Atomic Structure

Define Atom

basic unit of element

• Overall neutrally charged

• Made up of subatomic particles:

  • Nucleons -> protons and neutrons in nucleus

• Electrons -> in electron cloud

What is Atomic number

the number of protons in an atom

• If atom is neutral, is also the no. of electrons

What is a mole

6.022x1023 atoms in a substance

• Avogadro's number

  
  

What is atomic weight

weight of one mole

• Weight of 6.022x1023 atoms

• In g/mol

What are properties of Electrons

• Have wave and particle properties

• Exist in orbitals -> areas where electrons are likely to be found

• Orbitals are discrete and properties are described by quantum numbers

  • Principal -> no. of total orbitals

  • Angular momentum -> shape or orbital

  • Magnetic -> orientation of orbital

  • Spin -> magnetic field of electron

• Fill orbitals according to Pauli's exclusion principal

Define Valence electrons

Electrons in the outermost orbital of atom

• Determine stability of atom + likeliness to ionise

• Determines properties of atom

  • Bonding strength

  • Chemical reactions

  • Atomic charge

  • Optical parameters

  • Heat needed to break bonds?

what is Electronegativity

Degree to which an atom attracts electrons to itself

• Higher electronegativity = more accepting of electrons

  • Atom doesn't have enough electrons + is positively charged

  • Cations

• Electropositive = donate electrons

  • Anions

  • Too many electrons + negatively charged

• Some elements can be electronegative and positive

What is r₀ and its significance?

minimum/smallest bonding length

• Minimum radius for bonding

  • Equilibrium radius for energetic stability

  • Fnet = 0

• Maximum negative potential energy

  • Energy required to break bond

  • More energy required to break bond = stronger bond

• r < r0

  • negative F → repulsive force is stronger

  • positive GPE → repulsive

• r> r0

  • positive f → attractive force is stronger

  • negative GPE → attractive

  • weakens/decays exponentially till zero

what is lattice energy

energy required to break lattice

• Energy is released when broken

• Breaking bonds requires energy

○ Heat

○ Work (force)

• Small lattice energy = smaller melting point

• Large lattice energy = higher melting point

What is a Lattice

structured group of atoms

• In crystalline material

Types of bonding

• Ionic

• Covalent

• Metallic

Properties of ionic bonds

Form from the electrostatic attraction between anions (-) and cations (+)

• Mostly between metals and non metals

  • Metal = cation

  • Non-metal = anion

  • Increases bonding radius

  • E.g. magnesium oxide (MgO)

• Likely when there is a large difference in electronegativity

  • Electron from anion is transferred to cation

  • Further away from each other on periodic table

• At minimum energy = most stable

  • Balance of attractive and repulsive

• Very strong but brittle

Properties of Covalent bonding

Sharing of valence electrons between atoms

• Both atoms achieve a noble gas configuration

  • Fill up valence shell

  • Become most stable

• Likely when small differences in electronegativity

  • Close together on periodic table

• Closer atoms are to filling valence orbital = stronger the bond

• Common for molecules/materials of one kind of atom

  • E.g. diamond (carbon)

  • E.g. fluorine gas

    • Share one electron to fill valence shell in P orbital

Properties of Metallic bonding

Electrostatic attraction between delocalised electrons and positively charged metal ions in a metal lattice

• Metals are ordered in lattices

  • Atoms are closely packed

  • Strong bonds

  • E.g. aluminium

• Electrons are free to move between atoms

  • All electron clouds are connected

  • Electrons can be anywhere at any time

  • Bonds are non-directional

• Metallic bond lowers atom's individual energy

  • Min energy still at r₀

• More valence electrons = higher bonding energy = higher melting temperature

• Bonds are strong and ductile

  • Good for shaping, welding, moulding

• Increased strength by introducing alloys

Compare atomic bond types

Ionic - strong but brittle

Covalent - strong but not malleable or ductile

Metal - strong and ductile

How to calculate primary bonding

Calculating how many bonds are ionic or covalent in a substance

Properties of bonding

Melting temperature

Thermal expansion

Elastic / Young's modulus (E)

What is melting temp

temp required to break bonds in a solid

• We melt materials to break bonds in them

• Adding heat = potential energy required to break bonds is smaller

  • r0 does not change

  • we are just shifting to the right of the energy curve, so now: radius of bond length > r0

• When increasing temperature, melting temperature also reduces

Thermal expansion

The increase in size of a solid with the increase in temperature

• When adding heat into a system

  • Changes atomic vibrations

  • Changes thermal coefficient

  • Changes distance between atoms → increases

  • Changes bonding energy → decreases

• Metallic structures expand when heated + shrink when cooled

• Account for change in atomic bonds using coefficient of thermal expansion

What is the thermal expansion coefficient

measures how much a material expands or shrinks when its temperature changes

Elastic / Young's modulus (E)

Gradient of stress strain graph

• Higher young's modulus = steeper stress-strain slope = more force required to break bond

• E depends on melting temp, bond distance

  • Since these factors impact stress and strain → thus change E

• Stronger the bond = more force required to break it

Compare the … of ceramics, metals and polymers

• melting temp

• coeff. of thermal expansion

• E