The Periodic Table (VCE SSCE Chemistry): Revision Notes

The Periodic Table

Introduction

The periodic table is one of the most recognizable tools in modern chemistry. Developed over 150 years ago, it provides a systematic way to organize all known chemical elements and understand their properties.

Historical development

Mendeleev's contribution

Russian chemist Dimitri Mendeleev developed the first periodic table in 1869. His groundbreaking work was based on the observation that atomic properties seemed to vary in repeating patterns.

The power of prediction

When Mendeleev first proposed his periodic table, he made a bold decision: he left gaps for elements that had not yet been discovered. Even more remarkably, he predicted the properties of these unknown elements.

Initially, the scientific community largely ignored or ridiculed his work. However, when gallium was discovered five years later and its properties matched Mendeleev's predictions almost exactly, the scientific community began to take his periodic table seriously.

One remarkable aspect of the periodic table is that it has remained relevant even as our understanding of atomic theory has developed. It was created before protons, neutrons, or electrons were even proposed. Each new development in atomic theory has revealed underlying patterns in the table that Mendeleev could not have known about.

The modern periodic table structure

Today we know that the atomic number (the number of protons) is what makes one element fundamentally different from another. The modern periodic table arranges elements in order of increasing atomic number.

Key features

The modern periodic table has several important organizational features:

• Arrangement: Elements are arranged in order of increasing atomic number
• Periods: Horizontal rows numbered 1-7
• Groups: Vertical columns numbered 1-18
• Main group elements: Elements in groups 1, 2, and 13-18
• Transition metals: Elements in groups 3-12

Groups

Elements in the periodic table are arranged into vertical columns called groups. Groups are numbered from 1 to 18.

Valence electrons and groups

For main group elements, the group number can be used to determine the number of valence electrons (outer-shell electrons) in an atom of the element.

Why groups matter

The valence electrons are the electrons involved in chemical reactions. Since elements in the same group have the same number of valence electrons, they exhibit similar chemical properties.

Examples of groups with similar properties

Alkali metals (group 1, excluding hydrogen):

• All relatively soft metals
• Highly reactive with water and oxygen
• Electronic configurations show one electron in an s-subshell:
  • $\text{Li}: 1s^2 2s^1$
  • $\text{Na}: 1s^2 2s^2 2p^6 3s^1$
  • $\text{K}: 1s^2 2s^2 2p^6 3s^2 3p^6 4s^1$

Halogens (group 17):

• All colored elements
• Highly reactive
• Electronic configurations show highest-energy subshell of $s^2p^5$:
  • $\text{F}: 1s^2 2s^2 2p^5$
  • $\text{Cl}: 1s^2 2s^2 2p^6 3s^2 3p^5$
  • $\text{Br}: 1s^2 2s^2 2p^6 3s^2 3p^6 3d^{10} 4s^2 4p^5$
  • $\text{I}: 1s^2 2s^2 2p^6 3s^2 3p^6 3d^{10} 4s^2 4p^6 4d^{10} 5s^2 5p^5$

Noble gases (group 18):

• Very stable electron arrangement
• Helium and neon have full outer shells
• Other noble gases have a stable octet (8 electrons) in their valence shell
• Very low reactivity because they already have stable electronic configurations
• Do not tend to lose or gain electrons

The arrangement of electrons in atoms is responsible for the periodicity (periodic pattern) of element properties.

Special groups in the periodic table

The periodic table contains several special groups with specific names:

• Alkali metals: Group 1 (excluding hydrogen)
• Alkaline earth metals: Group 2
• Halogens: Group 17
• Noble gases: Group 18
• Lanthanoids: Elements 58-71
• Actinoids: Elements 90-103

Helium supplies at risk

On Earth, most helium is found underground with other natural gases. If a natural gas deposit contains commercially viable amounts of helium, it is extracted from the mixture. The largest commercial deposits are currently in the US, Qatar, and Algeria.

While helium is well known for use in balloons, it has many other important applications:

• Medical research and diagnostic equipment
• Cooling nuclear reactors and rockets
• Providing unreactive atmospheres for arc welding

Periods

The horizontal rows in the periodic table are called periods and are numbered 1-7.

