Classification of Elements and Periodicity in Properties - NEET MCQ

Explanation of Moseley's Straight-Line Graph

Moseley's law is an empirical law concerning the characteristic x-rays emitted by atoms. It states that the square root of the frequency of these x-rays is directly proportional to the atomic number of the element. In other words, the relationship between the atomic number and the frequency of the characteristic x-rays can be represented by a straight-line graph.

Option A: This would result in a curve, not a straight line, as the relationship between the atomic number and the frequency of characteristic X-rays is not linear.
Option B: This would also result in a curved graph, as the relationship between the square of the frequencies and the atomic number is not linear.
Option C: This is the correct answer because, according to Moseley's law, the square root of the frequency of characteristic X-rays is directly proportional to the atomic number of the element. Thus, plotting the square root of the frequencies against the atomic numbers will result in a straight-line graph.
Option D: This would not result in a straight-line graph, as the relationship between the reciprocal of the frequencies and the atomic number is not linear.

In summary, according to Moseley, a straight-line graph is obtained when plotting the square root of the frequencies of characteristic X-rays of elements against their atomic numbers. This relationship is expressed in Moseley's law and helps us understand the behavior of X-rays emitted by atoms.

Over 100 years ago, Henry Moseley carried out a systematic series of experiments that showed that the frequencies of the X-rays emitted from an elemental target under bombardment by cathode rays were characteristic of that element and could be used to identify the charge on its atomic nucleus.

• Mendeleev arranged the elements in order of their atomic masses while taking chemical properties into consideration.
• He left gaps predicting that some elements will be discovered later.
• When inert gases were discovered, they could be placed in a new group without disturbing his existing table.

Since statement A states that groups 7 and 8 were arranged on the basis of equivalent weights, it stands incorrect. 

In 1913, Henry Moseley showed that the atomic number of an element is a more fundamental property than its atomic mass.

• In 1829, Johann Dobereiner, a German scientist made some groups of three elements each and called them triads.
• He observed that the atomic mass of the middle element of a triad was nearly equal to the arithmetic mean of the atomic masses of the other two elements. 
• All three elements of a triad were similar in their properties.

• There are a total of 7 periods in the periodic table.
• The sixth period of the periodic table contains 32 elements.
• It is beginning with cesium and ends with radon.
• Lead (Pb) is currently the last stable element whereas all subsequent elements are radioactive.

• The horizontal row of the periodic table is called the Period.
• The vertical row of the periodic table is called the Group.
• There are seven periods and eighteen groups in a modern periodic table.

•  Third period of the modern periodic table contains 8 elements.

• John Newlands arranged the known elements in increasing order of their atomic masses.
• He started with the element having the lowest atomic mass ( hydrogen) and ended at thorium which was the 56th element.
• He found that every eighth element had properties similar to the first.

Döbereiner's Triads were groups of three elements that exhibited similar chemical properties. Johann Wolfgang Döbereiner, a German chemist, observed that when elements were grouped in sets of three (called triads), the atomic mass of the middle element was often approximately the average of the atomic masses of the other two.

One famous example of a Döbereiner's Triad is the Alkali Metals:

• Lithium (Li)
• Sodium (Na)
• Potassium (K)

These three elements have similar chemical behaviors, such as being highly reactive and forming strong bases when combined with water. Döbereiner also noticed that the atomic mass of Sodium (Na) (approximately 23) is about the average of Lithium (Li) (approximately 7) and Potassium (K) (approximately 39). This made them a classic example of a triad.

• The periodic table of today owes its development to two chemists namely the Russian chemist Dmitri Mendeleev and the German chemist J. Lother Meyer.
• In 1869, they independently proposed that when the elements are arranged in the increasing order of their atomic weights, similarities in both physical and chemical properties appear at regular intervals.

• Mendeleev predicted the existence of 'eka-silicon', which would fit into a gap next to silicon.
• The element germanium was discovered later.
• Its properties were found to be similar to the predicted ones and confirmed Mendeleev's periodic table.

Eka aluminum predicted by Mendeleev is Gallium.

Eka-aluminum and gallium are two names for the same element, as Eka-Aluminium has almost identical properties to the gallium element itself.

Additional information: Mendeleev names unnamed elements as EKA- Boron EKA- Aluminium and EKA Silicon which were later replaced as Scandium, Gallium, and germanium respectively.

• Eka boron is The element Scandium.
• Eka aluminum is the element Gallium.
• Eka silicon is the element Germanium.

Dobereiner's Triads were groups of three elements with similar chemical properties, where the atomic mass of the middle element was approximately equal to the average of the other two. Carbon, being a nonmetal with a unique set of properties, did not fit into any known triad.

In the long form of the periodic table elements are arranged in increasing order of their Atomic number.

Newland's Law of Octaves arranged elements in groups of seven, resembling musical octaves. However, this law did not leave room for the discovery of noble gases, which have very different properties compared to the elements in Newland's groups.

• Gallium and Germanium were the elements not discovered at that time and Mendeleev put gaps in the periodic table.
• Gallium was called Eka aluminium
• Germanium was called as Eka silicon

Mendeleev's Periodic Table was initially arranged based on increasing atomic mass. He noticed that elements with similar properties appeared at regular intervals when arranged in order of increasing atomic mass.

Periods:

• Elements are arranged in increasing the atomic number of elements in a period.
• One extra electron gets added to the outermost shell as we move along the periods from left to right.
• The electron gets added to the same shell or orbit and thus the electrons present for bonding increase by one unit.
• Thus, the shell number remains the same but the number of electrons present for bonding increases along a period.

Groups:

• Elements having the same number of outer electrons are put in the same group of the periodic table.
• When we move down a group, one extra shell gets added to the elements.
• The outermost shell has electrons present for bonding.
• Though the number of shells increases as we go down in a group, the number of electrons in the outermost shell remains the same.
• For example, the Halogens F, Cl, Br, I, At all belong to group 17 and have 7 electrons in the outermost shell.
• Similarly, Group 16 elements have 6 electrons in the outermost shell, group 15 has 5 electrons in the outermost shell, and so on.

Hence, group 17 is called halogens.

The Chalcogens are the elements found in Group 16 of the periodic table. The name "chalcogen" comes from the Greek words chalcos (meaning "ore") and gen (meaning "to form"), because many ores contain oxygen and sulfur, two of the chalcogens.

The elements in Group 16 include:

1. Oxygen (O)
2. Sulfur (S)
3. Selenium (Se)
4. Tellurium (Te)
5. Polonium (Po)

These elements share some chemical properties, such as forming compounds by gaining two electrons, leading to a common oxidation state of -2. Oxygen, the most well-known chalcogen, is a vital component of water and air, while sulfur is widely used in industries like rubber manufacturing and fertilizers.