Chemistry C1.1

Atoms

Atoms and elements are the building blocks of chemistry. Atoms contain protons, neutrons and electrons. When elements react they produce compounds.

All substances are made of atoms.A substance that is made of only one sort of atom is called an element. There are about 100 different elements. Elements are shown in the periodic table. The groups contain elements with similar properties.

Atoms of each element are represented by a chemical symbol, eg O represents an atom of oxygen, and Na represents an atom of sodium.

Atoms have a small central nucleus,which is made up of protons and neutrons and around which there are electrons.

In an atom,the number of electrons is equal to the number of protons in the nucleus. Atoms have no overall electrical charge.

All atoms of a particular element have the same number of protons. Atoms of different elements have different numbers of protons.

The number of protons in an atom of an element is its atomic number. The sum of the protons and neutrons in an atom is its mass number.

Electrons occupy particular energy levels. Each electron in an atom is at a particular energy level (in a particular shell). The electrons in an atom occupy the lowest available energy levels (innermost available shells). Candidates may answer questions in terms of either energy levels or shells.

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Elements in the same group in the periodic table have the same number of electrons in their highest energy level (outer electrons) and this gives them similar chemical properties.
The elements in Group 0 of the periodic table are called the noble gases. They are unreactive because their atoms have stable arrangements of electrons.
Candidates are not required to know of trends within each group in the periodic table, but should be aware of similarities between the elements within a group.
Candidates should know that the noble gases have eight electrons in their outer energy level, except for helium, which has only two electrons.

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When elements react, their atoms join with other atoms to form compounds. This involves giving, taking or sharing electrons to form ions or molecules. Compounds formed from metals and non-metals consist of ions. Compounds formed from non-metals consist of molecules. In molecules the atoms are held together by covalent bonds.
Chemical reactions can be represented by word equations or by symbol equations.
No atoms are lost or made during a chemical reaction so the mass of the products equals the mass of the reactants.
Metals lose electrons to form positive ions, whereas non-metals gain electrons to form negative ions

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