Atoms, Elements and Compounds
There are about 100 different elements from which all substances are made. The list of these elements is called the periodic table.
Element: A substance made up of only one type of atom. It is not possible to break an element down into a simpler substance.
Each element is made up of one type of atom.
Atoms are represented by chemical symbols e.g. Na for Sodium or O for Oxygen.
The elements in the periodic table are arranged in columns called groups. The elements in groups usually have similar properties.
Atom: An atom is the smallest particle that can exist on its own.
Element: An element is made up of one type of atom.
Compound: A compound needs two or more atoms or elements chemically bonded.
Mixture: Two or more atoms or compounds which are not chemically bonded (The elements are mixed)
- The nucleus at the centre of an atom contains two types of particle, called protons and neutrons. Protons have a positive charge, and neutrons have no charge.
- Electrons are tiny negatively charged particles that move around the nucleus. An atom has no overall charge. That is because the number of protons is equal to the number of electrons and their charges are equal and opposite (+1 and -1)
- Atoms are neutral because they have an equal number of protons and electrons.
- All atoms of an element contain the same number of protons. This number is called the atomic number, or the proton number. Elements are arranged in order of their atomic numbers in the periodic table.The atomic number is also the number of the electrons in an atom of the element.
- The mass number is the total number of particles in the nucleus of an atom, so it is the number of protons plus the number of neutrons.
The arrangement of electrons in an atom
- Each electron in an atom is in an energy level. Energy levels can be represented as shells, with electrons in the lowest energy level closest to the nucleus.
- The lowest energy level, or the first shell can hold two electrons, and the second energy level can hold eight. Electrons occupy the lowest possible energy levels. The electronic structure of neon, with 10 electrons is 2,8. Sodium with 11 electrons has the electronic structure 2,8,1.
- Elements in the same group of the periodic table have the same number of electrons in their highest energy level, e.g. group one elements have one electron in their highest energy level.
- Group 1 elements include lithium, sodium and potassium. These elements react quickly with water and with oxygen.
- The atoms of the unreactive Noble gases (In group 0) all have very stable arrangements of electrons.