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Atomic structure and the periodic table (The history of the atom (At the…
Atomic structure and the periodic table
Atoms
Atoms have radius of 0.1 nanometers. Nucleus has radius of 1 x 10-14 (1/ 10000 of radius of atoms). Nucleus has a positive charge. Almost whole mass of atom is concentrated in nucleus.
Atomic number= protons (which is equal to electrons).
Mass number= protons and neutrons
Neutrons= atomic number- mass number
Elements
Element- a substance that only contains atoms with same number of protons.
Isotopes- different forms of the same element. Same number of protons but different neutrons. Have same atomic number but different mass number.
Relative atomic mass- used instead of mass numbers when referring to element as a whole.
Compounds
Compounds- substances formed from 2 or more elements; the atoms are in fixed proportions- held together by chemical bonds.
A compound which is formed from metal and non-metal consists of ions. A compound formed from non-metals consists of molecules.
Properties of compounds are different from properties of original elements.
Chemical equations
Chemical changes shown using chemical equations.
Mixtures and chromatography
No chemical bonds between different parts of a mixture. Parts of mixture can either be elements or compounds. Can be separated using physical methods; filtration, crystallisation, simple distillation, fractional distillation and chromatography.
Air- mixture of gases; N2, O2, CO2, Ar- can be separated out easily.
Properties of a mixture- mixture of the properties of separate parts; chemical properties unaffected by it being a part of a mixture.
The history of the atom
At the start of 19th century, John Dalton- atoms as solid spheres and said different spheres made up different elements.
1897- J J Thompson concluded atoms weren't solid spheres. Made new theory- plum pudding model (ball of positive charge with negative electrons stuck in it.
1909- Ernest Rutherford and student Ernest Marsden conducted alpha particle scattering experiment. Fired positively charged alpha particles at thin sheet of gold. From plum pudding model, expected particles to pass straight through or slightly deflect- went straight through but some deflected more than expected and small amount deflected backwards.
New nuclear model created- tiny positively charged nucleus in centre where most mass concentrated. A cloud of negative electrons surround nucleus so most of atom is empty space. If alpha particles near concentrated positive charge of nucleus- deflected. If fired directly at nucleus- deflected backwards. Otherwise passed through empty space.
Niels Bohr- electrons contained in shells. Elements orbit nucleus in fixed shelld and aren't in between. Each shell in fixed distance from nucleus- supported by experiment and explained observations.
Rutherford- nucleus can be divided into smaller particles- same charge as H2 nucleus (protons).
James Chadwick- neutrons- this resulted in model of atom that is pretty close to modern one- nuclear model.
Development of periodic table
1869- Dmitri Mendeleev overcame problems of early periodic able by taking 50 known elements and arranging them into table of elements- had gaps. Elements put in order of atomic mass but did switch order if properties meant it should be changed.
Gaps left in table to ensure elements with similar properties stayed in same groups. These gaps indicated existence of undiscovered elements; allowed Mendeleev to predict their properties. When found and fitted pattern- confirmed Mendeleev's ideas.
Discovery of isotopes in early 20th century confirmed that Mendeleev was correct not to place elements in strict order of atomic mass but take account of properties. Isotopes of same element have different atomic masses but same chemical properties so occupy same position on periodic table.
The modern periodic table
Elements laid out in order of increasing atomic (proton) number- means in repeating patterns in properties of elements. Elements with similar properties form columns (groups)- electrons in outer shell.
Rows (periods)- each new period represents another full shell of electrons.
Metals and non-metals
Metals- elements that form +ions when they react- bottom and left of periodic table.
Non-metals- don't generally +ions when react.
Metals- strong but malleable. Good conductors of electricity and heat. High mp and bp.
Non-metals- brittle and aren't always solid at room temperature. Don't usually conduct electricity and how low density.
Group 1 elements
Alkali metals. 1 electron in outer shell- very reactive. Soft and have low density.
As you go down: reactivity increases- electron more easily lost as attraction between nucleus and electron decreases as distance increases. Lower bp and mp. Higher relative atomic mass.
They form 1+ ions. React to from ionic compounds- white solids that dissolve in water to form colourless solutions.
React vigorously in water. Li, S, Na float and move around surface, fizzing furiously.Produce H2. As you go down energy increases and can ignite H2.
Also form hydroxides that dissolve in H2O to give alkaline solutions.
React vigorously when heated in chlorine gas to form white salts- metal chlorides.
React with O2 to form metal oxides----- Li- lithium oxide, S- sodium oxide and sodium peroxide, Na- potassium peroxide and potassium superoxide.
Group 7 elements
Fl - reactive, poisonous yellow gas. Cl- reactive, poisonous dense green gas. Br- dense, poisonous, red brown volatile liquid. I- dark grey crystalline solid or purple vapour.
All exist as molecules- pairs of atoms. As you go down: less reactive- harde to gain extra electron as outer shell further from nucleus. Higher bp and mp. Higher relative atomic masses. They share electrons- covalent bonding- with non-metals for full outer shell. They form simple molecular structures.
Form 1- ions (hallides) when bond with metals- ionic structures. Displacement reaction can occur between more reactive halogen and salt of less reactive one.
Group 0 elements
Noble gases- full stable outer shell- unreactive. Exist as monatomic gases- single atoms not bonded. Colourless gas at room temperature, non-flammable.
As you go down: bp increases as increase in electrons in each atom- greater intermolecular forces.