Chemistry

About elements, bonding and properties

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Different types of bonding

1) Ionic bonding: metal + non metal. Holds oppositely charged ions together in giant structures. Strong electostatic forces act in all directions so each ion in a lattice is surrounded by ions with the opposite charge. Held firmly in place.

2) Covalent bonding: non-metal + non-metal. The atoms of non-metals need to gain electrons to achieve stability. They share electrons with other atoms. Each shared pair of electrons attracts the two atoms, forming covalent bonds.

3) Metallic bonding: Two metals form giant structures in which layers of atoms are in regular patterns. You can't see individual atoms but you can see metal crystals on the surface of some metals. Also, metal crystals can grow by displacement reactions. When metal atoms pack together, electrons in the highest energy level delocalise and move from one atom to another, producing positive ions in a sea of moving electrons. Delocalised electrons attract positive ions and hold the structure together.

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More on bonding

Sodium chloride has equal numbers of sodium and chloride ions-  NaCl. The ions alternate, making a cubic lattice. The ratio of ions in the formula and structure of an ionic compound depends on charges.

Magnesium = Mg2+, chlorine = Cl-, so the formula of magnesium chloride is MgCl2, as there are twice as many chloride ions to magnesium ions

Atoms of elements in Group 7 need one more electron to be a stable. They gain one electron so form a single covalent bond. Group 6 need two electrons and form two covalent bonds. And so on. Covalent bonds act only between the two atoms they bond and so many covalently bonded substances contain small molecules. Some atoms that can form several bonds make giant covalent structures.

When electrons in the highest energy level (the outer electrons) delocalise, the result is strong electrostatic forces between electrons and positively charged metal ions, so the metal is held together.

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Ionic compounds, simple molecules

Ionic compounds have giant structures in which very strong electrostatic forces hold ions tightly together. They are solids at room temperature. A lot of energy is needed to overcome the ionic bonds to melt the solids and so ionic compounds have high melting and boiling points. When they have been melted, the ions are free to move. This allows them to carry electrical charge, so liquids conduct electricity. Some ionic solids dissolve in water because water molecules split up the lattice. The ions are free to move so they conduct.

Atoms in molecules are held together by strong covalent bonds. These bonds act only between atoms within the molecule, so simple molecules have little attraction for eachother. Substances made of simple molecules have low melting points. Intermolecular forces are weak. These forces are overcome when a molecular substance melts/boils. This means that substances made of small molecules have low melting/boiling points. H2, Cl2 and CH4 etc have weakest intermeolecular forces and are gases at room temp. Larger molecules have stronger attractions and may be liquid at room temperature, or solids with low melting points.

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Giant covalent substances

Atoms of some elements form several covalent bonds. These atoms can join together in giant covalent structures.

Every atom in the structure is joined to several other atoms by strong covalent bonds. It takes an enormous amount of energy to break down the lattice and so these substances have high melting points.

Diamond (a form of carbon) and silica (silicon dioxide) have regular three-dimensional giant structures and so they are hard and transparent. They are both forms of carbon but have different properties.

Graphite is a form of carbon in which the atoms join in flat two-dimesional layers. There are only weak forces between the layers and so they slide over each other, making graphite slippery and grey.

Another name for giant covalent structures is macromolecules.

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Giant metallic structures

Metal atoms are arranged in layers. The layers of atoms slide over eachother so they are pliable and moveable. They can move into a new position without breaking apart, so the metal bends or stretches into a new shape. Metals are useful for making wires, rods or sheet materials.

Delocalised electrons hold the atoms in place. The delocalised electrons are free to move throughout the metal structure. This means that they can flow as an electric current without the metal changing. They can carry heat energy and so metals are also very good conductors of heat. The uses of metals depend on their ability to conduct heat and electricity.

1) Why can we change the shape of metals?

2) How do metals conduct electricity? [Higher only]

3) What allows metals to conduct heat? [Higher only]

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Nanoscience and nanotechnology

When atoms are arranged into very small particles they behave differently to ordinary materials made of the same atoms. A nanometre is one billionth of a metre. Nanoparticles are a few nanometres in size.

Nanoparticles contain a few hundred atoms arranged in a particular way. Their structures and very small sizes give them new properties that can make them very useful materials. Nanoparticles also have very large surface areas, exposing more atoms at their surface than normal materials. Electrons can move through them more easily than ordinary materials. They can be very sensitive to light, pH, electricity, and magnetism.

Nanotechnology uses nanoparticles as very selective sensors, highly efficient catalysts, new coatings and construction materials with special properties, and to make drugs more effective.

1) Why do nanoparticles have different properties to ordinary materials?

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Mass numbers

Protons and neutrons have equal masses. The relative masses of them are both one unit. The mass of an electron is very small in comparison, and so the mass of an atom is almost entirely made up of protons and neutrons. The total number of protons and neutrons is called mass number.

Atoms of the same element have the same atomic number. The number of protons and electrons in an atom must always be the same, but there can be different numbers of neutrons. Atoms of the same element with different numbers of neutrons (atomic number) are called isotopes. We can work out the atomic number this way:

Mass number (protons + neutrons), at the top, minus its atomic number, at the bottom.

Isotopes are atoms of the same element and have the same chemical properties they have different physical properties because the different numbers of neutrons gives them different masses. Some are unstable/radioactive.

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Masses of atoms and moles

Atoms are much too small to weigh so we use relative atomic masses. These are often shown in periodic tables.

We use an atom of 12C6 as a standard atom and compare the masses of all other atoms with this. The relative atomic mass of an element (Ar) is an average value that depends on the isotopes the element contains.

However, when rounded to a whole number, it is often the same as the mass number of the main isotope of the element.

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Calculating relative formula mass (Mr)

Adding up the relative atomic masses- it's really simple- gives us the relative formula mass.

Worked example:

Calculate the Mr of CaCl2

Solution:

Ar of Ca = 40, Ar of Cl = 35.5

So, 40 + (35.5 x 2) = 111

So the answer is 111.

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Calculating the mass of moles

Using moles allows us to calculate and weigh out in grams masses of substances with the same number of particles. One mole of sodium atoms contains the same number of atoms as one mole of chlorine. For example:

Worked example:

What is the mass of one mole of NaOH?

Solution:

Ar of Na = 23, Ar of O = 16, Ar of H = 1

So, 23g + 16g + g + 1g = 40 g

So the answer is 40 grams, as you can see. Use the periodic table to look up masses, and when calculating relative formula masses, make sure all the atomic masses are added up correctly.

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Making as much as we want

The yield of a chemical process compares how much you actually make with the maximum amount possible. When you actually carry out chemical reactions it is not possible to collect the amounts calculated from the chemical equations. Reactions may not go to completion and some product may be lost in the process.

The yield is often calculated as a percentage:

percentage yield = (amount of product collected divided by maximum amount of product possible) x 100

And, atom economy measures how much of the starting materials become useful products.

Atom economy = (relative formula mass of useful product divided by relative formula mass of all products) x 100

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Reversible reactions

If the products in a chem. reaction can react to produce the reactants the reaction can go in both directions. This is called a reversible reaction.

When there are no products, the reaction can only go forward, but as products are created, the reverse reaction may occur. In a closed system, nothing can escape and the rates of both forward and backward reactions will become equal. When this happens, the system reaches equilibrium.

If the conditions of the system are altered, the amount of reactant and product may change. Increased concentration of a substance will increase the rate of reaction away from that substance, as will a change in surface area and pressure. If the system is open and products are able to escape, the forward reaction continues only to completion.

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