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Chemistry

GCSE Chemistry - Fundamental Ideas
by

Bethany Pill

on 27 April 2014

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Transcript of Chemistry

Forming bonds..
Atoms, elements and compounds..
Chemical equations..
Atomic structure..
The arrangements of electrons in atoms..
Fundamental ideas
All substances are made of atoms. There are 100 different types of atom found naturally on Earth. These combine in a huge variety of ways to give us all those different substances.
Some substances are made of only one type of atom. We call these elements. Since there are around 100 different types of atom, there are roughly 100 elements.
Elements can have very different properties. Elements such as silver, copper and gold are shiny solids.
Other elements such as oxygen, nitrogen and chlorine are gases.
The name we use for an element depends on the language being spoken. For example, sulfur is called s
chwefel
in German and
azufre
in Spanish.
However, a lot of scientific work is international, so it is important that we have symbols for elements that everyone can understand.
These are shown in the periodic table.
The symbols in the periodic table represent atoms, for example O represents the atom of oxygen; Zn represents Zinc.
The elements in the table are arranged in columns called groups. Each group contains elements with similar chemical properties.
The line drawn in bold is the dividing line between metals and non-metals.
Most of the substances we come across are not pure elements. They are made up of different types of atom joined together, these are called compounds.
Chemical bonds hold the atoms tightly together in compounds, some compounds are made from just two types of atom, whilst other compounds consist of more different types.
A grouping of two or more atoms bonded together is called a molecule. Chemicals bonds hold hydrogen and oxygen together in a water molecule.
This is an example of a molecule.
In the middle of an atom there is a very small nucleus containing two types of particles, protons and neutrons. A third type particle orbits the nucleus, we call these electrons.
Protons have a positive charge, neutrons have no charge, so the nucleus has an overall positive charge.
Because any atom contains equal numbers of protons and electrons, the positive and negative charges cancel out, so any atom has no overall charge. Its charge is zero.
And the electrons that orbit the nucleus are negatively charged.
Therefore argon must have 22 neutrons.
As 18 + 22 = 40.
An atom has the same number of electrons orbiting its nucleus as it has protons in its nucleus. For example, a carbon atom has 6 protons, so we know it has 6 electrons.
REMEMBER
P
rotons are
P
ositive;
Neutr
ons are
Neutr
al;
so Electrons must be Negative.
The atoms of a particular element has the same number of protons. We call he number of protons in each atom of an element its atomic number.
The elements in the periodic table are arranged in order of their atomic mass. If you are told that the atomic number of an element is 1, it will be the 1st element listed.
The number of protons plus neutrons in the nucleus of an atom is called its mass number. For example, if an atom has 4 protons and 5 neutrons, its mass number will be 9.
Its atomic number is 18, so it has 18 protons. Remember that atoms have an equal number of protons and electrons. So argon also has 18 electrons.
Given the atomic number and mass number we can work out how many protons, electrons and neutrons are in an atom.
Let's do one together!
The mass number is 40, so we know that;

(The number of Protons)
+
(The number of Neutrons)
=
40
We now know that Argon has 18 protons, 18 electrons and 22 neutrons!
Forming bonds..
Atoms, elements and compounds..
Chemical equations..
Atomic structure..
The arrangements of electrons in atoms..
.
GCSE Chemistry unit 1
Bethany Pill - miss telfer
All substances are made of atoms. There are 100 different types of atom found naturally on Earth. These combine in a huge variety of ways to give us all those different substances.
Some substances are made of only one type of atom. We call these elements. Since there are around 100 different types of atom, there are roughly 100 elements.
Elements can have very different properties. Elements such as silver, copper and gold are shiny solids.
Other elements such as oxygen, nitrogen and chlorine are gases.
The name we use for an element depends on the language being spoken. For example, sulfur is called s
chwefel
in German and
azufre
in Spanish.
However, a lot of scientific work is international, so it is important that we have symbols for elements that everyone can understand.
These are shown in the periodic table.
The symbols in the periodic table represent atoms, for example O represents the atom of oxygen; Zn represents Zinc.
The elements in the table are arranged in columns called groups. Each group contains elements with similar chemical properties.
The line drawn in bold is the dividing line between metals and non-metals.
Most of the substances we come across are not pure elements. They are made up of different types of atom joined together, these are called compounds.
Chemical bonds hold the atoms tightly together in compounds, some compounds are made from just two types of atom, whilst other compounds consist of more different types.
A grouping of two or more atoms bonded together is called a molecule. Chemicals bonds hold hydrogen and oxygen together in a water molecule.
This is an example of a molecule.
In the middle of an atom there is a very small nucleus containing two types of particles, protons and neutrons. A third type particle orbits the nucleus, we call these electrons.
Protons have a positive charge, neutrons have no charge, so the nucleus has an overall positive charge.
Because any atom contains equal numbers of protons and electrons, the positive and negative charges cancel out, so any atom has no overall charge. Its charge is zero.
And the electrons that orbit the nucleus are negatively charged.
Therefore argon must have 22 neutrons.
As 18 + 22 = 40.
An atom has the same number of electrons orbiting its nucleus as it has protons in its nucleus. For example, a carbon atom has 6 protons, so we know it has 6 electrons.
REMEMBER
P
rotons are
P
ositive;
Neutr
ons are
Neutr
al;
so Electrons must be Negative.
The atoms of a particular element has the same number of protons. We call he number of protons in each atom of an element its atomic number.
The elements in the periodic table are arranged in order of their atomic mass. If you are told that the atomic number of an element is 1, it will be the 1st element listed.
The number of protons plus neutrons in the nucleus of an atom is called its mass number. For example, if an atom has 4 protons and 5 neutrons, its mass number will be 9.
Its atomic number is 18, so it has 18 protons. Remember that atoms have an equal number of protons and electrons. So argon also has 18 electrons.
Given the atomic number and mass number we can work out how many protons, electrons and neutrons are in an atom.
Let's do one together!
The mass number is 40, so we know that;

(The number of Protons)
+
(The number of Neutrons)
=
40
We now know that Argon has 18 protons, 18 electrons and 22 neutrons!
Now you try!
An energy level/shell can only hold a certain number of electrons.

