Chemistry- Fundamental ideas

Atoms, elements and compounds

• All substances are made of atoms 
• Elements are made of only one type of atoms 
• Chemical symbols are used to represent atoms
• Compounds contain more than one element
• There are about 100 different element from which all substances are made
• The element in the periodic table are arranged in columns, called groups. The elements in a group usually have similar properties
• Atoms have a tiny nucleus surrounded by electrons 
• When elements react, their atoms join with atoms of other elements.
• Compounds are formed when two or more elements combine together

Atomic structure

• The nucleus of an atom is made of protons and neutrons
• Protons have a positive charge, electrons a negative charge and neutrons are not charged 
• The atomic number (or proton number) of an element is equal to the number of protons in the nucleus of its atoms
• Elements are arranged in order of their atomic numbers in the periodic table 
• The mass number is the sum of the protons and neutrons in the nucleus of an atom
• Work out the number of each type of particle in an atom of fluorine from its atomic number of 9 and its mass number of 19
• Number of protons=atomic number=9
• Number of electrons=number of protons=9
• Number of neutrons=mass number-atomic number=19-9=10 

The arrangement of electrons in atoms 

• The atoms of the unreactive noble gases (in Group 0) all have very stable arrangements of electrons
• Electrons in atoms are in energy levels that can be represented by shells
• Electrons in the lowest energy level are in the shell closest to the nucleus
• Electrons occupy the lowest energy levels first
• All the elements in the same group of the periodic table have the same number of electrons in their highest energy level (outer-shell)

Forming bonds

• Compounds of metal bonded to non-metals have ionic bonds
• The formula of an ionic compund shows the simplest ratio of ions
• Compounds of non-metals have covalent bonda
• The formula of a molecule shows the number of atoms in the molecule
• For example, H20 shows that a water molecule contains two hydrogen atoms and one oxygen atom. Covalent bonds can be shown as lines between the atoms that are bonded together

Chemical Equations

• Atoms get re-arranged in chemical reactions
• The mass of the products is equal to the mass of reactants
• Symbol equations should always be balances 
• Atoms are neither created nor destroyed in a chemical reaction. So the number and type of atoms remains the same before and after the reaction
• This means that the mass of the products equals the mass of reactants 
• Symbol equations are balances by changing the large numbers in front of the formulae of the reactants and products. You should balance equations by changing only the large number. Never change the small ( subscript) numbers because this changes the formula of the substance
• In the formulae in symbol equations, small (subscript) numbers multiply only the atoms they follow, for example:
• In H2SO4 we have H2= 2 atoms of hydrogen, S=one atoms of sulfur, O4=4 atoms of oxygen

• If more than one atoms within a formula has to be multiplied, brackets are used
• Mg(NO3)2 (one magnesium ion and two nirate ions) is made from one atom of magnesium, 1*2 atoms of nitrogen and 3*2=6 atoms of oxygen