C1 - Atomic structure
Atoms
Each element is made of only one type of atom.
Compounds contain more than one element.
The periodic table lists all the chemical elements, with 8 main groups.
An atom has a tiny nucleus at its center, surrounded by electrons.
All substances are made of atoms.
The elements in a group usually have similar properties.
Chemical equations
State symbols give extra information in balanced symbol equations. (s), (l), (g), (aq) - aqueous solutions.
The total mass of reactants = the total mass of products.
There's the same number of each type of atom on each side of a balanced symbol equation.
Word equations give the names of the reactants and products.
No new atoms are ever created or destroyed in a chemical reaction.
Separating mixtures
The chemical properties of each substance in the mixture are unchanged.
Processes
Mixtures can be separated by physical means, such as filtration, crystallisation and simple distillation.
A mixture is made up of two or more substances that are not chemically combined together.
Fractional distillation and paper chromatography
Paper chromatography separates mixtures of substances dissolved in a solvent as they move up a piece of chromatography paper.
The different substances are separated because of their different solubilities in the solvent used.
The separation is possible because of the different boiling points of the liquids in the mixture.
An effective way of separating miscible liquids, using a fractionating column.
Structure of the atom
History of the atom
Key ideas were proposed successfully by Dalton, Thomson, Rutherford and Bohr.
New evidence has been gathered by scientists who have their model of the atom to explain their observations and calculations.
Ancient Greeks were the first to have ideas about atoms
Ideas have changed over time
The relative masses of a proton and neutron are both 1.
Atoms contain an equal number of protons and electrons so carry no overall charge.
Protons have a relative charge of +1 and electrons have a relative charge of -1. Neutrons have no electric charge. They are neutral.
Atomic number = number of protons (=number of electrons)
Atoms are made of protons, neutrons and electrons.
Mass number - number of protons + neutrons.
Ions, atoms & isotopes
You can represent the atomic number and mass number of an atom using the notion; 24/12 Mg, where magnesium's atomic number is 12 and is mass number is 24
Isotopes have identical chemical properties, but their physical properties can differ, e.g. density.
Atoms that gain electrons form negative ions. If atoms lose electrons they form positive ions.
Atoms of the same element always have the same number of protons but they can have different number of neutrons.
Electronic structures
After 8 electrons fill the 3rd shell the 4th starts to fill.
The number of electrons in the outermost shell of an elements atoms determines the way the element reacts.
The lowest energy level (1st shell) can hold up 2 electrons the second shell can hold up to 8.
The electrons in an atom are arranged in energy levels or shells.
Group 1 elements have 1 electron in their outer shell.
Law conservation of mass
States that no atoms are lost or made during a chemical reaction, so the mass of the products equals the mass of reactants.
Sometime the law is broken because the mass appears to change - usually when gases are reactants or products.
E.g. if a product is a gas it may escape into the air.
Sometimes substances increase in mass when heated in air - the extra mass is because oxygen from the air reacts.
Crystallisation
Distillation
Filtration
Separate substances insoluble in a solvent from those that are soluble in the solvent. E.g. sand separated from salt solution.
Separates a solvent from soluble solids dissolved in the solvent, - e.g. salt (sodium chloride) from salt solution
Separates a solvent from soluble solids dissolved in the solvent - e.g. seawater to useable water.
In the early 1800's, Dalton linked his ideas to strong experimental evidence. Dalton suggested atoms were tiny, hard spheres. These atoms could not be divided or split.
At the end of the 1800's, Thomson discovered a tiny negatively charged particle called the electron
Thomson proposed the 'plum pudding' model for the atom. The model suggested that negative electrons were embedded in a ball of positive charge.
10 years later, Geiger and Marsden were experimenting with alpha particles. They fired alpha particles at a very tin piece of gold foil.
A few alpha particles were repelled showing that there must be a tiny spot of positive charge in the center of the atom.
Rutherford then proposed the nuclear model.
In the nuclear model, electrons orbit around a nucleus. The nucleus contains positively charged protons.
Bohr then revised the nuclear model. He suggested that the electrons were orbiting the nucleus in energy levels or shells.
The electrons were a set distance from the nucleus. Bohr's theoretical calculations matched the experimental observations.
In1 932, Chadwick provided the experimental evidence that showed the existence of uncharged particles called neutrons in the nucleus.
Atoms of the same element with different numbers of neutrons are called isotopes.
Because all the elements in the same group have the same number of electrons in their outer shell, they all react chemically in the same way.
Noble gases are in group 0 - very unreactive cause of a full outer shell - they all have 8 electrons in their outer shell except for helium which has 2.