Atomic structure and the periodic table

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Atoms

Substances are made of tiny particles called atoms.

A small amount of substances are made up of only one type of atom. These are called elements. An atom is the smallest part of an element.

The symbols in the periodic table represent atoms.

Elements in the table are arranged in columns known as groups.

Many substances you come across are made up of different types bonded together, called compounds.

All atoms are made up of a tiny central nucleus with electrons orbiting around it.

Chemical equations

chemical equations show the reactants (the substances you start with) and the products (the new substances made) in a reaction.

Symbol equations show you how much of each substance is involved in a reaction.

Balanced equations- the same number of each type of atom on both sides of the equation. atoms cannot be created or destroyed in a chemical reaction. The total mass of the products formed in a reaction os equal to the total mass of reactants. This is called...

Law of conservation mass- In reactions involving gases, this law can appear broken when reactions are carried out in open containers.

State symbols- given to a balanced symbol equation- s=solids l=liquids g=gases aq=aqueous.

Separating mixtures

Remember to balance all equations Eg: 2H2+ 02- 2H20.

A mixture is made up of two or more substances (elements and compounds) that are not chemically joined together.

Before substances in a mixture are identified, they are separated from each other. You can use physical means to achieve the separation.

Filtration, crystallisation, distillation, chromatography. These techniques all rely on differences in the physical properties of the substances in the mixture, such as different solubilities in a solvent or different boiling points.

Filtration-Is used to separate substances that are insoluble in a particular solvent, from ones that are soluble in the solvent.

Crystallisation- To get a pure salt from the salt solution following filtration, you would need to separate the filtrate from the water. Do this by evaporating. Heat it on an evaporating dish on a water bath. Heat should be stopped at the point of crystallisation. When crystals are starting to appear pour the solution into something with a large surface area to evaporate off liquid and turn it into crystals.

Distillation-Allows us to collect the solvent. A solution is heated and boiled to evaporate the solvent. The vapour given off then enters a condenser. An outer glass tube with water running through it acting as a cooling jacket. The hot vapour is cooled and condensed back to a liquid to collect in a receiving vessel.

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History of the atom

John Dalton suggested that substances were made up of atoms that were like tiny herd spheres. Also that each chemical element habits own atoms that differed from others in mass. These atoms couldn't be divided or split. They were the fundamental building blocks of nature.

JJ Thomson- End of 1880’s he discovered the electron. Tiny negatively charged particle that was found to have a mass about 2,000 times smaller than the lightest atom.

He did experiments on the beams of particles, they were attracted to a positive charge.

The electrons had to come from inside the atoms so Dalton’s theory that atoms could not be split had to be revised.

He said the tiny electrons had to hidden in a cloud of positive charge.

Imagined the electrons as the bits of plum in a pudding, the plum pudding model.

Geiger and Marsden were doing an experiment with radio active particles. Fired dense positively charged particles at the thinnest piece of gold foil they could make.

They expected the particles to pass straight through the gold atoms with their diffuse cloud of positive charge. It was proposed that the electrons orbited around the nucleus which contains very dense positively charged protons.

Niels Bohr- revised the atomic model in 1914, when he noticed light was given out when atoms were heated only had specific amounts of energy.

He suggested that electrons must be orbiting the nucleus at set distances, in fixed energy shells/levels.

The energy must be given out when electrons fall from a high to low energy level.

Neutrons must have no charge and have the same mass as a proton. 1932 James Chadwick did an experiment that coulee only be explained by the existence of neutrons.

Structure of the atom- Protons have a positive charge, neutrons have no charge, electrons are negatively charged.

Every atom contains an equal number of protons and neutrons so there is no overall charge.

Atomic number- The number in each atom of an element is called its atomic number. Elements in the periodic table are ordered in order of their atomic number.

Mass number- The number of protons in the nucleus of an atom

number of neutrons=mass number - atomic number.

Ions, atoms and isotopes

Negative ion- if it gains a negative charge because it has more electrons than protons. if it loses one or more electron it becomes a positive ion.

Ion is a charged atom(group of atoms)

It has been estimated that there is about 7 billion billion atoms in a persons body. Atom is about a tenth of a billionth of a metre across.

Isotopes- Atoms of the same element with different numbers of neutrons are called isotopes.

Sometimes extra neutrons make the nucleus unstable, so it’s radioactive. not all are radioactive, some atoms of the same elements have different masses.

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Electronic structures

An energy level or shell can only hold a certain number of electrons.

The first and lowest energy level can hold up 2 electrons.

The second energy level can hold up to eight electrons. once there are eight in the third the fourth then fills up etc etc.

Elements in any of the main groups of the periodic table, their atoms all have the same number of electrons in their highest energy level. Often called the outer electrons, as they are in the outermost shell.

Chemical properties depend on how many electrons it has. The way the element reacts is determined by the number of electrons in the outer most shell. So they all react in similar way.

Elements in group 0 of the periodic table are called noble gases. very un-reactive elements. veery stable arrangement of electrons in the outer shell, except except for helium which has 2 in its outer shell.