Atoms and Elements

Atoms and Elements

Atoms have a small nucleus surrounded by electrons
1. The nucleus
  1. In the middle of an atom
  2. Contains protons and neutrons
    1. Protons are positively charged
    2. Neutrons have no charge (neutral)
  3. The nucleus has an overall positive charge
  4. Its tiny compared to the rest of the atom
2. The electrons
  1. Move around the nucleus
  2. They are negatively charged
  3. There tiny but cover a lot of space
  4. They occupy shells around the nucleus
Number of protons equals number of electrons
1. Atoms have no charge overall. They are neutral
2. The charge on the electrons is the same size as the charge on the protons but opposite
3. This means the number of protons always equals the number of electrons in an atom
4. If some electrons are added or removed the atom becomes charged and is then an ion
Elements consist of one type of atom only
1. Atoms can have different numbers of protons, neutrons and electrons. Its the number of protons that decides what type of atom it is
2. If a substance only contains one type of atom its called an element
3. There are about 100 elements
The periodic table
1. Atoms can be represented by symbols
  1. Atoms of each element can be represented by one or two letter symbol
2. The periodic table puts elements with similar properties together
  1. 1. The periodic table is laid out so that elements with similar properties form colums
    1. 2. These vertical columns are called groups
      1. 3. All of the elements in a group have the same number of electrons in their outer shell
        4. This is why elements in the same group have similar properties.
        5. For example the group 1 elements are Li, Na, K,Rb, Cs and Fr they are all metals and react the same way
        6. The elements in the final column (Group 0) are the noble gases. They have eight electrons in their outer shell apart from helium. This means there all stable and unreactive
Electron shells
1. 1. Electrons always occupy shells (sometimes called energy levels)
  1. 2. The lowest energy levels are always filled first - the ones closest to the nucleus
    1. 3. Only a certain amount of electrons are allowed in each shell: 1st shell - 2 2nd shell - 8 3rd shell - 8
      1. 4. Atoms are much happier when they have full electron shells
        5. In most atoms the outer shell is not full and this makes the atom want to react to fill it
2. Follow the rules to work out electronic structures
  1. 1. The periodic table tells us nitrogen has seven proton so it must have seven electrons
    1. 2. The first shell can only take 2 electrons and the seconed shell can take 8 so the electronic structure of nitrogen must be 2,5
Compounds
1. Atoms join together to make compounds
  1. 1. When different elements react atoms form chemical bonds with other atoms to form compounds. Its usually difficult to separate the two original elements out again
    1. 2. Making bonds involves atoms giving way taking or sharing electrons. Only the electrons are involved its nothing to do with the nuclei
      1. 3. A compound which is formed from a metal and a non-metal consists of ions. The metal atoms lose electrons to form positive ions and the non-metal atoms gain electrons to form negative ions. The opposite charges of the ions means that they are strongly attracted to each other. This is called ionic bonding
        4. A compound formed from non-metals consists of molecules. Each atom shares an electron with another atom. This is called a covalent bond. Each atom has to make enough covalent bonds to fill up its outer shell
        5. The properties of a compound are totally different from the properties of the original element
        6. Compounds can be small molecules like water or great lattices like sodium chloride
Formulas and reaction
1. A formula shows what atoms are in a compound
  1. 1. CO2 is a compound formed from a chemical reaction between carbon and oxygen. It contains 1 carbon atom and 2 oxygen atoms
    1. 2. H2SO4 so each molecule contains 2 hydrogen atoms, 1 sulfur atom and 4 hydrogen atoms
      1. 3. There might be brackets in a formula. The little number outside the bracket applies to everything inside the brackets
2. Atoms arent lost or made in chemical reactions
  1. 1. During chemical reactions things dont appear out of nowhere and things dont just disappear
    1. 2. You still have the same atom at the end of a chemical reaction as you had at the start. They are just arranged in different ways
      1. 3. Balanced symbol equations show the atoms at the start and the atoms at the end and how they are arranged
        4. Because the atoms arent gained or lost the mass of the reactants equals the mass of the products
Balancing equations
1. 1. There must always be the same number of atoms of each element on both sides they cant just disappear
  1. 2. You balance the equation by putting numbers in front of the formulas where needed

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