Additional Chemistry C2

Atomic structure

Atoms of any element are made from 3 smaller particles. The nucleus consists of protons and neutrons. On the outside moving particles known as electrons are present. Complete atoms have no charge because the number of positively charged protons is the same as the number of negatively charged electrons. So they cancel each other out and the atom is neutral.

Proton one mass unit Positive +1 Charge

Neutron one mass unit No charge 0

Electron Almost nothing Negative -1 Charge

The mass number is the number of positively charged protons plus the number of neutrons present.

The proton number or the atomic number is the number of protons which defines the element. The proton number does not change because if it did then it would become a different atom.

Isotopes

An isotope of an element has the same number of protons but the number of neutrons is different. This means that the atoms mass can be different giving it different properties to the element. Isotopes have the same chemical reactions as the element would. The properties made different would be density and diffusion.

Electronic Arrangement

The first shell is the smallest and can have a maximum of 2 electrons. From here onwards the next shells can hold 8 electrons and gradually get larger.

The number of electrons on the outer shell tells us what group number the element is in on the periodic table. All of the elements in the same group react and behave similarly in chemical reactions.

Ionic Bonding – METAL + NON METAL

New substances called compounds are formed when 2 or more elements are chemically joined or bonded together. In ionic bonding the metal will lose one or more electron(s) and the non metal will gain an electron.

Double Charged ions

When an element looses 2 ions and the non metallic element gains 2 electrons the metal has a positive charge of +2 and the non metallic element now an ion has a charge of -2. If these were magnesium and oxygen they would both end up with a filled second outermost shell. Therefore they would be the same size and they would form a lattice structure.

When an element like magnesium bonds with chlorine which only has 7 electrons on the outermost shell then to 2 magnesium ions split so there is 2 chlorine ions and one magnesium which is why it has the chemical formula MgCl₂.

Covalent Bonding – NON METAL + NON METAL

In covalent bonding the two atoms can bond together by sharing a pair of electrons. This is called a covalent bond. The two elements tend to be very similar and are usually 2 non metals. In a single covalent bond 2 atoms of the same element bond together and share electrons for example Hydrogen gives H₂. This is called a molecule.

In covalent bonding the atom needs to become stable by filling up its outermost shell. So if we had oxygen and hydrogen the oxygen atom has 6 electrons on its outer shell with 2 electrons needed to make it stable and filled. However as it bonding with hydrogen that only has 1 electron on its outer shell it needs 2 atoms of hydrogen to make both the hydrogen atom and the oxygen atom stable. This is why it has the chemical formula H₂O. Two single covalent bonds have been formed here.

A double covalent bond occurs when 2 atoms like oxygen bond together, because oxygen has 6 electrons on the outer shell it shares 2 pairs so both oxygen are stable. The chemical formula is O₂.

Metallic Bonding – METAL + METAL

Metals tend to be on the left and centre of the periodic table. Meaning they tend to have very few electrons on their outer shells one two or three at the most. Metals are usually solids at room temperature therefore their atoms or particles are tightly packed together. This is because they have so few electrons on their outer that they can overlap and form a strong bond. The electrons do not stay with their ion but are free to move all over the metal. This is the ‘sea’ of electrons or ‘delocalised’ electrons. The metals are positively charged because they have lost their electron but the electrons are negatively charged and act as a glue to hold them together. This is why metals have these certain properties: High melting and boiling point, good conduction of electricity, shiny surface.

Ionic compounds

Compounds are formed when two or more elements chemically join together. Ionically bonded compounds do not conduct electricity when they are in a solid state; ionically bonded substances are soluble in water. Each ion is so tightly held to its nearest neighbours in the lattice that each ion cannot move to carry the current. When water is present the compound dissolves and spreads throughout the substance. The free ions can therefore move and when the electrodes are added, the electrodes will attract oppositely charged ions. The cathode has a negative charge and will attract positively charged ions (normally the metal in ionic bonding). Then therefore the anode is positive and will attract negatively charged ions. Instead of dissolving in water we can melt the and till they become molten.

Simple covalent compounds

The atoms that are joined together have quite strong covalent bonds holding them together. But between the molecules there are usually quite weak forces of attraction. There are often intermolecular forces of attraction because they occur between the molecules. These weak forces can be easily overcome and this is the cause of the properties typical of simple covalent bonding. These have low melting and boiling points. This is why simple covalent bonds tend to be gases or liquids, if they are solids they tend to be soft like Vaseline. These do not dissolve in water; they do not conduct any electricity in any state of matter.

Giant covalent bonds

These have a very high boiling and melting points.

Metallic Compounds

Most metals are elements, this means they are pure and only made of one type of particle. Metals have tightly packed structures.