IX Atoms and Molecules

All materials and substances are based on 92 base elements.

The molecule is the smallest single particle of any substance. In a pure substance all molecules are the same kind and differ from those of other substances.

Forces referred to as bonds link molecules of solid and liquid substances together. They break when supplied with sufficient energy causing molecules to detach from each other.

Pure substance consisting of only one type of molecule.
Pure substances that can not be broken down any further.
The smallest unit of matter that shares the properties of an element
Two or more atoms joined by bonds. Each molecule has its own unique configuration of linked atoms.

The Structure of Atoms

At the core of any atom lies an atomic nucleus. The nucleus is composed of protons and neutrons. Protons are positively charged, neutrons are uncharged. The amount of protons always surmounts the number of neutrons in a nucleus. Therefore the nucleus is positively charged. The nucleus is surrounded by shells occupied by negatively charged electrons.
Most of the atom's mass is concentrated in the nucleus. While the proton is only slightly lighter than the neutron, an electron holds only about 12000\frac {1}{2000} of a proton's mass. Atoms almost entirely consist of the empty space between electrons and the nucleus.

Element Notation

AZX\begin{matrix} A \ Z \end{matrix} X

A=mass number=number of protons and neutronsA = \text{mass number} = \text{number of protons and neutrons}
Z=charge number=number of protonsZ = \text{charge number} = \text{number of protons}


The number of protons is fixed for each element. However the number of neutrons can vary. Individual configurations of proton:neutron ratios of elements are called their isotopes. The chemical properties among isotopes of specific elements are the same.

Atomic Mass Unit

u=1.66×1027kgu = 1.66 \times 10^{-27}kg

Defined as 112\frac {1}{12} the mass of 126C\begin{matrix} 12 \ 6 \end{matrix} C


The physical state of a substance depends on the strength of its bonds, that is the arrangement of its atom's electrons. Electrons orbit around the nuclei of atoms due to their electric attraction. They can only occupy certain orbits called shells. The maximum number of electrons a shell can hold is fixed and depends on the shell. The innermost shell of an atom can hold up to two electrons. The next one away from the nucleus can hold up to eight of them.

The energy of an electron depends on its host shell. The the shorter the shell's proximity to the nucleus, the lower is the energy of an electron occupying it. Electrons begin filling shells from the innermost shell outwards. They can escape towards an outer layer when given sufficient energy. They always prefer to make up full shells as the energy state of a full or empty atom is lower than that of one with semi-filled shells.

Periodic Table

The elements of each column form a group of elements with similar properties. Lithium, sodium, potassium etc. in column 11 are all highly reactive because they have only one electron sitting in their outer shells that is easily lost to another atom. Nobles gases such as helium, neon (2, 8), argon, xenon (2, 8, 18, 18, 8) and krypton have a full outer shell. They are unreactive.

Covalent Bonds

Formed when two atoms share a pair of electrons in their outermost shells.

Ionic Bonds

Formed due to electric attraction of oppositely charged ions in crystalline lattice structures.

Metallic Bonds

Positively charged ions on the inside are locked into position by their surrounding electrons.

Molecular Bonds

Act on uncharged molecules in close proximity to each other. Happens due to attraction of electrons to another atom's nucleus. Its the weakest kind of bond and found in liquids.

Solids and Structure

Objects are elastic when they return to their original state after bending or stretching. When deformed beyond its elastic limit an object is deformed permanently and has entered a plastic state.

Hooke's Law

The extension (length in meters) of a spring is directly proportional to the force that causes it.

F=keF = ke

Where kk is the spring constant and ee the extension. The spring constant can be calculated by measuring the force (e.g. weight pulling down on a spring) and length of the extension.

k=Fek = \frac F e

Classes of Solids

Crystalline Solids
Feature regular atom patterns. Not easy to break unless force is applied at just the right angle to cause chains of similar atoms to shift away from a chain of other atoms. Splits very cleanly.
Metals consist of tiny crystals (grains). Their boundaries are responsible for the strong bonds of metals. Also the size of grains varies with temperature.
Amorphous Solids
e.g. glass
Have no characteristic shape.
Their atoms are lockes together at random.
Long molecules consisting of chains of shorter identical molecules.
Cross links are responsible for strength of the polymer's bonds. Thus they vary in strength.


Molecules in a liquid are not locked to each other although they are in direct contact while moving about at random.

Occurs because of random motion of molecules. A liquid substance injected into another tends to spread out evenly because of the large numbers of molecules involed in the process.
The measure of a liquid's resistance to flow. Usually depends on temperature (e.g. oils at high temperatures are less viscous).
Happends because of friction between layers of molecules moving at different speeds. The slower molecules produce drag on the faster ones when they transfer into those faster moving layers.

When a liquid flows through a pipe the pipe's surface introduces drag slowing down the flow. Given a diameter DD of a pipe: D4D^4 equals the flow rate of a fluid through a pipe of diameter DD.


Pressure difference invokes a force causing substances to move from areas of high pressure to areas of low pressure. Pressure equals the force per unit area.

Pa=NAPa = \frac {N}{A}

The higher the pressure acting on a liquid the faster it flows.

Hydrostatic Pressure

The force of gravity exerts a pressure on a liquid that increases with depth below the surface.

Since density D=mV\text{density D} = \frac m V. Given a liquid filling a container up to height hh:

mgA=pressure on the base\frac {mg}{A} = \text{pressure on the base}

mghAh=total pressure\frac{mgh}{Ah} = \text{total pressure}

mghAh=mghV=pgh\frac{mgh}{Ah} = \frac{mgh}{V} = pgh where p=densityp = \text{density}

Gas Pressure

Occurs because gas molecules moving about randomly in a container keep hitting against its surface. The force exerted on the container evens out because of the very high number of molecules hitting against it each second. If the gas is heated up the pressure increases because the molecules gain speed. At 0K0K gas pressure equals 0Pa0Pa.
It's inversely proportional to the volume of the container as molecules hit the surface more frequently if they have less distance to travel from surface to surface.