Key concepts in chemistry Revision Notes

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.

Electron Transfer in Ionic Bonding

Ionic bonds are formed through the transfer of electrons from a metal atom to a non-metal atom. Metals lose electrons to become positively charged cations, while non-metals gain electrons to become negatively charged anions. This transfer allows both atoms to achieve a stable, full outer electron shell, similar to that of a noble gas.

Cations and Anions

A cation is a positively charged ion formed when an atom loses one or more electrons. An anion is a negatively charged ion formed when an atom gains one or more electrons. The electrostatic attraction between these oppositely charged ions is what holds them together in an ionic compound.

Dot and Cross Diagrams

Dot and cross diagrams visually represent electron transfer and ion formation. They show the outer shell electrons of each atom, using different symbols (dots and crosses) for electrons from different atoms. The diagrams then illustrate the transfer of these electrons and the resulting ions, including their charges and full outer electron shells, often enclosed in square brackets.

What is an Ion?

An ion is any atom or group of atoms that possesses an overall positive or negative electrical charge. This charge is due to the gain or loss of electrons, not protons or neutrons.

Formation of Positive Ions (Cations)

Positive ions (cations) are formed when an atom or group of atoms loses one or more electrons. Since electrons carry a negative charge, losing them results in more protons than electrons, giving a net positive charge. For example, a sodium atom (Na) loses one electron to become a sodium ion (Na+).

Formation of Negative Ions (Anions)

Negative ions (anions) are formed when an atom or group of atoms gains one or more electrons. Gaining electrons means there are more electrons than protons, resulting in a net negative charge. For example, a chlorine atom (Cl) gains one electron to become a chloride ion (Cl-).

Polyatomic Ions

Some ions are formed from a group of atoms covalently bonded together, which then collectively gain or lose electrons. These are known as polyatomic ions, such as the sulfate ion (SO4^2-) or the ammonium ion (NH4+).

Atomic Number and Mass Number

• The atomic number (Z) is the number of protons in an atom's nucleus. It determines the element's identity.
• The mass number (A) is the total number of protons and neutrons in an atom's nucleus.
• Number of neutrons = Mass number - Atomic number.

Electrons in Ions

• In a neutral atom, number of electrons = number of protons (atomic number).
• For positive ions (cations): electrons = protons - charge magnitude (e.g., for Fe2ⁱ, electrons = protons - 2).
• For negative ions (anions): electrons = protons + charge magnitude (e.g., for O²⁻, electrons = protons + 2).
• Protons and neutrons are unaffected when an ion forms; only electron count changes.

Calculating Particles for Ions

• Protons: Always equals the atomic number.
• Neutrons: Mass number minus the atomic number.
• Electrons: For a neutral atom, it's the atomic number. For a positive ion, subtract the charge value from the atomic number. For a negative ion, add the charge value to the atomic number.