The Periodic Table

The Periodic Table of The Elements

Elements are like a collection As more and more elements were discovered it became more important to organize and classify them

Between the late 1700’s and mid 1800’s scientists, using mostly atomic spectroscopy, doubled the number of known elements.

In early 1800’s, German chemist J.W. Dobereiner observed that several of the elements could be classified into groups of three which he called triads

Ca, Sr, and Ba Li, Na, K Cl, Br, I Dobereiner based his triads on similar chemical properties

In addition: Many of the properties of the middle element in each triad are approximate averages of the properties of the first and third element

Element Atomic Density Mass (amu) Cl 35.5 1.56 g/L Br 79.9 3.12 g/L I 126.9 4.95 g/L Ca 40.1 1.55 g/cm 3 Sr 87.6 2.6 g/cm 3 Ba 137 3.5 g/cm 3

In 1865, English chemist J.A.R Newlands presented another way to classify and organize the 62 elements known at the time

Newlands placed the elements in order of increasing atomic mass He noticed that the properties of the eighth element were like those of the first, the ninth like those of the second, and so on….

He called this repeating pattern of every eight elements THE LAW OF OCTAVES After the eight notes of the musical scale

Because he linked chemistry to music, he was not taken seriously! It took 20 years for him to receive credit for recognizing periodicity

In 1869 Russian Chemist Dimitri Mendeleev and German chemist Lothar Meyer published nearly identical ways of classifying

But Mendeleev is generally more credited with the 1 st periodic table for 2 reasons: He published first He was better at explaining it than Meyer

Mendeleev also saw the “periods” Credited with publishing the first “periodic table”

Mendeleev got lots of credit because he left gaps for missing elements!

In 1913, English Chemist James Moseley Presented a way of organizing the elements that we still use today.

Moseley was the first to put the elements in order of increasing Atomic #

Atomic # represents the number of protons in the nucleus of each element

When he did this, he saw the same repeating periodic pattern, without the exceptions that Mendeleev had to switch around

Periodic Law When the elements are arranged in increasing order by their atomic numbers, their properties repeat periodically

The Modern Periodic Table

Each Square Tells about a different element H Element Symbol Hydrogen Element Name Not always there 1 Atomic Number Represents the number of PROTONS in each atom of this element 1.009 Atomic Mass Represents the number of PROTONS AND AVERAGE NUMBER OF NEUTRONS in each atom of this element

Atomic Number 1: 1 proton (positive charge particles) in nucleus of EVERY hydrogen atom Atomic Number 1: ALSO means 1 electron (negatively charged particle) OUTSIDE the nucleus Atoms are NEUTRAL: Same number of positives as negatives Atomic Mass of 1.009 means that there is an average of 1.009 PROTONS AND NEUTRONS in the nucleus… Since there is already 1 proton (AND THAT CAN NOT CHANGE AND STILL BE HYDROGEN) That means that the average atom of Hydrogen has 0 Neutrons! H Hydrogen 1 1.009

Let’s do another 4 Particles 2 MUST BE Protons 4-2 = 2 Neutrons in nucleus He Helium 2 4.003

Each block tells us about the element… The periodic table arranges the elements into rows and columns based on similarities

Vertical columns are called groups.

Elements are placed in columns by similar properties.

Also called families

Alkali Metals

Group 1

ALL elements in Group 1 have 1 electron in their outer region

Alkali Earth Metals

Group 2

ALL elements in Group 2 have 2 electrons in their outer region

Boron Family

Group 13

Carbon Family

Group 14

Nitrogen Family

Group 15

Oxygen Family

A.K.A. the Chalcogens

Group 16

Halogens

Group 17

Noble Gasses

Group 18

Lanthanide Series

Actinide Series

Horizontal rows are called “Periods”

Each period also shows something in common

Period 1

Each element has one region of space for electrons around it

Period 2

Each element in this period has two regions of space around it for electrons

Period 3

Each element in this period has 3 regions of space around it for electrons

Period 4

Period 5

Period 6

The Lanthanides are part of period 6

Period 7

The Actinides are part of period 7

The periodic table can also show larger “Groups”

METALS Non-METALS METALLOIDS

Tall columns are collectively referred to as the “representative elements”

Short, center groups are collectively referred to as the “transition elements”

Two long rows on bottom are collectively referred to as the “inner transition metals”

Periodic Trends

Atomic Size

First problem where do you start measuring.

The electron cloud doesn’t have a definite edge.

They get around this by measuring more than 1 atom at a time.

Atomic Size

Atomic Radius = half the distance between two nuclei of a diatomic molecule.

} Radius

Trends in Atomic Size

Influenced by two factors.

Energy Level

Higher energy level is further away.

Charge on nucleus

More charge pulls electrons in closer.

Group trends

As we go down a group

Each atom has another energy level,

So the atoms get bigger.

H Li Na K Rb

Periodic Trends

As you go across a period the radius gets smaller.

Same energy level.

More nuclear charge.

Outermost electrons are closer.

Na Mg Al Si P S Cl Ar

The Periodic Table

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