Topic 1 - Atomic Structure and the Periodic Table

Topic 1 - Atomic Structure and the Periodic Table

1. Atoms

   • Atoms have radius of 0.1 NANOMETERS.
   • The NUCLEUS is in the MIDDLE of the atom and it contains PROTONS and NEUTRONS. The nuclus has a radius of 1/10000 of the atom. It has a POSITIVE CHARGE because of the protons and almost the WHOLE MASS of the atom is CONCENTRATED in the nucleus.
   • The ELECTRONS move AROUND the nucleus in electron SHELLS. They're NEGATIVELY CHARGED but cover A LOT OF SPACE. The volume of their orbits determines the size of the atom and electrons have virtually no mass.
   • Atomic number = number of protons Mass number = protons + neutrons protons = neutrons

2. Elements, Compounds and Mixtures

   • An ELEMENT is a substance made up of atoms that all have the SAME number of PROTONS in their nucleus. ISOTOPES are different forms of the same element, which have the SAME NUMBER of PROTONS but a DIFFERENT NUMBER of NEUTRONS. This means isotopes have the SAME ATOMIC NUMBER but DIFFERENT MASS NUMBERS.
   • Relative Atomic Mass (Ar) = sum of (isotope amount x isotope mass number)/ amount of all of the isotopes
   • When ELEMETS REACT, atoms COMBINE with other atoms to form COMPOUNDS. Compounds are a substance made up of TWO OR MORE elements.
   • A MIXTURE is a combination of ELEMENTS and COMPOUNDS with NO CHEMICAL BONDS.

3. The History of The Atom

   • 19th Century - JOHN DALTON described atoms as SOLID SPHERES, and said that different spheres made up the different ELEMENTS.
   • 1897 - JJ THOMSON concluded from his experiments that atoms WEREN'T solid spheres. His measurements of CHARGE and MASS showed that an atom must contain even smaller, negatively charged particles - ELECTRONS. The new theory was known as the PLUM PUDDING MODEL as it shown the atom as a BALL of POSITIVE CHARGE with ELECTRONS stuck in it.
   • 1909 - ERNEST RUTHERFORD and his student ERNEST MARSDEN conducted the ALPHA PARTICLE SCATTERING EXPERIMENT. They fired charged alpha particles at an extremely thin sheet of gold. From the plum pudding model, they were EXPECTTING the particles to PASS STRAIGHT THROUGH the sheet or be SLIGHTLY DEFLECTED at most. This was because the positive charge of each atom was thought to be very SPREAD OUT through the atom. But, whilst most of the particles DID go STRAIGHT THROUGH the sheet, some were deflected MORE THAN EXPECTED , and a small number were DEFLECTED BACKWARRDS. Rutherford came up with an idea to explain this new evidence and used THEE NUCLEAR MODEL of the atom. In this, there's a tiny positively charged NUCLEUS at the centre, where most of the MASS is concentrated, A 'cloud' of negative electrons surrounds the nucleus so most of the atom is EMPTY SPACE. When alpha particles came near the CONCENTRATED, POSITIVE CHARGE of the nucleus, they were DEFLECTED.
   • Scientists realised that electrons in a 'cloud' around the nucleus of an atom would be attracted to the nucleus, causing the atom to COLLAPSE. NIELS BOHR'S nuclear model of the atom suggested that all the electrons were contained in shells. Bohr proposed that electrons ORBIT the nucleus in FIXED SHELLS and aren't anywhere in between. Each shell is a fixed distance away from the nucleus.
   • Further experiments by Rutherford and other showed that the nucleus can be DIVIDED into smaller particles, which each have the SAME CHARGE as a HYDROGEN NUCLEUS. These particles were named PROTONS. About 20 years after scientists had accepted that atoms have nuclei, JAMES CHADWICK carried out an experiment which provided evidence for NEUTRAL PARTICLES in the nucleus which are now called NEUTRONS. The discovery of neutrons resulted in a model of the atom which was pretty CLOSE to the MODERN DAY accepted version, known as the NUCLEAR MODEL.

4. Electronic Structure

   • Electrons always occupy SHELLS. The LOWEST energy levels are ALWAYS FILLED FIRST and are closest to the nucleus. The 1st shell can only hold 2 electrons and the rest can hold 8. In most atoms, the OUTER SHELL is NOT FULL and this makes the atom want to REACT to fill it.

5. Development of the Periodic Table

   • In the EARLY 1800S elements were arranged by their PHYSICAL and CHEMICAL PROPRETIES and their RELATIVE AOTMIC MASS. Early periodic tables were not complete and some elements were placed in the wrong group. This is because ELEMENTS were placed in the ORDER OF RELATIVE ATOMIC MASSES and did not take into account their other properties.
   • 1896 - DMITRI MENDELEEV overcame some of the problems of the early periodic tables by taking 50 known elements and arranging them into his Table of Elements. MENDELEEV put the elements but MAINLY in order of ATOMIC MASSES but did switch that order if the properties mean it should be changed. GAPS were left in the table to make sure that elements with similar properties stayed in the same groups. Some of these GAPS indicated the existence of undiscovered elements and allowed Mendeleev to predict what their properties might be. When they were found and they FITTED THE PATTERN it helped confirm Mendeleev's ideas.

6. The Modern Periodic Table

   • In the periodic table, the elements are laid out in order of INCREASING ATOMIC NUMBER. Elements with SIMILAR PROPERTIES form COLUMNS . These VERTICAL COLUMNS are called GROUPS. The GROUP NUMBER tells you how many ELECTRONS are in the OUTER SHELL. The ROWS are called PERIODS and each period shows how many SHELLS the element has.

7. Metals and Non-Metals

   • Metals are elements which can FORM POSITIVE IONS when they react. MOST ELEMENTS in the periodic table are METALS
   • All metals have METALLIC BONDING which causes them to have SIMILAR basic physical properties. They're STRONG, but can be BENT or HAMMERED into different shapes, they're great at CONDUCTING HEAT and ELECTRICITY. They also have HIGH BOILING AND MELTING POINTS. As non-metals DON'T have metallic bonding, they are DULL, BRITTLE and AREN'T ALWAYS SOLID AT ROOM TEMPERATURE. They tend to have LOWER DENSITY and DON'T CONDUCT ELECTRICITY.

8. Group 1 and Group 7 Elements

   • Group 1 elements are known as the ALKALI METALS. They all have ONE ELECTRON in their outer shell which makes them VERY REACTIVE. As you go DOWN group 1, The REACTIVITY INCREASES, because the outer electron is MORE EASILY LOST as the attraction between the nucleus and electron decreases as it is FURTHER AWAY from the nucleus. Also, they have LOWER MELTING AND BOILING POINTS and a HIGHER RELATIVE ATOMIC MASS. Group 1 metals are much MORE REACTIVE than transition metals and react mote vigorously wit WATER, OXYGEN or GROUP 7 ELEMENTS. They're also much less DENSE, STRONG AND HARD.

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