Material Structure

All matter is considered to be composed of unit substances known as chemical elements. These are the smallest units that are distinguishable on the basis of their chemical activity and physical properties. The elements are composed of atoms which have distinct structure characteristic of each element.

Here are some definitions:

Matter: Anything that has weight and that takes up space.

Forms of Matter: Solid, Liquid, Gas

Molecule: The smallest particle of matter that retains the same properties of that matter. Example: Water

H2O = H2 + 1/2 O2

H2O (Water) = Molecule

H (Hydrogen) = Element

O (Oxygen) = Element


 Compound: A substance that can be broken down into two or more simpler substances. (A compound is made of millions of molecules)

Element: The smallest part of a substance that retains the same properties of that substance and can not be broken down any further. Elements are composed of atoms which have a distinct structure characteristic of each element.

Atom: The smallest particle of an element which retains the distinct structure characteristic of an element.

 Figure 1. Atomic Model


 Atomic Structure: The free atom is composed of electrons, protons, and neutrons. Almost the entire mass of the atom is concentrated in the nucleus, which contains the protons (positive charges) and neutrons (electrically neutral particles). An atom consists of a minute positively charged nucleus surrounded by a sufficient number of electrons (negative charges) to keep the atom as a whole neutral. Since the electron and proton have equal but opposite electrical charge, the neutral atom must contain an equal number of electrons and protons.


The electrons, spinning on their own axes as they rotate around the nucleus , are arranged in definite shells. The maximum number of electrons that can fit in each shell is 2n2 where n is the shell number. The maximum number of electrons that can fit in the first shell (K) is two, the second shell (L) eight, the third shell (M) eighteen, the fourth shell (N) thirty two, etc.

Elements are designated according to their chemical symbols. Thus iron (Fe) has an atomic number of 26 and an atomic mass number of 56. This means that each iron atom has 26 electrons, 26 protons and 30 neutrons.

 Atomic Mass Number:

Atomic Mass Number = A = Number of electrons + Number of neutrons or it can be equal to Number of Protons + Number of Neutrons. If we let A= Atomic Mass Number, then

A = e + n


A = p + n


where p = Number of Protons , n = Number of neutrons and e = Number of electrons

 Atomic Number:

Atomic Number = Z = Number of electrons = Number of protons

Z = e = p


If we look at Iron as an example, the atomic number of iron is 26 ( Z=26) and the atomic mass number is 56 ( A = 56).

Valence Electrons: The electrons that are largely responsible for the chemical behavior of the element and located in the unfilled shells.

Isotopes: Atoms of the same element that have different atomic mass numbers. Isotopes of an element have the same atomic number but differ in their atomic mass number because, even though they have the same number of electrons and protons (electrically neutral), they differ in the number of neutrons. Example: Hydrogen.

The following table shows the isotopes of hydrogen:

Hydrogen (H11)

Deuterium (H21)

Tritium (H31)

1 electron

1 electron

1 electron

1 proton

1 proton

1 proton

0 neutrons

1 neutron

2 neutrons


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Last updated: September 7, 1999

By: Serdar Z. Elgun