yr 8 - elements, compounds and mixtures

Elements

An element is a substance made up of only one type of atom. An easy way to determine whether something is an elementis to see if it appears on the Periodic table, as all elements can be found there. Elements include oxygen molecules (O2), helium atoms (He) inside party balloons and hydrogen molecules (H2) in the Sun's core. Notice that all of these things are only made up of a single type of atom.

Compounds

Compounds are made up of more than one type of atom chemically bonded together. Examples include carbon dioxide (CO2), water (H2O) and ethanol (C2H5OH), which are all made up of two or more types of atoms bonded together.

Mixtures

A mixture is a group of different elements or compounds which are not chemically bonded together. Air is a mixture, as it is made up of nitrogen, oxygen and argon (all elements) along with carbon dioxide and water vapour (compounds). Other examples of mixtures include cake batter and shampoo. Unlike elements and compounds, which have fixed melting and boiling points, mixtures will melt and boil over a range of different temperatures, as each component of the mixture will change state at different temperatures.

Mixtures can be separated by filtration, distillation, crystallisation, or chromatography. The technique used depends on the nature of substances which make up the mixture.

Molecule

Molecules are the smallest unit of compound. If you break up a molecule it seizes to exist as a compound and reverts back to its original state of an element. The atoms in the molecules are connected by chemical bonds. The arrangement and the strength of the bonds give molecules a certain shape.

Molecular Element: Molecules that consist of one type of atom (e.g. O2, P4)

Allotropes: Some elements can form more than one shape of molecules and each form are known as allotropes (P4 [White phosphorous], P14 [Red phosphorous]).

Substance: A matter which has a specific composition and specific properties, any type of material.

Element: A pure substance consisting of only one type of atom which all have the same numbers of protons in their nuclei

Periodic table: A tabular array of the chemical elements organized by atomic number.

Molecules: Two or more atoms connected by chemical bonds, which form the smallest unit of a substance that retains the composition and properties of that substance.

Atom: The basic building block of chemistry. It is the smallest unit into which matter can be divided.

Chemically bonded: The formation of a chemical bond between two or more atoms, molecules, or ions to give rise to a chemical compound

Filtration: The process of separating suspended solid matter from a liquid, by causing the liquid to pass through a filter.

Distillation: The process of separating the components of a liquid mixture through selective evaporation and condensation.

Crystallisation: The solidification of a liquid substance into a highly structured solid whose atoms or molecules are placed in a well-defined three-dimensional crystal lattice.

Chromatography: To separate the components of a substance to find out what it is composed of.

Elements and atoms

Anatomy of Atoms:

The atomic number in an atom defines the number of electrons an atom has.
The first shell in every atom always has 2 electrons.
The nucleus of an atom is always made of protons.
Each shell after the first shell can only hold 8 electrons, if the shell has less that 8 electrons, then the atom becomes highly unstable.
As the atomic number gets larger, the atom gets heavier and larger.
Outer shell: Electrons in the outer shell form bonds.
Octet rule: Atoms with low atomic numbers become full and stable when their outer shells contain eight electrons – the so-called octet rule. Anions, or positive ions are formed when atoms lose electrons, while cations or negative ions, are formed when atoms gain electrons.