bonding and shapes

Why do atoms form chemical bonds?

Atoms form chemical bonds to achieve a stable noble gas configuration. By obtaining a full outer electron shell, atoms decrease their potential energy and become more stable.

What is meant by the valency of an atom?

Valency is a measure of an atom's combining ability. It describes how many electrons an atom can gain, lose or share when forming chemical bonds.

How can the periodic table be used to determine valency?

The group number indicates the number of valence electrons present. This allows the number of electrons that must be gained, lost or shared to be determined.

Why are valence electrons important in bonding?

Valence electrons are the outer-shell electrons involved in chemical bonding. They determine how atoms interact with other atoms.

Why do Group 1 elements usually form 1+ ions?

Group 1 elements have one valence electron. Losing this electron produces a full outer shell and a stable noble gas configuration.

Why do Group 2 elements usually form 2+ ions?

Group 2 elements have two valence electrons. Losing both electrons produces a full outer shell and increases stability.

Why do Group 17 elements usually form 1− ions?

Group 17 elements have seven valence electrons and only require one additional electron to achieve a full outer shell.

Explain ionic bonding.

Ionic bonding is the electrostatic attraction between oppositely charged ions. These ions form when electrons are transferred from a metal atom to a non-metal atom.

Why do metals tend to form positive ions?

Metals have relatively low ionisation energies and lose valence electrons easily. This allows them to achieve a stable noble gas configuration.

Why do non-metals tend to form negative ions?

Non-metals have high electronegativity and strongly attract electrons. Gaining electrons allows them to complete their outer shell.

Why does ionic bonding produce a giant lattice instead of individual molecules?

Electrostatic attractions act in all directions. As a result, each ion attracts many oppositely charged ions and forms a giant three-dimensional lattice.

Why do ionic compounds have high melting points?

Large numbers of strong electrostatic attractions exist throughout the ionic lattice. Significant energy is required to overcome these attractions.

Why do ionic compounds have high boiling points?

Many strong ionic bonds must be overcome before ions can separate completely, requiring large amounts of energy.

Why do ionic compounds not conduct electricity in the solid state?

The ions are fixed in position within the lattice and cannot move. Since there are no mobile charge carriers, electricity cannot flow.

Why do molten ionic compounds conduct electricity?

When melted, the ions become free to move throughout the liquid. These mobile ions act as charge carriers.

Why do ionic compounds conduct electricity when dissolved in water?

Water separates the ions from the lattice. The ions become mobile and are able to carry electrical charge through the solution.

Why are ionic compounds brittle?

If the lattice is disturbed, ions of the same charge can become aligned. The strong repulsion between like charges causes the lattice to shatter.

Explain metallic bonding.

Metallic bonding is the electrostatic attraction between positive metal ions and delocalised electrons within a metallic lattice.

What are delocalised electrons?

Delocalised electrons are electrons that are free to move throughout the entire metallic structure rather than belonging to a single atom.

How does metallic bonding form?

Metal atoms lose their valence electrons. The atoms become positive ions while the electrons become delocalised throughout the lattice.

Why do metals conduct electricity?

Delocalised electrons are able to move freely through the metallic lattice and carry charge when a voltage is applied.

Why do metals conduct heat well?

Delocalised electrons transfer kinetic energy rapidly throughout the structure, allowing heat to spread efficiently.

Why are metals malleable?

Metal ions can slide past one another while remaining attracted to the sea of delocalised electrons. The bonding is maintained despite changes in shape.

Why are metals ductile?

The non-directional nature of metallic bonding allows the structure to stretch without destroying the electrostatic attractions.

Why do metals generally have high melting and boiling points?

Large numbers of strong metallic bonds must be overcome before the metallic lattice can be disrupted.

Explain covalent bonding.

Covalent bonding is the electrostatic attraction between two positively charged nuclei and a shared pair of electrons.

Why do non-metal atoms form covalent bonds?

Sharing electrons allows both atoms to achieve a stable noble gas configuration without transferring electrons completely.

What is a bond pair?

A bond pair is a pair of electrons shared between two atoms and involved directly in covalent bonding.

What is a lone pair?

A lone pair is a pair of valence electrons that is not involved in bonding and remains on a single atom.

Why are covalent bonds described as directional?

The shared electrons exist between specific atoms. This causes covalent bonds to exist in specific directions and produce definite molecular shapes.

Explain VSEPR theory.

VSEPR theory states that electron clouds repel each other because they contain negatively charged electrons. They arrange themselves as far apart as possible to minimise repulsion.

How does VSEPR theory predict molecular shape?

The number and arrangement of electron clouds around the central atom determine the shape of the molecule.

Why do lone pairs repel more strongly than bonding pairs?

Lone pairs are attracted to only one nucleus while bonding pairs are attracted to two nuclei. This allows lone pairs to occupy more space and exert stronger repulsion.

