Atomic Structure and The Periodic Table

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Atoms, elements and compounds

All substances are made up of atoms

Atoms-The smallest part of a chemical element that can exist

Compounds- Two or more substances that are chemically combined

Elements- An element is a substance whose atoms all have the same number of protons

Chemical reactions can be represented by word equations or equations using symbols and formulae

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Mixtures

Mixtures- Two or more elements or compounds that are not combined together

The chemical properties of each substance in the mixture are unchanged

Mixtures can be separated by physical processes such as filtration, crystallisation, simple distillation and fractional distillation. These processes do not involve chemical reactions so no new substances are made

Filtration- Used if your product is an insoluble solid that needs to be separated from a liquid reaction mixture. It can be used in purification as well.

Evaporation- Used to separate soluble solids from solutions. Is a quick way or separating a soluble salt from a solution.

Crystallisation- Used to separated separate soluble solids from solutions.

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The Development of The Model Of The Atom

Before the discovery of the electron, atoms were thought to be tiny spheres that could not be divided.

At the start of the 19th Century John Dalton described atoms as solid spheres and that the spheres made up different elements

In 1897 J J Thomson concluded from his experiments that atoms weren't solid spheres. His measurements of charge and mass showed that an atom must contain even smaller, negatively charged particles - electrons. The new theory was known as he "plum pudding model"

The plum pudding model showed the atom as a ball of positive charge with electrons stuck in it

The results from the alpha particle scattering experiment (Ernest Rutherford) led to the conclusion that the mass of an atom was concentrated at the centre (nucleus) and that the nucleus was charged. This nuclear model replaced the plum pudding model.

Niels Bohr adapted the nuclear model by suggesting that electrons orbit the nucleus at specific distances. The theoretical calculations of Bohr agreed with experimental observations

Later experiments led to the idea that the positive charge of any nucleus could be subdivided into a whole number of smaller particles, each particle having the same amount of positive charge. The name proton was given to these particles.

The experimental work of James Chadwick provided the evidence to show the existence of neutrons within the nucleus. This was about 20 years after the nucleus became an accepted scientific idea.

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Relative Electrical Charges Of Subatomic Particles

The relative electrical charge of the particles in the atom are

Proton- +1

Neutron- 0

Electron- -1

The is no overall charge because the charges from the proton and the electron cancel each other out

The number of protons in an atom of an element is its atomic number. All atoms of a particular element have the same number of protons. Atoms of different elements have different numbers of protons.

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Size and Mass Of Atoms

Atoms are very small, having a radius of about 0.1 nm (1 x 10-10 m).

The radius of a nucleus is less than 1/10 000 of that of the atom (about 1 x 10-14 m).

Almost all of the mass of an atom is in the nucleus.

The relative masses of protons, neutrons and electrons are:

Proton- 1

Neutron- 1

Electron- 1/1800

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

Atoms of the same element can have different numbers of neutrons; these atoms are called isotopes of that element.

The relative atomic mass of an element is an average value that takes account of the abundance of the isotopes of the element.

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Electronic Structure

The electrons in an atom occupy the lowest available energy levels (innermost available shells). The electronic structure of an atom can be represented by numbers or by a diagram. For example, the electronic structure of sodium is 2,8,1 or

showing two electrons in the lowest energy level, eight in the second energy level and one in the third energy level.

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The Periodic Table

The elements in the periodic table are arranged in order of atomic (proton) number and so that elements with similar properties are in columns, known as groups. The table is called a periodic table because similar properties occur at regular intervals.

Elements in the same group in the periodic table have the same number of electrons in their outer shell (outer electrons) and this gives them similar chemical properties

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Development Of The Periodic Table

Before the discovery of protons, neutrons and electrons, scientists attempted to classify the elements by arranging them in order of their atomic weights.

The early periodic tables were incomplete and some elements were placed in inappropriate groups if the strict order of atomic weights was followed.

Mendeleev overcame some of the problems by leaving gaps for elements that he thought had not been discovered and in some places changed the order based on atomic weights

Elements with properties predicted by Mendeleev were discovered and filled the gaps. Knowledge of isotopes made it possible to explain why the order based on atomic weights was not always correct.

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Metals And Non-Metals

Elements that react to form positive ions are metals.

Elements that do not form positive ions are non-metals.

The majority of elements are metals. Metals are found to the left and towards the bottom of the periodic table. Non-metals are found towards the right and top of the periodic table.

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Group 0

The elements in Group 0 of the periodic table are called the noble gases. They are unreactive and do not easily form molecules because their atoms have stable arrangements of electrons. The noble gases have eight electrons in their outer shell, except for helium, which has only two electrons.

The boiling points of the noble gases increase with increasing relative atomic mass (going down the group).

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Group 1

The elements in Group 1 of the periodic table are known as the alkali metals and have characteristic properties because of the single electron in their outer shell.

In Group 1, the reactivity of the elements increases going down the group.

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Group 7

The elements in Group 7 of the periodic table are known as the halogens and have similar reactions because they all have seven electrons in their outer shell. The halogens are non-metals and consist of molecules made of pairs of atoms.

In Group 7, the further down the group an element is the higher its relative molecular mass, melting point and boiling point.

In Group 7, the reactivity of the elements decreases going down the group.

A more reactive halogen can displace a less reactive halogen from an aqueous solution of its salt.