Key concepts in chemistry Revision Notes

Mendeleev's Approach and Gaps

Mendeleev arranged the known elements in order of increasing atomic mass. Crucially, he noticed recurring patterns in their chemical properties. To maintain these patterns, he deliberately left gaps in his table whenever an element with the correct properties for a particular position was not yet known. These gaps were not errors but intelligent placeholders for undiscovered elements, demonstrating his foresight in understanding periodicity.

Predicting Properties of Undiscovered Elements

For each gap he left, Mendeleev used the properties of the elements surrounding the gap (those above, below, and to the sides) to predict the physical and chemical properties of the missing element. He predicted atomic mass, density, melting point, reactivity, and the formulas of compounds (e.g., oxides and chlorides). For instance, he predicted 'eka-aluminium' (gallium) to have an atomic mass of about 68, a density of 5.9 g/cm³, and form an oxide with formula X₂O₃. The discovery of these elements with properties aligning closely with his predictions validated his periodic law.

Mendeleev's Initial Arrangement

Dmitri Mendeleev initially arranged elements in his periodic table based on their increasing relative atomic mass. This method allowed him to group elements with similar chemical properties together, and he even left gaps for undiscovered elements, predicting their properties accurately.

Anomalies and Isotopes

Mendeleev noticed some pairs of elements, such as tellurium and iodine, appeared in the 'wrong' order when strictly following increasing relative atomic mass. He correctly prioritised chemical properties, placing these elements according to their characteristic reactions. These anomalies are now understood to be due to the relative abundance of isotopes. Relative atomic mass is the weighted average of the masses of an element's isotopes. If an element with a lower atomic number has a higher proportion of heavier isotopes, its average relative atomic mass can be higher than an element with a greater atomic number but a higher proportion of lighter isotopes.

Impact of Isotopes

The existence of isotopes was unknown in Mendeleev's time. The discovery helped clarify why atomic number, which is the number of protons and defines an element, is a more fundamental property for ordering elements than relative atomic mass, which is an average influenced by isotopic composition.

Defining the Atomic Number

The atomic number (Z) is the number of protons in the nucleus of an atom. This number is unique to each element and determines its identity. For a neutral atom, the number of electrons is equal to the atomic number.

Atomic Number and the Periodic Table

Elements in the periodic table are arranged sequentially by increasing atomic number. This arrangement helps organise elements by their chemical properties and trends. The atomic number is always a whole number.

Significance of Protons

Protons are positively charged subatomic particles located in the nucleus. The number of protons not only defines the element but also dictates its chemical behaviour and reactivity, as it determines the electron configuration.

Periods and Electron Shells

The periodic table is organised into horizontal rows called periods. The period number an element belongs to directly corresponds to the number of electron shells its atoms contain. For example, elements in Period 2 have two occupied electron shells, while elements in Period 4 have four occupied electron shells. This regularity allows for a quick determination of shell count from an element's position.

Groups and Valence Electrons

Vertical columns in the periodic table are known as groups. The group number (for main group elements, excluding transition metals) indicates the number of electrons in the outermost shell, known as valence electrons. For example, elements in Group 1 possess one valence electron, and elements in Group 16 have six valence electrons. Elements within the same group tend to exhibit similar chemical properties due to having the same number of valence electrons, as these electrons are primarily involved in chemical reactions.

Predicting Configuration from Position

Knowing an element's position in the periodic table allows for a straightforward prediction of its electronic configuration. For example, an element in Period 3, Group 2 will have three electron shells and two valence electrons. This prediction can then be used to understand its reactivity and bonding behaviour because the valence electrons dictate most chemical interactions. The first shell can hold up to 2 electrons, the second up to 8, and subsequent shells can hold more, although 8 is still a key number for stability in the third shell for many elements.