What periods tell us

The period number provides information about the electronic configuration of an element. Specifically:

Period number = number of occupied electron shells in the element's atoms

Blocks

As understanding of atomic theory developed into Schrödinger's quantum mechanical model, a new pattern emerged in the periodic table. The table can be divided into four main blocks based on the type of subshell that contains the highest energy electrons.

The four blocks

• s-block: Groups 1 and 2 (highest energy electrons in s-subshells)
• p-block: Groups 13-18 (highest energy electrons in p-subshells)
• d-block: Groups 3-12, also called transition metals (highest energy electrons in d-subshells)
• f-block: Lanthanoids and actinoids (highest energy electrons in f-subshells)

Naming elements

Ancient elements

Twelve elements were known in ancient times, including gold, silver, mercury, and sulfur. The original names for these elements were derived from Latin. People at that time did not understand the modern definition of an element.

Searching for new elements

In the eighteenth and nineteenth centuries, scientists developed methods to extract gaseous elements from air and isolate solid elements from the ground. Elements often took names from Latin words describing where they were found:

• Silicon (Si) - from silex, meaning sand
• Calcium (Ca) - from calx, meaning limestone
• Mercury (Hg) - from hydrargyrum, meaning silver water

As analytical techniques improved, naming became more imaginative, with names derived from:

• Greek roots
• Detected characteristics (e.g., argon from Greek ἀργόν, meaning inactive)
• Places (e.g., germanium from germania, meaning Germany)
• Mythology (e.g., vanadium after the Scandinavian goddess Vanadis)

By 1869, when Mendeleev constructed the first modern periodic table, 64 elements were known.

Synthetic elements

The first synthetic element, technetium, was made in 1936. Many new elements were discovered after 1940, some during thermonuclear bomb tests.

More recently, elements have been discovered using particle accelerators. This has brought the total number of known elements to 118. Synthetic elements have large, unstable nuclei. Many were named after scientists or places (e.g., einsteinium, americium).

Critical and endangered elements

Of the 118 elements in the periodic table, many exist in only very small quantities on Earth. In recent decades, some rare elements have been used in bulk for the first time to produce new electronics, medicines, and catalysts.

What makes an element critical?

Critical elements are elements heavily relied upon by industry and society in areas such as renewable energy, electronics, food supply, and medicine. Some scientists believe unhindered access to these elements is required for society to function. Critical elements face supply uncertainty for several reasons:

1. Limited deposits (endangered elements)

Only small deposits are available on Earth, and these are disappearing rapidly. Examples include iridium, platinum, osmium, and palladium.

2. Supply centered in conflict areas (conflict elements)

These elements are mined in areas of war and conflict, often using child labor, making their use non-sustainable. Examples include tin, gold, tungsten, and tantalum, which are essential in mobile phone production.

3. Little to no recycling

There is minimal recovery of these elements, so reserves are being depleted. Examples include the rare earth elements (lanthanides).

4. Concentrated deposits (critical raw materials)

These elements have significant economic importance, no viable substitutes, and deposits concentrated in a small number of countries, putting supply at risk:

• Tungsten, antimony, molybdenum, germanium, gallium, and indium: concentrated in China
• Platinum, palladium, rhodium, ruthenium, and vanadium: concentrated in South Africa

5. Increased use due to new technologies

Recent expansion of new technologies has greatly increased demand for certain elements:

• Helium in medical technologies
• Phosphorus in fertilizers

Recycling campaigns

Recycling campaigns have been developed globally to make products more sustainable. One notable example was the 2020 Tokyo Olympic Games, when the Japanese people collected approximately 79,000 tonnes of old mobile phones and other electronic waste. This was recycled to recover precious metals to make 100% of the gold, silver, and bronze Olympic medals.

A new approach to recycling: phosphorus

Phosphorus plays a crucial role in producing the world's food. It is used to create fertilizers that enable greater crop yields.

However, it is estimated that phosphorus supplies will become depleted sometime this century. Experts predict this could have serious consequences for world food supplies. To maintain overall supply, it may be necessary to recover phosphorus from animal and human waste.

One suggested method is recovery from human urine using a toilet with a urine diversion and dehydration unit. The phosphorus can eventually be precipitated out as calcium phosphate.

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