Atomic structure, interatomic bonding & atomic arrangements

What forms an atom?

Nucleus & orbiting electrons

What forms the nucleus?

Protons & neutrons

What are the charges of subatomic particles?

Electrons: -1.602 × 10^-19 C

Protons: +1.602 × 10^-19 C

Neutrons: neutral

The mass of an electron to a proton (or neutron) is like the mass of:

a penny to a bowling ball

what are the three types of primary atomic bonding?

Ionic, covalent, and metallic

What are Ionic bonds?

• Transfer of electrons

• Strong attractive force (coulombic) between ions

• non-directional

What is non-directional bonding?

attraction is uniform in all directions

ex. like a magnet in the middle of a room, no matter where you stand around it the force is the same

What are the properties of materials with ionic bonds?

• high melting point

• hard and brittle

• electrical/thermal insulators when solid

• conductive when dissolved in water (aqueous)

• soluble in polar solvents

What are covalent bonds?

• sharing of electrons

• small differences in electronegativity of atoms (so one atom can’t pull electron away)

• varying bonding energies ( not all equally strong)

what are the properties of materials with covalent bonds?

• varying melting temperatures

• varying mechanical properties

• typically electrical insulators, but some are conductive

• can be solid, liquid or gas

What are metallic bonds?

• free electrons shared by atoms

what are the properties of materials with metallic bonds?

• good conductors

• can be strong or weak

• mostly ductile

What are secondary (Van der Waals) bonds?

• weak bonds

• formed by dipoles (uneven electron distribution

• intermolecular

What is bonding energy?

amount of energy needed to separate two bonded ions

What does high bonding energy mean for melting points?

High melting points. The higher the bonding energy the harder it is to break bonds.

What is short range order?

Atoms are arranged in a pattern near each other, but that pattern doesn’t last very far

ex. People standing in a crowd

What is long range order?

Atoms are arranged in a repeating, regular pattern that goes on and on over a large area.

ex. Soldiers lined up

Are amorphous materials short range or long range order?

only short-range order

Amorphous materials vs. Polycrystalline materials

Amorphous: short-order, disordered

Polycrystalline: made of many small crystals, ordered

Describe crystal structure

Lattice: divides space into segments

Unit cell: individual segment that makes up the lattice, retains the characteristics of the entire lattice

What is the atomic packing factor?

how much space in the unit cell is actually occupied by atoms

What is allotropy?

same element, different strucutres

• applies to elements only (one atom)

• same element can exist in different crystal structures depending on conditions (temperature or pressure)

  ex. carbon → diamond (3D tetrahedral structure) vs graphite (layered structure)

What is Polymorphism?

same compound, different structures

• applied to compounds (more than one type of atom)

• same compound can exist in different crystal structures

ex. Silicon dioxide (SiO₂) can exist as quartz, cristobalite, or tridymite

What is X-ray Diffraction?

technique used to look at the atomic arrangement of atoms in a crystal by measuring how X-rays scatter when they hit a crystal

Why can X-rays be used to study crystals?

X-rays have similar wavelengths to the distance between atoms, so when they hit a crystal, they scatter in patterns that reveal atomic spacing

How do atoms in a crystal affect X-ray scattering?

Atoms scatter X-rays because of their electrons.

• More electron → stronger scattering

• arrangement of atoms determined constructive or destructive interference

What is constructive and destructive interference in XRD?

Constructive: waves align → add → bright spot

Destructive: waves oppose → cancel → dark spot

pattern depends on atomic plane spacing and X-ray wavelength

How does crystal structure affect X-ray diffraction intensity?

Type of atoms: more electrons → stronger scattering

Number of atoms in a place: more atoms → stronger peak

Spacing of planes: determines diffraction angles

Wave interference: constructive = strong, destructive = weak

What is Bragg’s Law

describes the angle at which a beam of x-rays of a particular wavelength diffracts

Primary chemical bonds found in solids are ionic, metallic, and Van der Waals bonds. True or False?

False.

The primary chemical bonds found in solids are ionic, covalent, and metallic

What is the smallest repeating unit in a crystalline material?

a unit cell