Atomic Number: Do you know what is unique about each one of us? For a majority of people, it is not their name, neither their physical description. However, it could be our cell phone numbers, email addresses, or our DNA. Similarly, elements possess a unique number that contributes to their identity. This number is known as the Atomic Number. The atomic number of an element can be described as the number of protons present in the nucleus of each atom of that element. Let’s explore it in detail.

Atomic Number Definition

The atomic number or proton number of an element is defined as the number of protons present in the nucleus of each atom of that element. The number of protons is the characteristic feature that imparts uniqueness to each element compared to all other elements. Elements are different because of their atomic number. 

The Modern Periodic table displays all of the known elements arranged in the order of increasing atomic number. 

Representation of Atomic Number

The atomic number is represented by the letter ‘Z.’ The conventional symbol Z comes from the German word Zahl which means ‘number.’ Hence, Atomzahl means atomic number.

The atomic number of an element is indicated as a left subscript to its elemental symbol. For example, an atom or a nucleus of nitrogen (chemical symbol N) is written as_–

Similarly, hydrogen, which belongs to the upper left of the Periodic Table, has an atomic number of \(1\). Every hydrogen atom has one proton in its nucleus, followed by Helium, which has two protons. Lithium atoms have three protons, and so forth.

In a neutral atom, the number of both electrons and protons are equal.

Hence, Atomic number \(=\) number of protons \(=\) number of electrons

Atoms are neutral; hence, the number of electrons is equal to the number of protons. For example, the atomic number of manganese is \(25\). Hence, an atom of manganese would have twenty-five protons and twenty-five electrons.

Atomic Number and Periodic Table

As elements were being discovered, efforts were made to see if any kind of similarity or regularity was present in the information available for these elements. Mendeleev made an early attempt to organize these data, which led him to discover the first periodic table. The early periodic table was based on atomic weights and was instrumental in providing clues about the possible identity of new elements. After discovering the nucleus, Mendeleev went on to upgrade his periodic table based on the number of protons present in the nucleus of an atom, i.e., the atomic number. 

In the modern periodic table, the elements are arranged in the order of increasing atomic number.  The atomic number determines the number of protons, which determines the number of electrons surrounding the nucleus. It is the arrangement of these electrons that determines most of the chemical behaviour of an element. Hence, atomic number is vital to an element’s identity and chemical property.

Elements with similar chemical properties line up in the same column (group) ascending order of atomic numbers in the modern periodic table.  For instance, all of Group \({\rm{1A}}\) elements are relatively soft metals, form \(1+\) charges, and react violently with water. All of Group \({\rm{8A}}\) elements are monoatomic gases, stable and unreactive at room temperatures, etc.  Hence, there is a periodic repetition of the properties of the chemical elements with increasing atomic numbers.

Elements are also arranged in horizontal rows with increasing atomic numbers. These rows are called Periods. The maximum elements that a period can hold depending on the maximum electron present in the outermost shell of an atom. The period number represents the total number of shells present in the atom of an element. On moving across a period from left to right, the electrons get added to the shells gradually. The elements move to the next period when the outermost shell is saturated with eight or eighteen electrons.

Period number \(=\) number of shells	Number of elements
\(1\)	\(2\)
\(2\)	\(8\)
\(3\)	\(8\)
\(4\)	\(18\)
\(5\)	\(18\)
\(6\)	\(32\)
\(7\)	\(32\)

The classification of elements by atomic number allows us to understand many properties of the atom and makes it possible to predict behaviours instead of memorizing everything.

Examples of Atomic Number

The atomic number is equal to the number of protons in the nucleus of an atom or the number of electrons in an electrically neutral atom.

Atomic number \(=\) number of protons

For example, in a sodium atom, there are \(11\) electrons and \(11\) protons.

Thus the atomic Number of Na atom \(=\) Number of electrons \(=\) Number of protons \(=11\).

Atomic Number and Energy Levels

The atomic number gives the number of electrons, and electrons are distributed in different energy shells. Hence, there is a relation between atomic number and the energy levels present in the atom. A maximum of two electrons can be found in the first orbital of every element.

