Periodic table

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Chemistry

In the beginning a long time ago before cars and mobile phones, students spent all day learning about the matter that made up the earth. They had long lessons, wore funny clothes and only had religious holidays.

Alchemists people who made up weird potions to live forever or make gold.

At the same time a man named Democritus (A teacher) suggested everything in the world was made up of tiny particles that could not be seen. He called them atoms

2000 years later Democritus idea was correct.

Scientists today don’t rely on weird potions and secret spaces with cauldrons instead they rely on investigations carried out with careful observations.  

Today there are 112 known elements

Building Blocks

Atoms are joined together in groups called molecules

Atoms can join together to form elements or large molecules of many different shapes

Polymer scientific word to describe large molecules e.g. Rubber, Cotton

Questions

  1. What is a molecule? Name the 2 compounds that are made up of molecules?
    Groups of atoms joined together, Carbon dioxide (CO
    2) and Water (H20)
  2.  Are all compounds made up of molecules? explain
    No some can exist in their natural form on their own
  3. Name 4 elements that occur naturally as molecules
    Carbon Dioxide (CO
    2) and Water (H20) Methane and ozone
  4. What are polymers? Give three examples
    Large Molecules eg plastics. glass and paint

Atoms

The particles in the centre of the atom are called protons (+) and neutrons (neutral)

Together they form the nucleus

Moving rapidly around the nucleus are tiny little electrons (-) arranged in shells

Number of protons = Number of electrons

The number of protons in an atom is called its atomic number

The atomic number tells us what element it is

Naming Elements as individuals

O = Oxygen, C = Carbon, N = Nitrogen

When there is more than one element

Cl = Chlorine
Ca = Calcium

Cu = Copper

If there is an element that has a symbol that doesn’t match

Na = Sodium

Pb = Lead

Ag = Silver

Periodic table

As early scientist discovered more and more elements they had to think of a way to organise them. Each element was given a name and a number

Dmitiri Meneleev a Russian scientist organised a system called the periodic table

Scientist today used a modified version of this table      

The group number is the number of electrons in the outside shell

The period number refers to the outermost shell containing electrons

The atomic number is the Protons and the neutrons in an atom

Inert Gases gasses which don’t change or mix

Noble gasses which cannot be mixed with other gasses

First 20 elements

  1. Hydrogen H
  2. Helium He
  3. Lithium LI
  4. Beryllium Be
  5. Boron B
  6. Carbon C
  7. Nitrogen N
  8. Oxygen O
  9. Fluorine F
  10. Neon Ne
  11. Sodium NA
  12. Magnesium Mg
  13. Aluminium Al
  14. Silicon Si
  15. Phosphorus P
  16. Sulphur S
  17. Chlorine Cl
  18. Argon Ar
  19. Potassium K
  20. Calcium Ca

The Nucleus

The atomic number is the number of protons present in each atom. Atoms with same atomic number have identical chemical properties. Mass number is the sum of the number of protons and neutrons in the atom. The number of neutrons in an atom can be calculated by subtracting the atomic number from the mass number.

A = the mass number

Z = the atomic number

E = symbol of the element

E.g. find the number of neutrons calcium has.

Atomic number - 40.08

Mass number – 20

Number of neutrons = 40.08 – 20 = 20.08

It is difficult to measure and compare the mass of atoms because of their extremely small size. So we try to compare the equal numbers of atoms rather than trying to measure the mass of a single atom. However a problem arises when not all atoms of an element are identical. Some elements may contain atoms with slightly different masses called isotopes. This is worked out as an average rather than a whole number.

Most elements exist as two or more isotopes. Isotopes all have the same chemical properties, but slightly different masses. When the protons and neutrons are held tightly together the nuclei are stable when they are not they are unstable.

Unstable isotopes disintegrate to form other substances which are radioactive. Some atoms are naturally radioactive; others have to be made active and are called artificial.

The energy emitted by radioactive substances is called nuclear radiation.

3 types of nuclear radiation:

Alpha – alpha particles are helium nuclei that contain 2 protons and 2 neutrons. Positively charged and cannot travel through materials easily. E.g. human skin and paper. Little hazard

Beta: beta particles are the same size and mass as electrons. They can have a positive or negative charge. Can penetrate human skin and damage cells. Do not penetrate thin layers of plastic, wood or aluminium

Gamma rays: not particles but bursts of energy which is released when alpha or beta particles are emitted. They travel at the speed of light and highly penetrate. They cause serious and permanent damage to the human body.

Shell Diagrams

  • When atoms come in contact with each other, they often join together to form molecules. Other atoms join together to from giant crystals that contain billions of atoms.
  • It is the electrons in each atom that account for the chemical behaviour of all matter, because they form the outermost part of the atom.
  • An electron shell diagram is a simplified model of an atom. In these diagrams the nucleus of the atom, containing protons and neutrons is drawn in the middle. Electrons are arranged in a series of energy levels around the nucleus in shells.
  1. First shell holds only 2 electrons (K)
  2. Second shell holds up to 8 electrons (L)
  3. Third shell holds up to 18 electrons (M)
  4. Fourth shell holds up to 32 electrons (N)
  • The Outside shell can only ever hold 8 electrons
  • The following equation is used to calculate the maximum number of electrons in each shell. The xth shell holds a maximum of 2x2 

  • The number of electrons in a shell corresponds to the number of period numbers on the periodic table. When Mendeleev and Meyer grouped the elements they had no idea that electrons existed. E.g. Group number 1 electrons contain 1 electron in the outer shell.

  • If an atom is given enough energy e.g. heat, the electrons will be able to move from one shell to another. Electrons move from a place of either higher energy to a place of lower energy and vice versa.

  • Knowledge of the outer shell helps us understand how compounds form. E.g. Sodium chloride (salt) forms when Sodium gives and electron to Chlorine, therefore giving chlorine a full outer shell. This then forms an ion.
  • Ions form when an atom looses or gains an electron to get a full outer shell. Atoms without 8 electrons in their outer shell are unstable whereas atoms with a full outer shell are stable.
  • Positive ions are called Cations and negative ions are called anions
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Drawing and Writing Configurations

  • Ionic compounds form when atoms gain or lose electrons. Atoms can gain stability by sharing electrons. When two or more atoms share electrons a molecule is formed.
  • A chemical bond formed by the sharing of electrons is called a covalent bond. Non-metals share electrons to form covalent bonds. E.g. oxygen works best when two oxygen atoms share.
  • Electron dot diagrams are used to show how electrons are arranged. The atoms are drawn showing the outer shell only.
  • Most covalent compounds have the following properties.
  • They exist as gases, liquids or ...

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