
  • They exist within the dense nucleus, the centre of an atom.
  • They remain within the nucleus repelling each other proton within the bound of the neutrons.

Electrons 

  • Electrons are the negative particles present within an atom. 
  • They exist as clouds around the atoms in the 3D configurations called the orbits, the lighter periphery of the atom. 
  • They are continuously revolving around the nucleus. 
  • They remain bound within the atom due to the force of attraction. The force of attraction is between the electron’s negative and positive charges of protons in the nucleus.

Neutrons

  • They share the nuclear space of an atom with the protons.
  • They are neutral in charge. 
  • The mass of neutrons and protons makes the nucleus a denser space.
  • The number of neutrons in an atom is always equal to the count of protons.

Basic properties of an atom

Atomic number

  • The overall count of either electrons or protons present in an atom is the Atomic number of an atom.
  • The atomic number of an atom is denoted by ‘Z’.
  • It is written as a subscript in the element symbol.
  • The atomic number differs with each element of the periodic table.

Mass number

  • Mass number definition is the collective sum of the number of protons, and electrons in an atom are the atom’s mass number.
  • The mass number is also called Atomic Mass.
  • It is denoted by the alphabet ‘A’.
  • It is written as a superscript in the element symbol.

What is the Mass Number(A) of an atom?

  • The mass number is the weight of the nucleus of an atom. It gives the mass of the atom because the weight of an electron is too small for considering it for major calculations.
  • For example, in a Carbon atom, the number of electrons and protons is six each. The atomic number of the carbon atom is Z = 6. The mass number of the carbon atom is A = 6+6 = 12.
  • It is also calculated by doubling the atom’s atomic number(Z) of the atom. But not in all cases.
  • For example, the mass number of a carbon atom is A = 12, i.e., double the time the atomic number[Z = 6; A = 2(6) = 12].
  • The exception is in the cases of isotopes of an atom. 
  • For example, an isotope of Carbon-12(12 denotes the mass number) is Carbon-13. In C-13, the number of protons is, as the normal carbon, 6. At the same time, the number of neutrons is 7.
  • Thus the atomic mass of C-13 is A = 6+7 = 13. Thus, it is C-13.
  • Isotopes are atoms having the same Atomic number (Z) and a different Mass number(A). They have the same number of protons and electrons as the usual elements but differ in the total count of neutrons. 

Calculating the Mass number

  • The mass number of an atom is calculated by adding the number of protons and the number of neutrons in an atom.
  • Example – If Protons = 5 and neutrons = 5, then the Mass number is

A = No. of protons + No. of neutrons

A = 5 + 5

A = 10

  • Subtracting the atomic number from the atomic mass gives the value of the neutrons of an atom.
  • No. of neutrons = Atomic number(Z) – Atomic mass(A)
  • Example – If the atomic number of an atom is 15 and the mass number is 31,

No. of neutrons = 15 – 31

No. of neutrons = 16

  • When the mass number and atomic number are known, the element from the periodic table can be found because these are mostly fixed and verified values. They are often unique to every element.
  • Here, never finalise the value as a negative number. A mass number or atomic number is never a negative value because if it is negative, it means it is less than 0. 
  • It means the atom has a mass of less than zero. That is practically impossible and logically never makes sense.

Conclusion

An atom consists of subatomic particles like electrons, protons, and neutrons. The number of electrons is generally equal to protons. Protons are, generally, equal to neutrons in number. As per the mass number definition, it is the total weight of an atom. The electrons are neglected in calculating the mass of an atom. Thus, the mass number is calculated by the sum of the neutrons and protons in an atom.