- 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
- Elements are the simplest complete chemical substances. Each element corresponds to a single entry on the periodic table. An element is a material that consists of a single type of atom. Each atom type contains the same number of protons.
- Chemical bonds link elements together to form more complex molecules called compounds. A compound consists of two or more types of elements held together by covalent or ionic bonds.
- Elements cannot be divided into smaller units without large amounts of energy. Compounds, on the other hand, can have their bonds broken with practical amounts of energy, such as the heat from a fire.
- Matter can be broken down into two categories: pure substances and mixtures. Pure substances are further broken down into elements and compounds. Mixtures are physically combined structures that can be separated back into their original components.
- 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
- 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 gain 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 Cl1–)
- 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 (Cl1–) gains one electron that was given by the sodium ion (Na1+).
Common Elements and Symbols to Learn
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