5.1 History of the Periodic Table

Summary

This video covers the history and structure of the periodic table, focusing on key figures like Dmitri Mendeleev and Henry Moseley. It explains how Mendeleev arranged elements by atomic mass, discovering periodic properties, and how Moseley later improved this system by ordering elements by atomic number. The video also introduces key groups, such as alkali metals, halogens, noble gases, and the lanthanides and actinides, explaining their properties and significance. The periodic law and periodicity of elements are highlighted, showcasing the recurring chemical and physical properties across periods and groups.

Takeaways

• 📜 Chemists faced challenges before 1860 due to various systems of measurement, making communication difficult.
• 🧪 In 1860, the first International Congress of Chemists in Germany standardized the method of measuring atomic mass.
• 🔬 Dmitri Mendeleev created a chemistry textbook and arranged elements by increasing atomic mass, discovering periodic patterns.
• 📏 Mendeleev noted that certain properties of elements repeated at regular intervals when ordered by atomic mass.
• 🔮 Mendeleev predicted three elements and their properties, which were later confirmed, demonstrating the accuracy of his system.
• ⚛️ Henry Moseley refined Mendeleev's system by relating periodic properties to the atomic number rather than atomic mass.
• 🧮 The periodic law states that chemical and physical properties of elements are periodic functions of their atomic number.
• 📊 The modern periodic table is organized by ascending atomic number, grouping elements with similar properties in vertical columns (groups).
• 🏅 Group names include alkali metals, alkaline earth metals, halogens, and noble gases, with noble gases being highly unreactive.
• 📉 Lanthanides and actinides are placed separately at the bottom of the table due to their similar properties, but to save space.

Q & A

What was the major challenge in chemistry before 1860?

Before 1860, different measurement systems were used for masses, making it difficult for scientists to communicate their results effectively.

What was the outcome of the 1860 International Congress of Chemists in Germany?

The 1860 Congress standardized a system for measuring atomic mass, enabling scientists to investigate the relationship between atomic mass and other properties more effectively.

How did Dmitri Mendeleev contribute to the development of the periodic table?

Dmitri Mendeleev organized elements by their atomic mass and discovered that their properties repeated periodically, leading him to predict the existence and properties of three undiscovered elements.

What key discovery did Henry Moseley make regarding the periodic table?

Henry Moseley discovered that the periodic law worked better when elements were arranged by atomic number (number of protons) rather than atomic mass, establishing the modern understanding of the periodic table.

What is the periodic law as defined by Henry Moseley?

The periodic law states that the physical and chemical properties of elements are periodic functions of their atomic numbers.

How are elements grouped in the modern periodic table?

In the modern periodic table, elements are arranged by ascending atomic number and grouped into vertical columns, or groups, based on similar chemical properties.

What are alkali metals, and where are they found in the periodic table?

Alkali metals are highly reactive elements found in Group 1 of the periodic table.

Why are noble gases referred to as 'noble,' and where are they located in the periodic table?

Noble gases are called 'noble' because they are chemically unreactive. They are located in the far-right column of the periodic table.

Why are the lanthanides and actinides placed separately from the rest of the periodic table?

The lanthanides and actinides are placed separately to save space, as they would otherwise stretch the table horizontally, but they are part of periods 6 and 7 and share similar chemical properties.

What is periodicity in the context of the periodic table?

Periodicity refers to the recurring pattern of chemical properties at regular intervals, as elements in the same group display similar characteristics periodically across different periods.

Outlines

00:00

🧪 Evolution of the Periodic Table

This paragraph introduces the historical context of the periodic table's development. It starts with the difficulties faced by chemists in the 1860s due to varied measurement systems, which led to the first international congress of chemists in Germany where atomic mass measurement was standardized. Dmitri Mendeleev, a Russian scientist, is highlighted for organizing elements by atomic mass, discovering that certain properties reoccurred periodically. His confidence in this discovery led to predictions of three elements and their properties, which were later confirmed. The paragraph also introduces Henry Moseley, who refined Mendeleev's work by using atomic numbers instead of atomic mass to organize elements, leading to the modern understanding of the periodic table and the formulation of the periodic law.

