Embedded Files
Development of atomic theory
Development of atomic theory
You should be able to describe the main stages in the development of atomic structure.
Dalton's atomic theory
Dalton's atomic theory
John Dalton developed an atomic theory in the 1800s. He did experiments, worked out some atomic weights and invented symbols for atoms and molecules. His most important conclusions are summarised below:
All matter is made of atoms
Atoms cannot be broken down into anything simpler
All the atoms of a particular element are identical to each other and different from the atoms of other elements
Atoms are rearranged in a chemical reaction
Compounds are formed when two or more different kinds of atoms join together
Dalton's theory was developed and changed as new evidence was discovered.
JJ Thomson's discovery of the electron
JJ Thompson discovered the electron in 1897. This showed that the atom contained smaller pieces, whereas Dalton had thought that atoms could not be broken down into anything simpler.
Rutherford's nuclear atom
Rutherford's nuclear atom
In 1911 Ernest Rutherford used experimental evidence to show that an atom must contain a central nucleus. This was further evidence that an atom contained smaller pieces.
Bohr's electron orbits
Niels Bohr further developed Rutherford's nuclear atom model. He used experimental evidence to support the idea that electrons occupy particular orbits or shells around the nucleus of an atom.
Atomic structure
Atomic structure
All substances are made from atoms. Each atom is made of a tiny nucleus - containing protons and neutrons - surrounded by electrons, which orbit the nucleus in shells.
Atomic structure
Atomic structure
All material things are made from atoms. There are just over one hundred different types of atom, called elements. Atoms can join together in millions of different combinations to make all the substances on Earth and beyond.
Every atom is made of a nucleus consisting of protons and neutrons. The nucleus is surrounded by electrons. Protons and electrons are oppositely charged. Neutrons have no charge. This means the nucleus of an atom is always positively charged.
An atom has a neutral overall charge because it has the same number of electrons as protons.
Protons and neutrons have the same mass. Electrons have such a small mass that this can usually be taken as zero.
Structure of the atom
John Dalton (1766-1844)
Comparing the charge and mass of electrons, protons and neutrons
Comparing the charge and mass of electrons, protons and neutrons
Every element on the periodic table is given a symbol, which has two numbers corresponding to it. One of the numbers is always bigger than the other (except with hydrogen, where they are both the same).
The atomic number (also called the proton number) is the number of protons in an atom. This is the smaller of the two numbers.
The mass number (also called the nucleon number) is the total number of protons and neutrons in the nucleus of an atom. This is the larger of the two numbers.
So in the example above, an atom of helium has..........
2 protons (atomic number is 2)
2 electrons (because the number of electrons in an atom is always the same as the number of protons - to make the atom neutral)
2 neutrons (worked out by taking away the atomic number from the mass number)
In this example, an atom of lithium has;
3 protons,
3 electrons and
4 neutrons
The elements are arranged in the periodic table in ascending order of atomic number so it's easy to find the name or symbol for an atom if you know the atomic number. They scan from left to right across a period (the row).
Electron Arrangements
You also need to know where elements are placed within the shells of an atom. If you see the diagram below, we call the shell the closest to the nucleus the 'first shell'. It can hold a maximum of 2 electrons. The second shell holds a maximum of 8 electrons.
Up to and including GCSE level, you only need to arrange electrons in shells for the first 20 elements. That is hydrogen (1 electron) to calcium (20 electrons).
The electron configuration (or arrangement) can be used to work out where an element is placed in the Periodic Table.
Group number: Number of electrons in the outer shell
Period number: Number of occupied electron shells
For example, sulfur has the electron arrangement of 2,8,6. It is found in Group 6 of the Periodic Table because it has 6 electrons in its outer shell and it is found in period 3 because it has 3 shells which have electrons in them.
Page updated
Report abuse