The first, and lowest, energy level hold 2 electrons.
We can draw diagrams to show the arrangement of electrons in an atom. A carbon atom has 6 protons, which means it has 6 electrons.
Each shell represents a different energy level. The lowest energy level is shown by the shell nearest to the nucleus. The electrons in an atom occupy the lowest available energy level.
One model of the atom which we use has electrons arranged around the nucleus in shells.
This is how we represent an atom of carbon.
The second can hold up to 8 electrons. Once there are 8 electrons in the third energy level, the fourth begins to fill up, and so on.
To save drawing atoms all the time, we can simply write down the numbers of electrons in each energy level, this is called electronic structure.
For example, the carbon atom...




Has an electronic structure of 2,4.
The elements in any one of the main groups of the periodic table, will have the same number of electrons in their highest energy level.
These electrons are often called the outer electrons as the are in the outermost shell. Therefore, all the elements in Group 1 have one electron in their highest energy level.
The chemical properties of an element depend on how many electrons it has. The way an element reacts is determined by the number of electrons in its outermost shell.
So as the elements in a particular group all have the same number of electrons in their highest energy level/outermost shell, they all react in a similar way.
The elements in group 0/8 of the periodic table are called the noble gases because they are completely unreactive. Their atoms have a very stable arrangement of electrons.
They all have 8 electrons in their outermost shell, except for helium, which has 2 electrons. We say that these elements have a
full
outer shell to avoid confusion.
Sometimes atoms react together by transferring electrons to form chemical bonds. This happens when metals react with non-metals.
If the reacting atoms are all non-metals, then the atoms share electrons to form chemical bonds.
When a metal bonds with a non-metal, the metal atom gives one or more electrons to the non-metal atom. Both atoms become charged particles called ions.
Metal atoms form positively charged
ions (+)

Non-metal atoms form negatively charged
ions (-)
Opposite charges attract each other, there are strong attractions between positive and negative ions in a compound of a metal and a non-metal.
These strong forces of attraction are the chemical bonds that form. They are called ionic bonds.
To see how ions are formed we can look at an example. Lithium metal will react with the non-metal fluorine. They make the compound lithium fluoride.
Lithium atoms have 3 electrons, each negatively charged. As all atoms are neutral, we know it also has 3 positive protons in its nucleus; the charges are balanced.
When lithium reacts with fluorine it loses 1 electron. This leaves it with only 2 electrons. However, if there are 3 protons in the nucleus, the lithium ion is left with a 1+ charge.
The electron lost from lithium is accepted by a fluorine atom. A fluorine atom has 9 electrons and protons. With the extra electron from lithium, the fluorine ion now has a -1 charge.
In compounds between metal and non-metal, the charges on the ions always cancel each other out. This means that their compounds have no overall charge.
Each pair of shared electrons forms a chemical bond between the atoms. These are called covalent bonds. Molecules are formed instead of ions.
Non-metal atoms bond to each other in a different way. The outermost shells of their atoms overlap and they share electrons.
So, the formula of lithium fluoride is written as...

LiF
Methane
In covalent molecules we can just count the number of each type of atom in a molecule to get its formula.
The chemical formula of an ionic compound tells us the ratio of each type of ion in the compound. We use a ratio because when ions bond they form structures made of millions of ions.
The ratio depends on the charge of each ion. The charges
must
cancel each other out, to keep it neutral.
Chemical equations show the reactants (the substances we start with) and the products (the new substances made) of a reaction.
In this presentation, you will be taught everything you need to know about Fundamental Ideas. I suggest you follow the presentation, as initially the map is rather jumbled. Other good revision resources include;

- www.bbc.co.uk/schools/gcsebitesize/science/aqa

- http://www.aqa.org.uk/subjects/science/gcse/chemistry-4402/past-papers-and-mark-schemes

- https://www.youtube.com/playlist?list=PLA91D37E416C975B2

In chemical reactions the atoms get rearranged. You can think of them as 'swapping partners'.
Using symbol equations helps us to see how much of each substance is reacting. Representing reactions this way is better than using word equations for three reasons:
This also means that:
The total mass of the products formed in a reaction is equal to the total mass of reactants.
This equation is balanced - there is the same number of each type of atom on both sides of the equation. This is important because atoms cannot be created or destroyed in a chemical reaction.
For example, calcium carbonate decomposes upon heating, we can show the reaction using a symbol equation like this:
3) Word equations can get very complicated when lots of chemicals are involved.
2) Word equations do not tell us how much of each substance is involved in the reaction; symbol equations do.
1) Word equations are only useful if everyone reads them speaks the same language.
How to balance an equation:
We can check if an equation is balanced by counting the number of each type of atom on either side of the equation. If the numbers are equal, then the equation is balanced.
Full transcript