Atomic Structure: The basics
Some key words:
- Atom: The smallest part of an element that can still be recognised as that element
- Element: A substance made up of only one type of atom. It cannot chemically be broken down into any simpler substances
- Molecule: Two or more atoms bonded together. (e.g. HCl)
- Compound: A group of different atoms joined together. (e.g. NaCl)
All compounds are molecules but NOT all molecules are compounds.
Atoms contain protons, neutrons and electrons. The protons and neutrons are found in the nucleus. The electrons are in energy levels/shells around the nucleus.
- Positive Protons- mass 1
- Negative Electrons- mass 1/1836
- No charge Neutrons- mass 1
The periodic table is organised by the number of protons each element has- The atomic number .
In a neutral atoms (overall charge: 0), the number of protons will be the same as the number of electrons.
The atomic mass is the number of protons + the number of electrons.
Number of neutrons= atomic mass- atomic number
REMEMBER: Mass , Number (Mass is on the top, Number on the bottom)
Electron arrangement and the group number
In the periodic table, the group number is the same as the number of electrons in the outer shell. (e.g. Group 1 atoms have 1 electron in their outer shells.)
Because the chemical properties of an element depends on the number of electrons in its outer shells, this means that all the elements in a particular group react in a similar way.
Atoms like to gain a stable structure (i.e. a full outer shell). If they are in group 1,2,3 they lose electrons. If they are in group 5,6,7 they gain electrons. Group 4 shares electrons. Atoms of elements in the same group behave in similar ways to either lose/gain/share electrons and thus get a full outer shell.
Notice that group 8 electrons have a full outer shell. As a result they are unreative.
The number of shells an element needs to accomodate all its electrons is the same as its period number.
Ions and Ionic Bonding
An ion is a charged particle. It is formed when an element wants to aquire a stable configuration. Metals (in group 1,2,3) form ions with apositive charge (+1,+2,+3). Non metals (in group 5,6,7) form ions with a negative charge (-3,-2,-1)
- If electrons are lost from an atom, the resulting ion is positive.
- If electrons are gained from an atom, the resulting ion is negative.
Metals form positive ions and non metals form negative ions. These opppsitely charged ions attract each other- this is Ionic Bonding. The resulting compound is held together by strong electrostatic forces. The oppositely charged ions cancel each other and as a result the overall charge is zero.
REMEMBER: Ionic Bonding only occurs between metals and non metals.
Covalent Bonding is when non- metal atoms bond to each other in a different way to Ionic Bonding. Their outermost shells overlap and they share electrons. Each pair of shared electrons form a chemical bond between the atoms.
- No ions are EVER formed.
- It occurs only in non metals, not in metals.