For example, the atomic number of carbon is \(6\). Hence, there are six electrons distributed as \(2, 4\) in two shells. The innermost shell consists of two electrons. Similarly, an atom with the atomic number \(12\) has an electronic structure \(2, 8, 2\), with two electrons in the inner energy level, then eight in the next energy level, and two in the outer highest energy level. 

Atomic Number and Isotopes

Isotopes are two or more types of atoms of a chemical element with the same atomic number and position in the periodic table but different mass numbers. Hence, they differ in nucleon number in their nuclei. Due to the difference in nucleon number, all isotopes of a given element have almost the same chemical properties, but they differ in their atomic masses and physical properties.

For example-  \({}_{\rm{8}}^{{\rm{16}}}{\rm{O,}}\,{\mkern 1mu} {}_{\rm{8}}^{{\rm{17}}}{\rm{O,}}\,{}_{\rm{8}}^{{\rm{18}}}{\rm{O}}\)

Atomic Number and Valency

Valency is the combining capacity of an element. It is governed by the number of electrons present in the outermost shell of an atom. The atomic number of an element gives the number of electrons. Hence, atomic number plays an important role in determining the valency of an element. Elements in the same group of the periodic table have the same valency.

Valency is given by-

For elements that belong to group I to group IV: Valency \(=\) number of electrons in the outermost shell. 

For elements that belong to group V to group VIII: Valency \(= 8\) – Number of electrons in the outermost shell. 

For example – Calcium \((Z = 20)\) belongs to group II of the Periodic Table has two electrons in its outermost shell. Hence, its valency is \(2\).

Nitrogen \((Z=7)\) belongs to group V of the Periodic Table has five electrons in its outermost shell. Hence, its valency is \(8 -5 = 3\).

Calculating Number of Neutrons

We know that the atomic number of an atom of an element gives its number of electrons and protons. Then, what about the number of neutrons? 

The number of neutrons of an atom of an element can be calculated as follows:

Mass Number \(–\) Atomic Number \(=\) number of neutrons

Or, Mass number \(–\) Number of protons \(=\) Number of neutrons

The mass number of an element is represented by A.

Hence, Number of neutrons \(=\mathrm{A}-\mathrm{Z}\)

Summary

The atomic number represented by Z refers to the number of protons in the nucleus of an atom and the number of electrons that surround the atom’s nucleus. Each chemical element has a different number of protons, so the atomic number is a unique identifier for an element. It is the governing principle behind the arrangement of elements in the periodic table of elements. However, the atomic number should not be confused with the mass number. Atomic Number represents the number of protons and electrons in a neutral atom; the mass number represents the sum of protons and neutrons in a particular atom. In this article, we learned the definition of atomic number, its representation, and the arrangement of elements in the Modern Periodic Table. We also learned how to calculate an atom’s neutron number and valency when its atomic number is given.

Frequently Asked Questions (FAQs) on Atomic Number

Q.1. Why does Z denote the atomic number?
Ans: The atomic number is represented by the letter ‘Z’ because it is the conventional symbol that comes from the German word Zahl which means ‘number.’ Hence, Atomzahl means atomic number.

Q.2. Is Lithium neutral?
Ans: Yes, a lithium atom is neutral because all atoms of a given element have the same number of protons and electrons given by the atomic number. Lithium atoms contain \(3\) positively charged protons in their nucleus and \(3\) negative electrons outside the nucleus.

Q.3. Which element has 15 electrons, 15 protons and 16 neutrons?
Ans: If there are \(15\) protons in an atom, this means the atomic number of the given element is also \(15\). The element with the atomic number \(15\) is Phosphorus. The mass number of Phosphorus is \(31\) (Atomic Number + Number of neutrons).

Q.4. What happens if the number of protons is being changed?
Ans: Changing the number of protons in an atom will change the atom from one element to a different element. Sometimes, adding a proton to an element makes the element radioactive. 

Q.5. What happens if an atom loses a neutron?
Ans: When you remove or add a neutron to the nucleus of an atom, the resulting substance is a new type of the same element and is called an isotope.

Learn About Atomic Number & Mass Number

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