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Chapter 3: Matter and Change

Ch. 3.1 Matter

Atoms are the basic building blocks of all matter.


Matter- anything that has volume and mass. Everything that surrounds you is made of matter.

Usually occurs as solid, liquid, or gas. All matter made of elements. Elements- a substance that cannot be broken down into simpler substances by physical or chemical means.

Each element has unique physical and chemical properties. Elements are made up of atoms. Atoms are made up of smaller particles: proton, neutron, electron.

Nucleus- the center of an atom, made up of protons and neutrons. Proton- a tiny particle that has mass and a positive charge. Neutron- a particle with approximately the same mass as a proton, that has no charge.

All atomic nuclei has a positive charge.

Electron- smaller particles with little mass surrounding the nucleus, has a negative electric charge.

Atom is neutral when protons are equal to electrons. Electrons are in constant motion to form a cloud-like region surrounding the nucleus.

Symbols for Elements

92 elements occur naturally on Earth and stars. Others produced in lab experiments. Each element is identified by a abbreviation known as a chemical symbol.

Elements are classified and arranged by their chemical properties in the periodic table.

Mass Number

The number of protons and neutrons of elements vary.

Atomic number- the number of protons in an atom. Mass number- the sum of the protons and neutrons.

Electrons have little mass, they are not included in the mass number. For example uranium 238: mass 238 ( 92 protons + 146 neutrons)


Isotopes- atoms of the same element with the same number of protons but different number of neutrons. The atomic mass of an element is the average of the mass numbers of the isotopes of an element.

Radioactive Isotopes

Nuclei of some isotopes are unstable and tend to break down, emitting energy in form of radiation. Radioactive decay is the process in which a nucleus loses protons and neutrons, change a proton to a neutron, or change a neutron to a proton.

Changing protons, changes the element. Radioactive decay is often used to calculate the ages of rocks.

Electrons in Energy Levels

Electrons occupy areas called energy levels. the size of an atom depends on the number and arrangement of its electrons.

Filling Energy Levels

Each energy level can hold a limited number of electrons. 1st level holds 2 electrons, 2nd holds 8 electrons, 3rd holds 18 electrons, 4th holds 32 electrons.

Valence Electrons

The outermost electrons are called valence electrons and determine the chemical behavior of different elements. elements with the same number of valence electrons have similar chemical properties.

Elements with full outermost levels are highly nonreactive.


Ions- an atom that gains or loses an electron has a net electric charge. When atom loses electrons it has a positive charge. When atom gains electrons it has a negative charge.

What Elements Are Most Abundant

Hydrogen and helium are most abundant elements in the universe. All other elements are less than 1% of all atoms in the universe.

Earth's crust is different; 98.5% is made up of only eight elements. 75% of these are oxygen and silicon which forms silicates. Aluminum and iron are the next two most abundant.

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Ch. 3.2


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Ch. 3.3


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Ch. 3.4


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Page last updated April 3, 2017.