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Atomic Number and Mass Number - Understanding Protons, Neutrons and Isotopes

Last Updated on Feb 18, 2025
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Decoding Atomic Number and Mass Number

Atomic number and mass number are fundamental concepts in physics and chemistry. These numbers are always integers because they are derived from counting the number of protons, neutrons, and electrons in an atom. The sum of the mass number and the atomic number for an atom (often symbolized as A-Z) represents the total number of subatomic particles present in the atom. The mass number, specifically, denotes the mass of the atom's nucleus in atomic mass units (amu).

It's interesting to note that isotopes of an element share the same atomic number but differ in their neutron numbers and consequently their mass numbers. Most naturally occurring elements exist as a mixture of isotopes, and the standard atomic weight of an element is determined by the average isotopic mass of this isotopic mixture in a specific environment. This average isotopic mass is also referred to as the relative atomic mass. The concept of atomic weights of elements was first explored by scientists in the 19th century.

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Atomic Number

The atomic number of an element is determined by the number of protons in its nucleus. It serves as a unique identifier for each element. The mass number, on the other hand, is the sum of the number of protons and neutrons in an atom.

The modern periodic table is arranged such that elements are listed in order of increasing atomic number and subsequently, increasing mass number. But what exactly are these numbers and how are they calculated?

As we know, an atom consists of electrons, protons, and neutrons. The number of electrons in the outermost shell determines the valency of the atom. Meanwhile, the atomic number and mass number of the atom are associated with the number of protons and neutrons, respectively.

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Defining Atomic Number
  • The atomic number of an atom is the total number of protons in its nucleus.
  • It is represented with the letter ‘Z.’
  • All atoms of a particular element have the same atomic number, i.e., the same number of protons.
  • Different elements have different atomic numbers.
  • For instance, every carbon atom has an atomic number of 6, whereas every oxygen atom has 8 protons in its nucleus.

Defining Mass Number
  • The mass number of an atom is the sum of the number of protons and neutrons in its nucleus.
  • It is denoted using the letter ‘A.’
  • Protons and neutrons, which are located in the nucleus of an atom, are collectively called nucleons.
  • For instance, a carbon atom has 6 protons and 6 neutrons. So, its mass number is 12.
  • While all atoms of an element have the same number of protons, the number of neutrons can vary. Thus, atoms of the same element can have different mass numbers. These variants are known as isotopes.
  • The weight of an electron is almost negligible. Hence, the atomic mass of an atom is approximately the same as its mass number.

Tracing the History of Atomic Number

The concept of atoms as the fundamental building blocks of matter has not always been widely accepted. Around 2500 years ago, the Greek philosopher Leucippus and his pupil Demokritos proposed that the universe is composed of tiny, indivisible particles, which they termed atoms. However, the renowned Greek philosopher Aristotle disagreed with this theory, and his views prevailed for nearly 2000 years, effectively sidelining the concept of atoms.

Today, we understand that atoms form the basis of all matter. They combine in various ways to create different substances. With the exception of the common form of hydrogen, all atoms contain protons, neutrons, and electrons. The atomic number of an element equals the number of protons in its nucleus. In a neutral atom, the number of protons equals the number of electrons in the energy levels surrounding the nucleus.

Isotopes are atoms of the same element that have different numbers of neutrons, and hence, different mass numbers. The standard atomic weight of an element is determined by the average isotopic mass of an isotopic mixture in a specific environment on Earth.

Examples of Atomic Number

The atomic number of an atom equals the number of protons in its nucleus or the number of electrons in a neutral atom.

Atomic number = Number of protons

For instance, a sodium atom has 11 electrons and 11 protons. Therefore, the atomic number of a sodium atom = number of electrons = number of protons = 11.

Atomic Number and Orbital Energy Levels

Electrons at a specific energy level are more likely to be found in certain areas of that level, termed as orbitals. Sublevels consist of orbitals with the same energy. Each orbital can accommodate a maximum of two electrons.

The most common method of illustrating the arrangement of electrons in an atom is to draw diagrams like the one depicted below.

To write down the numbers of electrons in each energy level, we use the atomic number of an element, which tells us how many electrons there are in its atoms. For example, carbon has an atomic number of 6, so its electronic structure is 2,4, meaning it has two electrons in the inner energy level and four in the next energy level. An atom with an atomic number of 12 has an electronic structure of 2, 8, 2, with two electrons in the inner energy level, then eight in the next energy level and two in the outer highest energy level. To understand these arrangements better, it helps to examine several examples.

Distinguishing Between Valency, A, and Z

When calculating valency, we only consider the electrons in the outermost shell of the atom. However, to determine the atomic number or the mass number, we consider the total number of protons and neutrons.

Notation of an Atom

To write the notation of an atom, we need to know the symbol of the element, the atomic number, and the mass number. The mass number is written above the symbol, and the atomic number is written as a subscript.

Therefore, the notation of Carbon would be:

Calculating Number of Neutrons

If we know the number of protons and the mass number of an element, we can calculate the number of neutrons by subtracting the atomic number from the mass number.

 


 

Solved Example

Problem:

Given an atom with an atomic number of 9 and a mass number of 19, determine the following:

  1. The number of protons present
  2. The number of neutrons present
  3. The number of electrons present

Solution:

  1. The atomic number is always equal to the number of protons present. Therefore, there are 9 protons.
  2. The number of neutrons is obtained by subtracting the atomic number from the mass number. Therefore, there are 10 neutrons.
    (protons + neutrons) – protons = neutrons
  3. In an atom, the number of protons and the number of electrons are always the same. Therefore, there are 9 electrons.

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Frequently Asked Questions

With atomic number 118, oganesson (symbol Og) is the highest atomic number element discovered or synthesised. This is a “magic” number in the sense that it completes the seventh row of the periodic table of elements.

The number of protons (positive charges) in the nucleus of an atom is given by its atomic number. This term was first introduced by Henry Gwyn-Jefferies Moseley.

Add the mass of protons and neutrons to calculate the atomic mass of a single atom of an element. The number of protons and the number of neutrons together determine the mass number of an element: mass amount = protons + neutrons.

The atomic number symbol, Z, stands for “Zahl,” meaning German number. The symbol Z denoted an element’s place in the periodic table before 1915.

The complete amount in the protons and neutrons in the nucleus of an atom. For example, nitrogen has seven protons in its nucleus and seven neutrons, supplying it with 14 masses.

The number of protons and neutrons combines to give us the mass number of an atom. It is represented using the letter ‘A.’ The atomic number of an atom is equal to the number of protons in the nucleus of an atom or the number of electrons in an electrically neutral atom. It is represented using the letter ‘Z.’

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