Terms

• element Any one of the simplest chemical substances that cannot be changed in a chemical reaction or by any chemical means. Made up of atoms that all have the same number of protons.
• compound A substance made from two or more elements. Consists of a fixed ratio of chemically bonded atoms. Has unique properties that are different from the properties of its individual elements.
• chemical bond Any of several attractive forces that serve to bind atoms together to form molecules.

Elements and Compounds

An Atom

• Atom – smallest unit of all matter, that is composed of 3 sub-atomic particles called protons, electrons and neutrons
• Proton – the ‘heavy’ positively-charged particle in the nucleus of an atom
• Electron – the very ‘light’ negatively-charged particle that orbits the nucleus of an atom
• Neutron – the ‘heavy’ uncharged particle in the nucleus of an atom

• Uncharged or unreacted atoms have the same number of positive protons and negative electrons.
• Approximate size of atoms – Millions of atoms could fit on the sharp point of a needle. Also, if you imagine that an atom is the size of an oval, a proton and a neutron would be the size of footballs in the middle of the oval, and the electron would be the size of a rice grain racing around the running lane.

Atomic Number and Atomic Mass

• An Example from the Periodic Table

• Atomic Number – the number of protons in an unreacted atom
• Mass Number – the number of protons and neutrons together

Atomic Diagrams

• Protons and neutrons are in the nucleus.
• Protons are p+
• Neutrons are n
• Electrons orbit the nucleus in electron levels or ‘rings’.
• Electrons are e–
• A limited number of electrons are situated in each electron ‘ring’.
• First ring – maximum of 2 electrons
• Second ring – maximum of 8 electrons
• Third ring – maximum of 8 electrons
• Fourth ring – maximum of 18 electrons
• Electron rings close to the nucleus are filled first.

Elements and Compounds

• Element – An element is a substance composed of the same type of atoms (e.g. gold Au, oxygen O₂).
• Compound – A compound is a substance made of more than one type of atom (e.g. water H₂O, carbon dioxide CO₂).
• Molecule – A molecule is the smallest particle of either an element or a compound.

Inert or Noble Gases

• Inert or Noble Gases are unreactive gases. They do not corrode or react.
• Examples of Noble Gases are:
• He – Helium
• Ne – Neon
• Ar – Argon
• Kr – Krypton
• Xe – Xenon
• Rn – Radon
• The electron rings of these unreactive gases are full, therefore they become stable.

Ions (Charged Atoms)

• When atoms react, they may either gainCOMPOUND OF ELEMENTS ASSIGNMENTCOMPOUND OF ELEMENTS ASSIGNMENT or lose electrons. Electrons have a negative charge. An atom gaining or losing electrons will get an overall charge.
• Positive Ions are atoms that have lost electrons (e.g. sodium Na¹⁺)
• Negative Ions are atoms that have gained electrons (e.g. chlorine Cl^1–)
• In chemical reactions, atoms tend to gain or lose electrons to resemble the electron numbers of the stable Noble Gases.

Covalent and Ionic Compounds

• Covalent Compound– a compound where electrons are shared between the atoms (e.g. carbon dioxide CO₂)
• Ionic Compound – a compound formed from the attraction between positive and negative ions. For example in the ionic compound sodium chloride NaCl, the chlorine ion (Cl^1–) gains one electron that was given by the sodium ion (Na¹⁺).

Common Elements and Symbols to Learn