05:01

🔍 Structure and Groups of the Modern Periodic Table

This paragraph explains the modern periodic table's structure, including vertical columns called groups and horizontal rows called periods. It discusses how elements in the same group, such as the alkali metals, alkaline earth metals, halogens, and noble gases, share similar chemical properties. The discovery of noble gases like argon and helium, which were identified as unreactive, is highlighted. The lanthanides and actinides, two rows located at the bottom of the table, are mentioned for their chemical similarities despite not being in the same group. The concept of 'periodicity' is also covered, explaining that regular intervals of similar properties recur one period apart and that the spacing in atomic numbers between elements in the same group is consistent.

Keywords

💡Atomic Mass

Atomic mass refers to the mass of an atom, typically measured in atomic mass units (amu). In the video, the concept of atomic mass is crucial because, before 1860, scientists used various systems to measure it, which caused difficulties in communicating their results. The development of a standardized system for measuring atomic mass was a key factor in understanding relationships between elements.

💡Dmitri Mendeleev

Dmitri Mendeleev was a Russian scientist who is credited with developing the periodic table. The video highlights how Mendeleev organized elements by their atomic mass and noticed that certain properties repeated periodically. His ability to predict the existence and properties of undiscovered elements solidified the importance of his work.

💡Periodic Law

The periodic law is the principle that the chemical and physical properties of elements are periodic functions of their atomic number. In the video, this concept is tied to both Mendeleev's and Henry Moseley's contributions, showing how the arrangement of elements by atomic number rather than mass revealed recurring properties at regular intervals.

💡Henry Moseley

Henry Moseley was an English scientist who improved upon Mendeleev's periodic law by showing that the properties of elements align better with atomic number, rather than atomic mass. His work with the spectrums of elements helped establish the modern periodic law, as described in the video.

💡Groups

Groups are vertical columns in the periodic table where elements share similar chemical properties. In the video, groups such as the alkali metals (Group 1) and halogens (Group 17) are discussed to show how elements in the same group react similarly due to periodicity in their properties.

💡Periods

Periods are horizontal rows on the periodic table, where elements are arranged by increasing atomic number. The video explains that as you move across a period, the properties of elements change but recur in the same pattern from one period to the next, a concept known as periodicity.

💡Periodic Table

The periodic table is a tabular arrangement of elements according to their atomic number, with elements in the same column (group) having similar properties. The video explains its evolution, from Mendeleev's early arrangement by atomic mass to the modern form that organizes elements by atomic number.

💡Alkali Metals

Alkali metals are the elements in Group 1 of the periodic table, such as lithium, sodium, and potassium. They are highly reactive, especially with water. The video uses alkali metals as an example of how elements in the same group share similar properties.

💡Noble Gases

Noble gases are the elements in Group 18, including helium, neon, and argon. These gases are known for being unreactive, which is why they are called 'noble.' The video describes how the discovery of argon and its unreactive nature led to the creation of this group in the periodic table.

💡Lanthanides and Actinides

The lanthanides and actinides are two rows of elements placed below the main body of the periodic table. These elements, despite being placed separately, are part of periods 6 and 7. The video explains that they are chemically similar and are positioned this way to save space in the table.

Highlights

Before 1860, chemists used various measurement systems, making communication difficult.

In 1860, the first International Congress of Chemists standardized atomic mass measurement.

Dimitri Mendeleev used the standardized atomic mass to investigate element properties.

Mendeleev organized elements by mass and discovered periodic properties.

Mendeleev's periodic law predicted the existence and properties of elements yet to be discovered.

Henry Moseley improved the periodic law by relating it to atomic numbers.

The modern periodic table groups elements by similar chemical properties.

Elements in the same group of the periodic table share common properties.

The discovery of argon led to the formation of the noble gases group.

Helium was identified as a noble gas through its spectrum in the sun.

Lanthanum and actinium are the first elements in the lanthanides and actinides rows.

Lanthanides and actinides are chemically similar and placed below the main table for space.

The periodic table is divided into groups and periods, reflecting periodicity.

Noble gases are found at the end of each period, indicating regular intervals of properties.

Halogens occur one space before the end of each period, demonstrating periodicity.

The atomic number difference between elements in the same group is consistent across periods.