How do lone pairs affect molecular shape?

The stronger repulsion from lone pairs pushes bonding pairs closer together, changing molecular shape and reducing bond angles.

What is electronegativity?

Electronegativity is a measure of an atom's ability to attract the shared electrons within a covalent bond.

How does electronegativity change across a period?

Electronegativity generally increases across a period because nuclear charge increases and atomic radius decreases.

How does electronegativity change down a group?

Electronegativity generally decreases down a group because atomic radius increases and the nucleus attracts bonding electrons less strongly.

What is a non-polar covalent bond?

A non-polar covalent bond forms when electrons are shared equally between two atoms due to little or no electronegativity difference.

What is a polar covalent bond?

A polar covalent bond forms when electrons are shared unequally because one atom is more electronegative than the other.

How does electronegativity difference affect bond polarity?

As electronegativity difference increases, electrons are pulled increasingly towards one atom, creating a larger bond dipole.

Why is there no clear boundary between covalent and ionic bonding?

Bonding exists on a spectrum. As bond polarity increases, a bond gains more ionic character rather than changing suddenly from covalent to ionic.

How can electronegativity difference be used to classify bonds?

A difference less than 0.5 is usually non-polar covalent, 0.5 to 1.6 is usually polar covalent, and values above about 2 are generally ionic.

Why are some molecules with polar bonds still non-polar overall?

If the polar bonds are arranged symmetrically, their dipoles cancel each other out and produce no overall dipole.

Why is carbon dioxide a non-polar molecule?

Carbon dioxide contains polar bonds, but its linear and symmetrical structure causes the bond dipoles to cancel completely.

Why is water a polar molecule?

Water contains polar bonds and a bent shape. The bond dipoles do not cancel, producing a net dipole across the molecule.

How can molecular polarity be determined?

Draw the molecular shape, identify bond dipoles and determine whether the dipoles cancel or produce a net dipole.

What is a Lewis structure?

A Lewis structure is a diagram that shows valence electrons, bonding pairs and lone pairs within a molecule or ion.

Why are Lewis structures useful?

They show how atoms achieve full outer shells and help predict bonding, molecular shape and polarity.

Why are only valence electrons shown in Lewis structures?

Only valence electrons participate in bonding. Inner-shell electrons do not normally influence bonding behaviour.

What is the octet rule?

The octet rule states that atoms tend to gain, lose or share electrons until they have eight electrons in their outer shell.

How many shared electron pairs form a single bond?

One shared pair of electrons forms a single covalent bond.

How many shared electron pairs form a double bond?

Two shared pairs of electrons form a double covalent bond.

How many shared electron pairs form a triple bond?

Three shared pairs of electrons form a triple covalent bond.

Why does oxygen form a double bond in O₂?

Each oxygen atom requires two additional electrons to complete its octet, so two electron pairs are shared.

Why does nitrogen form a triple bond in N₂?

Each nitrogen atom requires three additional electrons to complete its octet, so three electron pairs are shared.

How is the central atom chosen when drawing a Lewis structure?

The central atom is usually the least electronegative atom, excluding hydrogen which is always terminal.

What are covalent molecular substances?

These are substances made up of individual molecules containing small numbers of covalently bonded atoms.

Why do covalent molecular substances usually have low melting points?

Only weak intermolecular forces need to be overcome when the substance melts.

Why do covalent molecular substances usually have low boiling points?

The weak attractions between molecules require relatively little energy to overcome.

Why do covalent molecular substances not normally conduct electricity?

Their electrons are localised within covalent bonds and cannot move freely through the substance.

What is a covalent network substance?

A covalent network substance consists of a giant lattice of atoms joined by covalent bonds throughout the entire structure.

Why do covalent network substances have very high melting points?

A large number of strong covalent bonds must be broken before the structure can be disrupted.

Why are covalent network substances generally hard?

Strong covalent bonds extend throughout the structure, making deformation difficult.

What is an allotrope?

An allotrope is a different structural form of the same element in the same physical state.

Why are diamond and graphite allotropes of carbon?

Both contain only carbon atoms but differ in the arrangement and bonding of those atoms.

Why is diamond extremely hard?

Each carbon atom forms four strong covalent bonds in a giant three-dimensional network.

Why does diamond not conduct electricity?

All valence electrons are involved in covalent bonds and there are no mobile charge carriers.

Why does graphite conduct electricity?

Each carbon atom contributes one delocalised electron that is free to move through the structure.

Why is graphite soft?

The layers are held together by weak forces and can slide over each other easily.

Why do both diamond and graphite have very high melting points?

Both contain giant covalent structures with many strong covalent bonds that must be broken.

What are nanoparticles?

Nanoparticles are particles between 1 and 100 nanometres in size.

Why do nanoparticles often have different properties from bulk materials?

Their extremely small size produces quantum effects that alter physical and chemical properties.