Structure and bondng NCEA level 2 CHEM Flashcards

Molecular solids are made of molecules. There are weak intermolecular forces or van der Waals forces (forces BETWEEN molecules) holding the molecules together, and they require a small amount of energy to be overcome. This means the substances have low melting and boiling points. Since the weak van der Waals forces allow the molecules to be easily separated this makes them soft or brittle - easy to break the solid.

Polar molecules are usually always soluble in polar solvents such as water. Since both types of molecules have similar weak intermolecular forces between them, then weak forces can also exist between the two different molecules. (Like dissolves like).

IMPORTANT There are strong covalent bonds between the atoms in the molecules (intramolecular forces) but it is NOT these that are being broken when the solids melts or the liquid boils.

covalenty bonded to eachother, with no free charged particles.

ATOMS

In order to melt covalent network substances strong covalent bonds between atoms must be broken. Because of their strength, a lot of energy is required to break these bonds and separate atoms. Therefore diamond, graphite and SiO2 all have high melting points.

In graphite (solid) each carbon atom is covalently bonded to 3 others. This leaves one electron that is delocalised and is free to move. Because this electron is free to move, it carries charge and so graphite conducts electricity.

Graphite consists of C atoms each covalently bonded to 3 other C atoms (in a trigonal planar shape) in a 2D arrangement, forming layers with weak van der Waals forces between them. The weak attractions between the layers of atoms in graphite are easily broken. Therefore, graphite is soft and slippery and the layers can slide over each other. This makes it useful as a lubricant.

Ionic solids are made of positive ions and negative ions, held together by strong electrostatic attractions in a 3– D lattice structure. As these electrostatic attractions (ionic bonds) are strong, they require a large amount of energy to overcome them so their melting and boiling points are high

Solid ionic substances are a 3D lattice of ions held together by strong ionic bonds. These forces hold the ions in a fixed position. As the ions cannot move, the solid will not conduct electricity. When melted (or dissolved in water) the ions become free to move, and this allows the liquid to conduct electricity.

The bonds within ionic solids are directional so they are not malleable or ductile. Instead they are brittle. If oppositely charged ions are forced to shift position then like charges are lined up and repel, causing the crystal to shatter.

Ionic solids are soluble in water as the ions are separated from the lattice due to attraction towards the polar water molecules which is sufficient to pull the ions from the lattice. Molecules of a non–polar solvent like cyclohexane are not attracted towards the ions and so ionic solids are insoluble in this

Metal atoms are held together in a 3–D lattice by metallic bonding, in which loosely held valence electrons are attracted to the neighbouring ions. (Metal cations in a sea of delocalised electrons). The valence electrons are free to move throughout the structure in both the solid and liquid state. This is why metals are good conductors of electricity (and heat)

The attraction of the metal atoms for the valence electrons is not in any particular direction (a non-directional force), therefore metal atoms can move past one another (without breaking the metallic bond and disrupting the structure and breaking the metal), and so metals are both malleable and ductile.

Metals are not soluble in water, as the metallic bonds are too strong to be broken by the attraction to the water molecules.

Many metals are hard and strong, and have high melting and boiling points as there is a strong attraction to overcome between the valence electrons and the nuclei of neighbouring atoms.

CATIONS+ ANIONS- IONIC BOND. IONS FREE TO MOVE WHEN MELTED OR DISSOLVED IN WATER, STRONG IONIC BONDS, MANY ARE SOLUBLE.

enthalpy change, negative means exo thermic and postivie means endo thermic.- the amount of energy absorbed or released.

In chemical reactions, endothermic processes absorb heat from their surroundings, while exothermic processes release heat into their surroundings. Essentially, endothermic reactions feel cold as they take in heat, and exothermic reactions feel hot as they release heat.

CO₂ has two regions of electron density around the central carbon atom.
According to VSEPR theory, these regions repel each other equally to minimise repulsion. There are no lone pairs on the carbon, and each oxygen atom is double-bonded to the central carbon atom. This arrangement creates a parent geometry of linear, with a bond angle of 180°.