Chemical Elements

Forms of existence of chemical elements

Chemical elements are rarely found in isolation. In nature, they can exist in three main forms:

• As part of compound substances. Most elements tend to form diverse compounds. For example, oxygen is a structural component of water, carbon dioxide, sand, sulfuric acid, sugars, and even complex drugs (such as aspirin).
• In the form of simple substances. Some elements can exist in a free state, forming molecules from identical atoms. A clear example is gaseous oxygen ($O_2$) and ozone ($O_3$).
• Noble (inert) gases. A unique group of elements: helium, neon, argon, krypton, xenon, and radon. In nature, they exist exclusively as separate, isolated atoms. They can be found as impurities in atmospheric air, dissolved in water, and in the voids of crystal lattices of various minerals.

Isotopes and symbols

Every chemical element has a symbol, which consists of one or two initial letters of its Latin name. Interestingly, in a number of cases, this symbol bears no phonetic resemblance to the common English name. For example, iron is designated as Fe (Ferrum), and mercury as Hg (Hydrargyrum).

Chemical elements often have atom varieties known as isotopes. Isotopes are variants of atoms of the same element that differ from each other only in atomic mass, while remaining practically identical in all other chemical properties. A classic example is ordinary (light) and heavy hydrogen. Water containing heavy hydrogen ("heavy water") differs from natural water in its physical characteristics, but chemically it remains the exact same substance.

Abundance in the Earth's crust

Elements are distributed extremely unevenly in nature. Below are data on the content of some key elements in the Earth's crust (% by mass):

Periodic law and table variants

To date, several variants of the Periodic Table exist in chemistry:

1. Long-period. All elements of a period are arranged in a single row, while main (A) and subgroup (B) groups form separate columns.
2. Short-period. The fourth, fifth, and sixth periods are divided into two rows, and groups A and B with the same numbers are combined into common columns.
3. IUPAC variant (IUPAC). Recommended by the International Union of Pure and Applied Chemistry. In this variant, groups are simply numbered from 1 to 18, and D. I. Mendeleev's name is usually omitted from the title.

Each period of the table contains exactly 10 transition metals. According to Mendeleev's classical concept, the table contains 8 groups, with group VIIIB including three transition metals from each period simultaneously.

Predictive function and the analogy method

For over 100 years, the Periodic Law has served as the primary guide in chemical research. It dramatically reduces the volume of material to be memorized and possesses a predictive function — allowing the properties of yet unstudied substances to be predicted based on their known analogues.

Let us examine the analogy method in practice. Suppose we need to write the reaction between rubidium hydroxide ($RbOH$) and hydrogen selenide ($H_2Se$). The compounds of these elements may be unfamiliar to us, so we look for analogues within the group:

• Rubidium ($Rb$) is an analogue of sodium ($Na$) and potassium ($K$), meaning $RbOH$ is a typical base (alkali).
• Hydrogen selenide ($H_2Se$) is an analogue of hydrogen sulfide ($H_2S$), therefore it exhibits pronounced acidic properties.
• Result — the reaction equation: $2RbOH + H_2Se = Rb_2Se + 2H_2O$.

It is important to remember that elements of the same group are analogues, not absolute copies. Moving down a group, properties change quantitatively, and metallic properties intensify. Thus, in group IV, the non-metal carbon ($C$) is replaced by the metal lead ($Pb$), and in group V, nitrogen ($N$) is replaced by bismuth. An interesting analogy is also observed: compounds of transition metals and non-metals in their highest oxidation states show marked similarities (for example, potassium permanganate $KMnO_4$ resembles potassium perchlorate $KClO_4$).

Unexpected anomalies sometimes occur. Until 1962, noble gases were considered completely inert with a valence of zero. However, the first chemical compound of xenon was then synthesized — xenon hexafluoroplatinate ($XePtF_6$). Today, dozens of xenon and krypton compounds are known. Anomalous, uncharacteristically high oxidation states (+5 and +7) have also been discovered for heavy metals — corresponding gold fluorides have been obtained.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Determination of an element and isotope based on atomic nucleus structure
• Comparative characteristics and chemical affinity of elements within the same group
• Calculation of chemical equivalents of simple substances
• Reasons for the enhancement of metallic properties in subgroups from top to bottom

Other topics in the section "Chapter 1. Substances. Properties and Structure"

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