Topic 4: Atoms

CORE CONTENT:

Atoms – building blocks of matter

• all matter is made up of tiny particles called atoms

• an element is made up of atoms of the same kind. For example all the atoms of an element, such as copper, are identical

• an element is a substance that cannot be broken down into two or more substances by chemical means (An element cannot be changed into another element by means of a chemical reaction)

• atoms of one element differ from the atoms of all other elements

• all known elements are listed on the Periodic Table of the Elements

Sub atomic particles

• atoms are made up of smaller sub-atomic particles (protons, neutrons and electrons)

• the central region of the atom is called the nucleus

• the nucleus is made up of positively charged particles called protons and neutral particles called neutrons

• negatively charged particles called electrons move around the nucleus

• atoms are neutral because the number of negatively charged particles (electrons) is equal to the number of positively charged particles (protons)

Pure substances

• elements and compounds are pure substances

Elements

• an element is a material that consists of atoms of only one kind, such as hydrogen (H), oxygen (O), carbon (C), sodium (Na) and chlorine (Cl)

• all known elements are listed on the Periodic Table of Elements. They are limited in number and are the building blocks of millions of compounds

• some elements on the Periodic Table of Elements form diatomic molecules for example hydrogen (H2), nitrogen (N2), oxygen (O2), chlorine (Cl2). These are called molecules of elements

• sometimes atoms react together chemically to form molecules of compounds (such as H2O, CO2)

Compounds

• a compound is a material that consists of atoms of two or more different elements chemically bonded together, such as water (H2O), carbon dioxide (CO2), salt (NaCl)

• the atoms in a given compound are always combined/bonded in a fixed ratio such as, in water, where the ratio is always two hydrogen atoms (H) to one oxygen atom (O)

• a chemical bond is the force that holds atoms together

• compounds [such as water (H2O), carbon dioxide (CO2), salt (NaCl)] are formed by chemical reactions

• compounds can be broken down in a decomposition reaction into other compounds or their original elements by heating or electrolysis. For example, electrolysis decomposes water (H2O) to form hydrogen (H2) and oxygen (O2)

Mixtures of elements and compounds

• elements and compounds are often found mixed together, such as in air, sea water, rocks, and in living things

• mixtures are separated by physical means; compounds can be separated by chemical means