Atomic Structure

Atomic Structure

1. Isotopes

   • The full chemical symbol for an element shows its mass number at the top and atomic number at the bottom
   • Isotopes of Hydrogen
     • Most hydrogen atoms consist of just one proton and one electron, but some also have one or two neutrons.
   • Isotopes of Chlorine
     • Chlorine atoms contain 17 protons and 17 electrons. About 75 per cent of chlorine atoms have 18 neutrons, while about 25 per cent have 20 neutrons.

2. Energy shells

   • Electrons are arranged in different shells around the nucleus.
   • The innermost shell - or lowest energy shell - is filled first.
   • Each succeeding shell can only hold a certain number of electrons before it becomes full.
   • The innermost shell can hold a maximum of two electrons, the second shell a maximum of eight, and so on.

3. Electronic structure

   • The electronic structure of an atom is a description of how the electrons are arranged. It can be shown as numbers or a diagram.
   • A diagram shows each energy shell as rings around the nucleus, with each electron shown by a dot or sometimes a cross.
   • The radius of a typical atom is about 10–10 m and its mass is about 10–23 g.
   • An atom contains equal numbers of positively charged protons and negatively charged electrons, so it is neutral overall.
   • You need to be able to work out the electronic structure of the first twenty elements in the periodic table (from hydrogen to calcium).

4. Development of Atomic Theory

   • John Dalton atomic - 1800s. Found some atomic weights & invented symbols for atoms & molecules: 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 & 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
   • JJ Thompson discovered the electron in 1897. Showing that the atom contained anything simpler, whereas Dalton thought that atoms couldn't be broken down
   • 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.
   • 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. The development of the theory of atomic structure is an example of: How a theory may change as new evidence is found How a scientific explanation is provisional but may become more convincing when predictions based on it are confirmed later on

5. All material things are made from atoms.

6. There are just over one hundred different types of atom, called elements.

7. Atoms can join together in millions of different combinations to make all the substances on Earth and beyond.

8. Every atom is made of a nucleus consisting of protons and neutrons.

9. The nucleus is surrounded by electrons.

10. Protons and electrons are oppositely charged.

11. Neutrons have no charge. This means the nucleus of an atom is always positively charged.

12. An atom has a neutral overall charge because it has the same number of electrons as protons.

13. Protons and neutrons have the same mass. Electrons have such a small mass that this can usually be taken as zero.

14. The atomic number is the number of protons in an atom.

15. The mass number: total number of protons & neutrons in an atom.

16. The elements are arranged in the periodic table in ascending order of atomic number