Transcripts

in this video we're covering chapter one

section five

uh history of the periodic table and

just some background before 1860

people use various different measurement

systems masses etc

to carry out the results in chemistry

and this made it very hard for

scientists to communicate the results to

one another so

in 1860

they convey a bunch of chemists conveyed

in germany

for the first international congress of

chemists

and they standardized a way for

measuring atomic mass

and this allowed them to investigate the

relationship between mass

and other properties

so one scientist we're going to be

looking at today who used this

relationship

between atomic mass and other properties

was a man by the name of dimitri

mendeleev now while main delayev was

making a chemistry textbook because he

was a russian scientist

he

decided to lay out the elements by their

mass

and see if there was any correspondence

to various properties

so we wrote all the elements on a note

card

on different note cards like he had

hydrogen or gold

or whatever

and he laid them out in order of their

mass

and then what he found

was that if he did it the right way

and restarted his rows at certain

intervals

that

certain properties

of elements would recur

at what are called periodic intervals

intervals

in other words

he found that the same properties of

elements

occurred over

over and over again

as the mass increased

they occurred

let's say

in the example of these boxes

hypothetically you'd find the same

shininess or chemical reactivity or what

have you

every fourth box

now this isn't the case in actual

chemistry but the point is he figured

out a system

for a corresponding

mass with

various properties at regular intervals

and he was so confident in this

that he predicted

you know three other elements

that would be discovered and their

properties

and all of these were later confirmed a

few decades

after he predicted them

and though this sort of periodic law as

it were

the periodicity of

ascending atomic mass

was quite evident to chemists after

mendeleev they still didn't know why

this was the case so now we get to a

english scientist by the name of henry

mosley who was working with rutherford

and investigating spectrums of the

periodic table

and he found that this periodic law

that

mendeleev had discovered

worked better

when

you laid out the elements by their

number of protons

not their mass

and this came to be known as the

periodic law

which states that physical and chemical

properties of elements

are periodic functions of their atomic

numbers

so basically the atomic number

which is again the number of protons in

an element

dictates

when certain chemical and physical

properties will show up

so every so often when you're going

along by ascending atomic number you'll

come across

x uh chemical property so that could be

you know extreme reactivity with

uh halogens or something

basically what mosley did was he

improved on mendeleev's law by relating

it to the atomic number

which we know is the more correct

sort of answer to the problem as we'll

see later so now we get to the modern

periodic table which

groups elements in vertical columns

called groups

by similar chemical properties so you'll

find that

elements in say call column one or group

one share very common properties as do

elements in

two

or

as do

elements that are in rows over here

basically it's arranged by ascending

atomic number

from left to right

and then broken up

over here

at regular intervals to provide

similar properties going down the

columns and these groups are often given

names for example group one is

called the alkali metals

group two is called the alkaline earth

metals

over here

with fluorine chlorine bromine

and iodine

this group is known as the halogens

and

for a long time chemists had known about

you know sort of

these upper regions of the periodic

table

except for this column over here

it wasn't until the late 1800s that

chemists started separating the

atmosphere

to find its various components and their

exact

proportions

and they came across this gas right here

argon

and they noticed it was highly

unreactive now they match this again

with an element spectrum that they found

in the sun years earlier

helium

to form a new group on the end

known as the noble gases

noble because they're unreactive

now down here at the bottom you may have

been wondering why these two rows are

separated from the table

so they actually are

in periods six and seven

and they are known as the lanthanides

and the actinides

named after

the first element

in each

row down here

and they are very similar chemically

despite the fact that they're not in the

same

group you can still have chemically

similar elements that are near each

other

and they're usually

placed below the rest of the table to

save space

because otherwise you would have to fit

that whole horizontal box

right in here where these two blank

spaces are on the periodic table

so as i mentioned before the table is

divided up into vertical

columns

known as groups

and horizontal rows

known as periods now we'll study periods

more later

but the important thing about periods

is that

the

regular intervals

of

similar properties occur one period

apart which is why the table is laid out

like this

and this is a property known as

periodicity

and this can be observed in all groups

so you'll notice

that in each period there is noble gas

on the end

and these all form a group because

they're chemically very similar

but they recur regularly

at the end of each period similarly the

halogens occur one space before the end

of each period

you'll also notice that the space in

atomic number

between

elements of the same group

meaning like the length of each period

is the same regardless of what group

you're in so it's the same for the

halogens right here as it is

for the noble gases and it always

follows the same

difference in atomic number

goes 8 8

18

18

32 apart and we'll see why that is later