Matter can be classified into two general groups; mixtures and pure substances.

Chemical changes

Chemical changes occur when substances change and a new substance is created. Signs that a chemical change has occurred include a change of temperature. If the substance experiences an increase in thermal energy than an exothermic reaction is taking place, in this type of reaction heat is released by the breaking of chemical bonds. If there is a decrease in thermal energy than a endothermic change is taking place, in this type of reaction thermal energy is absorbed into chemical bonds. A change of color or release of gases, as long as the gas is a new substance not just a change in state of matter, can indicate either a chemical or a physical change.


People once thought that the atom was the smallest particle of matter in the universe However, scientists now know that atoms are made up of even smaller parts. There are three different kinds of particles. They are: protons, neutrons, and electrons. Most of the mass of an atom is found in the central part of the atom, called the nucleus. The nucleus of an atom is made up of protons and neutrons. These particles are packed very tightly together in the nucleus.

Electrons are found outside the nucleus. They circle the nucleus very, very quickly. Electrons are very small and have almost no mass. The number of electrons in an atom is always equal to the number of protons in the nucleus of that atom. Scientists have discovered that protons, electrons, and neutrons have different charges. You probably know that the word “charge” has something to do with electricity. There are two kinds of charges. There are positive (plus) charges and negative (minus) charges. By studying atoms, scientist, have learned that:

  • PROTONS have positive (+) charges.
  • ELECTRONS have negative (-) charges.
  • NEUTRONS have no charges. They are neutral.

Since atoms have the same number of protons and electrons, the number of positive charges equals the number of negative charges. The opposite charges cancel each other out. Therefore, the whole atom has no overall charge.

The arrangement of electrons In the past, scientists believed that electrons circled the nucleus the same way the planets circled the sun. Today, however, scientists know that there is no exact path of an electron. The quick moving electrons form a “loud” around the nucleus.

In the modern atomic theory, electrons are arranged into energy levels, or shells. Each electron shell is labelled with a capital letter. The first shell is the “K” shell. It is the shell closest the the nucleus. The “K” shell has the least amount of energy. The next shell is the “U’ shell. After the “L” shell, comes the “M” shell, and so on.

Each shell can only hold a certain number of electrons.

  • The “K” shell can hold up to 2 electrons.
  • The “L” shell can hold up to 8 electrons.
  • The “M” shell can hold up to 18 electrons.

The number of shells an atom has depends upon the number of electrons the atom has. In general, each shell must have its full number of electrons before a new shell starts. If there are mom electrons than a shell can hold, a new shell starts.

The difference between atomic mass and atomic number Atoms of different kinds of matter have different numbers of protons and electrons. When scientists talk about different kinds of matter, they often refer to the matter by it’s atomic number. The atomic number of an atom is the number of protons (and usually, the number of electrons) in the atom. Scientists also describe atoms by their atomic mass. Scientists do not measure the mass of atoms in grams or ounces. They measure the mass of atoms in atomic mass units (a.m.u.). You can figure out the atomic mass of an atom by using the following information:

  • Each proton has a mass of 7 a.m.u.
  • Each neutron has a mass of I a.m.u.
  • The atomic mass of an atom is the total number of protons and neutrons in the nucleus of the atom.

What about the electrons Don’t they count? Electrons are very light. Their mass is not counted in the atomic mass. Sometimes two moms of the same kind of matter do not have the same atomic mass. How is this possible? They have a different number of neutrons. All atoms of the same kind of matter always have the same number of protons. Thus, they all have the same atomic number. Atoms of the same kind of matter that have different numbers of neutrons are called isotopes.

Properties of matter are characteristics that describe matter and can be used to identify different types. A general property, such as mass, weight, size, or shape can change for a given type of matter. A characteristic properties are always the same for a specific kind of matter and stay the same despite the size or shape of the sample. Some examples of characteristic properties include hardness, flammability, boiling point, freezing point, melting point and density.

Physical changes

Physical changes occur when a change happens to matter which does not alter the type of matter that it is . A physical change will alter the form of a substance but not its identity. The physical properties of an object or substance change but it does not change into a new substance. A change of color or release of gases can indicate either a physical or a chemical change has occurred.


Gases have particles that are spaced far apart and will expand away from one another to fill their container. These particles travel very quickly and bounce of the solid surfaces they come in contact with. Gases do not have a definite volume because they are so easily compressed and decompressed. When a gas is heated and charged with electrons it will change into a plasma, when it is cooled it will change into a liquid.


Liquids are less dense than solids. They have a definite volume but unlike solids they take the shape of their container. The particles of liquids have some space between them and roll over one another in a fluid or flowing motion. They also have an acute magnetism which causes the particles of a liquid to group together or puddle on a surface. When a liquid is heated it will change into a gas, when it is cooled it will change into a solid.


Solids are the most dense forms of mater. There particles are closely spaced and can only vibrate. Solids have a definite volume and shape. Unlike liquids liquids, gases and plasmas, solids have a fixed shape and do not change to take up the space of their container. When a solid is heated it will change into a liquid.


Plasma is an ionized gas. When a plasma’s source of heat or electricity is taken away it will change into a gas.