1b Flashcards
History of the Atom - 1 - John Dalton
1800’s - John Dalton suggests that each of the elements are made from just one type of atom: tiny spheres that could not be divided
History of the Atom - 2 - JJ Thomson
1897 - J Thomson discovers the electron and proposes the plum pudding model: the atom is a ball of positive charge and the negative charge are embedded in it (like blueberries in a blueberry muffin)
History of the Atom - 3 - Ernest Rutherford vs thompson
1911 - Ernest Rutherford fired alpha particles at a piece of very thin gold foil(about 10,000 atoms thick).
Thomson’s plum pudding model predicted that…
All alpha particles… passed straight through
However what was observed was:
Most alpha particles… passed straight through
A very few alpha particles… were deflected by more than 90 degrees
Most of the atom is empty space
All the positive charge and most of the mass is concentrated in a small volume (the nucleus)
History of the Atom - 4 - Niels Bohr
1913 - Niels Bohr adapted the nuclear model by suggesting that electrons orbit the nucleus at specific distances The theoretical calculations of Bohr agreed with experimental observations.
History of the Atom - 5 - James Chadwick & other
1920-1932 - Later experiments led to the idea that the positive charge of any nucleus could be subdivided into a whole number of smaller particles, each particle having the same amount of positive charge. The name protons was given to these particles.
About 20 years after scientists had accepted that atoms have nuclei, in 1932 James Chadwick carried out an experiment which provided evidence for neutral particles in the nucleus. These became known as neutrons. The discovery of neutrons resulted in a model of the atom which was pretty close to the modern-day version
Atoms & its contents
Atoms are the tiny particles that everything is made up of- they have a radius of about 1 x 10-10 metres.
The nucleus is in the middle of the atom. It contains protons and neutrons. The nucleus has a positive charge overall because of the protons
The atom is made up of three subatomic particles - protons, neutrons and electrons
atoms 2
Almost all of the mass of an atom is concentrated in the nucleus. But size-wise it’s tiny compared to the rest of the atom. The radius is 1 x 10-14m, about 1/10 000 of the size of the atom
electrons
The electrons are negatively charged. They move around the nucleus in electron shells
The number of protons always equals the number of electrons in an atom. The charge on the electrons (-1) is the same size as the charge on the protons (+1) - but opposite. This means atoms have no charge overall - they are neutral.
If some electrons are added or removed, the atom becomes charged and is then an ion. In an ion, the number of protons doesn’t equal the number of electrons. For example, an ion with a 2- charge has two more electrons than protons
nuclear symbol of an atom
The nuclear symbol of an atom tells you its atomic number and mass number.
The atomic number of an atom tells you how many protons it contains. Every atom of an element has the same number of protons. In a neutral atom the number of protons is equal to the number of electrons, so a neutral atom will also have a number of electrons equal to its atomic number.
The mass number of an atom tells you the total number of protons and neutrons that it contains.
Isotopes
Isotopes are different forms of the same element, which have the same number of protons but a different number of neutrons. This means they have the same atomic number but a different mass number.
calculating the relative atomic mass of an element from the abundance of its different isotopes.
Different isotopes of an element occur in different quantities, or abundances You need to know how to calculate the relative atomic mass of an element from the abundance of its different isotopes.
Mendeleev’s periodic table
- Arranged the 50 or so known elements at the time in order of mass so that they formed columns of elements that had similar properties
- He swapped some elements around so that they were not in order of mass but their properties fitted with the other elements in the column
- He used his table to predict the existence and properties of yet undiscovered elements (eg germanium)
- When these elements were discovered, scientists became convinced that Mendeleev’s table was a very important discovery.
Modern Periodic Table
The Modern Periodic Table is different to Mendeleev’s as atomic number (number of protons in an atom) is used to order the elements not atomic mass. This gives a perfect pattern, and no elements have to be ‘swapped’ around. It contains 118 elements.
Electronic structure and the periodic table - period number
The period number of an element tells you how many electron shells it has.
e.g. Nitrogen is a period 2 element. Nitrogen atoms have two electron shells. Bromine is a period 4 element. Bromine atoms have four electron shells.
Electronic structure and the periodic table - period number
The group number of an element tells you how many electrons there are in its outer shell.
e.g. Group 1 elements, like lithium, sodium and potassium, all have one electron in their outer electron shell.
The exception to the rule is the elements in Group 0. These elements all have full outer shells of 8 electrons (or 2 in the case of helium). If you know the position of an element in the periodic table, you can work out its electronic configuration
metals or non-metals in the periodic table
The elements in the periodic table can be classified as metals or non-metals. The non-metals are found in the top right of the periodic table (except for hydrogen). The metals make up the rest of table, and are separated from the non-metals by a zig-zag line running from boron to astatine
Metals tend to form positive ions (by losing electrons) and non-metals tend to form negative ions (by gaining electrons).
Electrons & shells
Electrons always occupy shells (sometimes called energy levels). The electron shells with the lowest energy are always filled first - these are the ones closest to the nucleus. Only a certain number of electrons are allowed in each shell.
