Valency and chemical bonding

In the study of chemistry, especially at a basic level, it is important to understand how atoms interact with each other. This section explores the concepts of valency and chemical bonds, which provide the necessary foundation for understanding how elements combine to form compounds.

What is an atom?

An atom is the smallest unit of matter that retains the properties of an element. Each atom consists of a nucleus made up of protons and neutrons, surrounded by electrons that revolve around the nucleus in different energy levels or shells.

Electron configuration and energy levels

Electrons occupy specific energy levels around the nucleus, and these levels can hold a certain number of electrons. The first level can hold up to 2 electrons, the second up to 8, the third up to 18, and so on. However, for the sake of simplicity, you'll often encounter only the first two levels in basic chemistry:

• First level: up to 2 electrons
• 2nd level: up to 8 electrons

The arrangement of electrons in these levels is called the electron configuration, which determines how an atom will interact, or form bonds, with other atoms.

Valency

Valency is a measure of an atom's ability to bond with other atoms. It is determined by the number of electrons in an atom's outermost shell. Atoms are more stable when their outermost shell is full. Most atoms try to achieve a full outer shell, either by sharing electrons or transferring them between atoms to form compounds.

Explanation of valency

If the outer shell is not filled, the atom has the possibility of forming bonds with other atoms to achieve stability. For example, oxygen with a configuration of 2, 6 (two electrons in the first shell and six in the second) needs two more electrons to fill its outer shell. Thus, the valency of oxygen is 2.

The electron configuration visually looks like this:

Oxygen (O) 1s² 2s² 2p⁴

Chemical bond

Atoms bond with each other to achieve a stable electron configuration, usually a full outer shell. There are three main types of chemical bonds: ionic, covalent, and metallic bonds.

Ionic bond

Ionic bonds form when one atom donates electrons to another atom, resulting in the formation of positive and negative ions. These oppositely charged ions attract each other to form a stable compound.

For example, sodium (Na) can give up one electron to gain a full outer shell, while chlorine (Cl) needs another electron to gain the same shell. Sodium donates one electron to chlorine, resulting in Na⁺ and Cl^-, which bond to form sodium chloride (table salt):

Na → Na⁺ + e⁻ Cl + e⁻ → Cl⁻ Na⁺ + Cl⁻ → NaCl

Covalent bonds

Covalent bonds are formed when atoms share pairs of electrons. This type of bond usually occurs between nonmetals. Both atoms achieve complete outer shells by sharing electrons.

For example, two hydrogen atoms can form a hydrogen molecule (_H2) by sharing their single electrons:

H• + •H → H:H (or HH)

Metal bonding

In metallic bonds, the electrons in the outer shell of metal atoms move freely. This movement allows metals to conduct electricity and heat. The 'sea of electrons' is shared between the lattice of metal ions, giving metals unique properties such as ductility and conductivity.

In the example of a metal such as iron (Fe), the bond is depicted as follows:

Fe → Fe²⁺ + 2e⁻ (delocalized electrons)

Understanding valency with examples

The valency of different elements varies depending on the number of electrons in their outermost shell. Here are some examples:

• Hydrogen: Valency 1, because it needs an extra electron to fill its shell.
• Carbon: Valency 4, needs four electrons to fill its outer shell (2, 4).
• Oxygen: Valency 2, needs two electrons to fill its outer shell (2, 6).
• Chlorine: Valency 1, needs one extra electron to fill its outer shell (2, 8, 7).

Consider the compound water (_H2O):

Oxygen combines with two hydrogen atoms through covalent bonding to form a stable configuration:

O + 2H → H₂O

Octave rule

A principle known as the octet rule is often used to predict how atoms will bond. This rule states that atoms form bonds in such a way that each atom has eight electrons in its outer shell, resembling the electron configuration of a noble gas. However, there are exceptions to this rule, and some elements can be stable with fewer or more than eight electrons.

Polarity and bonding

Covalent bonds can be polar or nonpolar depending on the electronegativities of the atoms involved. Electronegativity is a measure of an atom's ability to attract electrons. When two atoms have different electronegativities, the electrons in a covalent bond can be closer to one atom, resulting in a polar covalent bond. For example, the bond within the water molecule (_H2O) is polar because the oxygen atom is more electronegative than hydrogen, which attracts the shared electrons.

H(δ⁺) — O(δ⁻) — H(δ⁺)

Summary and concepts to remember

• Valency refers to the combining strength of an atom, usually determined by the number of electrons needed to fill its outer shell.
• The formation of chemical bonds helps atoms reach a more stable electron configuration.
• There are three main types of chemical bonds: ionic, covalent, and metallic.
• The octet rule is an important concept in understanding how atoms form compounds, although there are exceptions to it.
• The electronegativities difference affects the type of bond formed (polar or nonpolar covalent bond).

By understanding valency and the different types of chemical bonds, we can predict and explain how different substances form, shedding light on the molecular structure of the world around us.

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