Crash Course Chemistry
Revision for GCSE Chemistry (pretty much everything)
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| 🇬🇧 | 🇬🇧 |
| What are atoms? | It is the smallest particle that can exist that still retains properties of its element |
| What are compounds? | Substance that contain atoms of two or more different atoms that are chemically joined together |
| What is an isotope? | It is atoms of the same element with different number of neutrons but same number of protons (and electrons) |
| Define relative atomic mass? | The average weighted mass of the atoms of an element compared to 1/12th of the mass of C-12 atom |
| What is the overall charge of an atom? | It does not have a charge as the number of electrons is equal to the number of protons |
| Atomic number | It is the number of protons in the central nucleus |
| Atomic mass | Number of protons and neutrons in the central nucleus |
| In an atom the number of electrons is equal to what? | Is equal to the number of protons |
| What is a mixture? | It is made up of two or more different substances that are not chemically joined |
| What is the difference between a mixture and a compound? | - Mixtures are separated by physical means, compounds can only be separated by chemical means - Substances in mixtures are not joined chemically (only joined physically) whereas in compounds, the elements are joined chemically - In a mixture, each substance retains its own properties, in a compound, the compound has different properties from the elements it contains - Mixtures do not have a set ratio of each substance whereas the ratio of the atoms of each element in a compound does not change |
| What are the physical processes used to separate mixtures? | - Distillation - Fractional distillation - Filtration - Evaporation - chromatography |
| Distillation | - Used to separate solute and solvent - Only the solvent evaporates, it enters the condenser and cools down |
| Fractional distillation | - Used to separate different liquids with different boiling points - The liquid with the lowest boiling point evaporates first and it enters the condenser and it cools down and is collected |
| Filtration | Used to separate liquid and insoluble solid The solid is collected in the filter paper and the liquid is collected in the conical flask |
| Evaporation | Used to separate liquid from soluble solid The water evaporates and is removed leaving the solid crystals behind |
| Chromatography | Used to separate dissolved substances such as dyes and food colouring |
| Rf value | Distance travelled by substance/ distance travelled by solvent |
| Plum pudding model vs nuclear model | The negatively charged electrons are found on shells (and they orbit the central nucleus) in the nuclear model whereas in the plum pudding model the electrons are found randomly distributed in the positively charged “soup” The nuclear model features neutrons whereas the plum pudding model does not (why?) Mass is concentrated in the centre of the atom in nuclear model whereas mas is evenly distributed in the plum pudding model nuclear model the atom mainly empty space whereas plum pudding model is a ‘solid’ mass nuclear model positive charge occupies only a small part of the atom whereas in the plum pudding model positive charge spread throughout the atom |
| Describe how the atomic model has changed over time due to new experimental evidence? | Dalton’s model… - Atoms cannot be broken down into anything simpler - Atoms of the same element are identical to each other - Atoms of different elements are different from each other - During chemical reactions atoms rearrange to give different substances |
| Thomson’s model… | - Thomson discovered the electron - Atoms are neutral overall so he thought that electrons had to be evenly distributed within a positively charged “soup” |
| Geiger-Marsden experiment | - They aimed beams of positively charged particles at thin gold foil - These particles should have passed straight through however many bounced back (were deflected) as the central nucleus is positive and repelled the positive alpha particle beams - As most went through, most of the atom is empty space - Atoms have a central positively charged nucleus with most of the mass as only a few were deflected - It proved that most of the mass is concentrated in a central nucleus and it proved that the nucleus is positive |
| Bohr’s model: | - He discovered that electrons orbit on shells which are at set distances from the central nucleus - He heated the atoms, and when the electrons gained energy (it is excited), they moved up to a higher energy level and when the electrons move back down, energy is released |
| Chadwick’s model: | - Chadwick discovered neutrons - It was hard to discover as it had no charge - Neutrons have to be present in the central nucleus to account for the missing mass and also to stabilise the atom |
| Period number (electronic) | Number or number of circles |
| Group number (electronic) | Number of electrons in outermost shell |
| Atomic number (electronic) | Total number of electrons in all shells |
| How are elements in the periodic table arranged? | In ascending atomic number |
| How are elements with similar properties placed in the periodic table? | Elements with the same number of outer shell electrons are placed in a group |
| Why do elements in the same group have similar properties? | They have the same number of valence electrons and the electronic configuration determines the chemical properties (what determines the physical properties?) |
| Early attempts to classify elements: | - First it was classified by atomic weight, however similar elements were not grouped together and there were errors in the table |
| Dobereiner’s triads | He grouped similar elements into group of three The middle element has an atomic weight that is the average of the other two elements |
| Newland’s law of octaves | He arranged the elements into increasing atomic weight He noticed that every eighth elements have similar chemical properties and he grouped elements into groups of eight (triads) However, the pattern eventually broke down |
| Mendeleev’s table | He arranged the elements in order of increasing atomic weight. he left gaps where no element fitted the repeating pattern. These gaps were for undiscovered elements. He mendeLEAVES the gaps…. |
| Henry Moseley | He discovered the isotopes of elements. This discovery established that “the properties of the elements varied periodically with atomic number,” not atomic weight |
| Metals vs nonmetal | Metals tend to lose electrons to form positive ions whereas non-metals tend to gain electrons to form negative ions |
| Noble gases | ● They are non-metal monoatomic gases ● They have low boiling points ● Boiling point increases down the group as the atoms gets larger down the group and the intermolecular forces between the atoms get stronger so more energy is needed to overcome the forces ● Density increases down the group (density is low) ● They are inert, very unreactive and inflammable |
| Why are noble gases unreactive? | They have a full outer shell- (they have eight electrons in the outer shell) so they are already very stable, it is hard to lose or gain an electron from a noble gases |
| Group one alkali metals | - Has one outer electron - Very reactive so must be stored away from water and oxygen - They are called alkali metals as they dissolve in water to form hydroxides which are alkali - They are soft and shiny (can be cut with a knife) - Good conductors |