Atomic Number

All atoms of a particular element have the same number of protons in the nucleus.  This number is a basic property of an element, called its atomic number and given the symbol Z:

Z = number of protons

In a neutral atom, the number of protons in a nucleus is exactly equal to the number of electrons outside the nucleus.  Consider, for example, the elements hydrogen (Z = 1) and uranium (Z = 92).  All hydrogen atoms have one proton in the nucleus, all uranium atoms have 92.  In a neutral hydrogen atom there is one electron outside the nucleus; in a uranium atom there are 92.

H atom:  1 proton, 1 electron                          Z=1

U atom:  92 protons, 92 electrons                   Z=92

Atoms and the Atomic Theory

In 1808, an English scientist and schoolteacher, John Dalton, developed the atomic model of matter that underlies modern chemistry.  Three of the main postulates of modern atomic theory, all of which Dalton suggested in a somewhat different form, are stated below.

1. An element is composed of tiny particles called atoms.  All atoms of a given element show the same chemical properties.  Atoms of different elements show different properties.
2. In an ordinary chemical reaction, atoms move from one substance to another, but no atom of any element disappears or is charged into an atom of another element.
3. Compounds are formed when atoms of two or more elements combine.  In a given compound, the relative numbers of atoms of each kind are definite and constant.  In general, these relative numbers can be expressed as integers or simple fractions.

On the basis of Dalton's theory, the atom can be defined as the smallest particle of an element that can enter into a chemical reaction.

Separating Components of a Mixture

Many different methods can be used to separate the components of a mixture from one another.  A couple of methods that you may have carried out in the laboratory are:

• filtration used to separate a heterogeneous solid-liquid mixture.  The mixture is passed through a barrier with fine pores, such as filter paper.  Copper sulfate, which is water-soluble, can be separated from sand by shaking with water.  On filtration the sand remains on the paper and the copper sulfate solution passes through it.
• distillation used to resolve a homogeneous solid-liquid mixture.  The liquid vaporizes, leaving a residue of the solid in the distilling flask.  The liquid is obtained by condensing the vapor.  Distillation can be used to separate the components of a water solution of copper sulfate.

What is a Mixture?

A mixture contains two or more substances combined in such a way that each substance retains its chemical identity.  When you shake copper sulfate with sand, the two substances do not react with one another.  In contrast, when sodium is exposed to chlorine gas, a new compound, sodium chloride, is formed.
There are two forms of mixtures:
1.  Homogeneous or uniform mixtures are ones in which the composition is the same throughout.  Another name for a homogeneous mixture is a solution, which is made up of a solvent, usually taken to be the substance present in largest amount, and one or more solutes.  Most commonly, the solvent is a liquid, whereas solutes may be solids, liquids, or gases.  Soda water is a solution of carbon dioxide (solute) in water (solvent).  Seawater is a more complex solution in which there are several solid solutes, including sodium chloride; the solvent is water.  It is also possible to have solutions in the solid state.  Brass is a solid solution containing the two metals copper (67%-90%) and zinc (10%-33%).
2.  Heterogeneous or nonuniform mixtures are those in which the composition varies throughout.  Most rocks fall into this category.  In a piece of granite, several components can be distinguished, differing from one another in color.

How Compounds are Resolved into Their Elements

Many different methods can be used to resolve compounds into their elements.  Sometimes, but not often, heat alone is sufficient.  Mercury(II) oxide, a compound of mercury and oxygen, decomposes to its elements when heated to 600 degrees Celsius.  Joseph Priestly, an English chemist, discovered oxygen more than 200 years ago when he carried out this reaction by exposing a sample of mercury(II) oxide to an intense beam of sunlight focused through a powerful lens.  The mercury vapor formed is a deadly poison.  Sir Isaac Newton, who distilled large quantities of mercury in his laboratory, suffered the effects in his later years.

Another method of resolving compounds into elements is electrolysis, which involves passing an electric current through a compound, usually in the liquid state.  By electrolysis it is possible to separate water into the gaseous elements hydrogen and oxygen.  Several decades ago it was proposed to use the hydrogen produced by electrolysis to raise the Titanic from its watery grave off the coast of Newfoundland.  It didn't work.  

What is a Chemical Compound?

A compound is a pure substance that contains more than one element.  Water is a compound of hydrogen and oxygen.  The compounds methane, acetylene, and naphthalene all contain the elements carbon and hydrogen, in different proportions.  

Compounds have fixed compositions.  That is, a given compound always contains the same elements in the same percentages by mass.  A sample of pure water contains precisely 11.19% hydrogen and 88.81% oxygen.  In contrast, mixtures can vary in composition.  For example, a mixture of hydrogen and oxygen might contain 5, 10,  25, or 60%  hydrogen, along with 95, 90, 75, or 40% oxygen.

Characteristic properties of compounds:

1.  Elements in a compound are present in a definite proportion 

Example:  2 atoms of hydrogen + 1 atom of oxygen becomes 1 molecule of compound-water.

2.  Compounds have a definite set of properties

Elements of the compound do not retain their original properties

Example-Hydrogen(element{which is combustible and non-supporter of combustion}) + Oxygen(element{which is non-combustible and supporter of combustion}) becomes  Water(compound{which is non-combustible and non-supporter of combustion})

3.  Elements in a compound cannot be separated by physical methods.

The properties of compounds are very different from those of the elements they contain.  Ordinary table salt, sodium chloride, is a white, unreactive solid.  As you can guess from its name, it contains the two elements sodium and chlorine.  Sodium (Na) is a shiny, extremely reactive metal.  Chlorine (Cl) is a poisonous, greenish-yellow gas.  Clearly, when these two elements combine to form sodium chloride, a profound change takes place.

Science Fact

A textbook discussion from 1870 suggests matter is what is made up of atoms:

Three divisions of matter are recognized in science:  masses, molecules, and atoms.
A Mass of matter is any portion of matter appreciable by the senses.
A Molecule is the smallest particle of matter into which a body can be divided without losing its identity.
An Atom is a still smaller particle produced by division of a molecule.

What is Matter?

Matter is anything that has mass and occupies space.  It exists in three phases:  solid, liquid, and gas.  A solid has a fixed shape and volume.  A liquid has a fixed volume but is not rigid in shape; it takes on the shape of the container.  A gas has neither a fixed volume nor a rigid shape; it takes on both the volume and the shape of the container.
This DNA molecule is an example of matter.

In context of relativity, mass is not a conserved quantity.  Thus, in relativity usually a more general view is taken that it is not mass, but the energy-momentum tensor that quantifies the amount of matter.  Matter therefore is anything that contributes to the energy-momentum of a system, that is, anything that is pure gravity.  

Origins

The pre-Socratics were among the first recorded speculators about the underlying nature of the visible world. Thales regarded water as the fundamental material of the world.  Anaximander posited that the basic material was wholly characterless or limitless:  the Infinite.  Anaximenes posited that the basic stuff was pneuma or air.  Heraclitus seems to say the basic element is fire, though perhaps he means that all is change.  Empedocles spoke of four elements of which everything was made:  earth, water, air, and fire.  

Aristotle was the first to put the conception on a sound philosophical basis, which he did in his natural philosophy, especially in Physics I book.  The word Aristotle uses for matter, hyle or hule, can be literally translated as wood or timber, that is, "raw material" for building.  

Later Developments

The modern conception of matter has been refined many times in history, in light of the improvement in knowledge of just what the basic building blocks are, and in how they interact.  

In the late 19th century with the discovery of the electron, and in the early 20th century, with the discovery of the atomic nucleus, and the birth of particle physics, matter was seen as made up of electrons, protons and neutrons interacting to form atoms.