- Created by: Mel
- Created on: 24-05-12 10:13
Nucleus at the centre of an atom contains two types of particle, protons (positive charge) and neutrons (no charge). Electrons are negatively charged particles that move around the nucleus. An atom has no overall charge because the number of electrons is equal to the number of protons and their charges are equal and opposite.
All atoms of an element contain the same number of protons. Elements are arranged in order of their atomic numbers (number of protons) in the periodic table.
Arrangement of Electrons in Atoms
Each electron in an atom is in an energy level. Electrons in shells (energy level) further away from the nucleus have more energy than electrons in shells closer to the nucleus. The lowest energy level (closest to the nucleus) can hold two electrons, and the second can hold eight. Elements whose atoms have a full outer shell are very stable and unreactive (noble gases - helium, neon, argon).
E.g. Sodium with 11 electrons has an electronic structure of 2,8,1.
Atoms of elements can achieve stable electronic structures by gaining or losing (ionic bonding) electrons to form ions, or by sharing electrons to form covalent bonds. When an element in Group 1 reacts with an element in Group 7 an electron is transferred netween atoms to form ions with the electron structure of a noble gas.
Atoms of elements in Group 1 lose their single outer electron, e.g. Sodium Na (2,8,1) forms sodium ions, Na+ (2,8)
Ionic compounds are usually formed when metals react with non-metals. Metal atoms form positive ions (cation), while non-metal atoms form negative ions (anion). Ions are electrically charged particles formed when atoms lose or gain electrons. Compounds made of ions have giant structures that are very regular. Ionic bonding holds oppositely charged ions together in giant structures. Other elements that can form ionic compounds include those in Groups 2 and 6.
Ionic Bonding (cont.)
When Sodium (2,8,1) reacts with Chlorine (2,8,7), Sodium Chloride is formed.
Atoms of non-metals need to gain electrons to achieve stable arrangements of electrons. They do this by sharing electrons with other atoms. Atoms of elements in Group 7 need to gain one electron and so form a single covalent bond.
Covalent bonds act only between the two atoms they bond, and so many covalently bonded substances consist of small molecules (e.g. H2O). Some atoms that can form several bonds, like carbon, can join together in giant covalent structures. Diamond and silicon dioxide (silica) have giant covalent structures.
We can represent a covalent compound by showing the highest energy level.
Bonding in Metals
Atoms in a metallic element are all the same size. They form giant structures with layers of atoms arranged in regular patterns. The outer electrons in each atom can easily move from atom to atom and form a 'sea' of free electrons surrounding positively charged metal ions. Electrostatic forces (between electrons and positive ions) hold metal atoms in place in their giant structures. Metal is made up of a number of small crystals called grains and the places where they join are the grain boundaries.
When metal atoms pack together the electrons in the highest energy level delocalise and move from one atom to another. This produces positive ions in a 'sea' of moving electrons. The delocalised electrons strong attract the positive ions and hold the structure together.
Sometimes metal crystals on the surface of steel has been dipped in zinc to prevent it from rusting (galvanizing).