C1.1 Fundamental Ideas

C1.1 Fundamental Ideas

1. 1.1 Atoms, elements and compounds

   • There are about 100 elements from which all substances are made. The periodic table is a list of the elements
   • Each element is made up of one type of atom. Atoms are represented by chemical symbols
   • The elements 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

2. 1.2 Atomic Structure

   • The nucleus at the centre of an atom contains 2 types of charged particle, called protons and neutrons. Protons have a positive charge and neutrons have no charge
   • Electrons are tiny negatively charged particles that move around the nucleus. An atom has no overall charge. That is because the number of protons is equal to the number of electrons an their charges are equal and opposite (proton +1/electron -1)
   • All atoms of an element contain the same number of protons. This number is called the atomic number
   • Elements are arranged in order of their atomic numbers in the periodic table. The atomic number is also the number of electrons in an atom of the element
   • The mass number is the total number of particles in the nucleus of an atom, so it is the number of protons plus the number of neutrons

3. 1.3 The arrangement of electrons in atoms

   • Each electron in an atom is in an energy level. Energy levels can be represented as shells, with electrons in the lowest energy level closest to the nucleus
   • The lowest energy level (first shell) can hold 2 electrons, and the second can hold 8. Electrons occupy the lowest possible energy levels
   • Elements in the same group in the periodic table have the same number of electrons in their highest energy level
   • Group 1 elements include lithium, sodium and potassium. These elements react quickly with water and with oxygen

4. 1.4 Forming Bonds

   • When different elements combine they form compounds
   • When a metal reacts with a non-metal, ions are formed. Metal atoms lose one or more electrons to form positively charged ions. Non-metal atoms gain electrons to form negatively charged ions. The oppositely charged ions attract each other strongly and the compound had ionic bonds
   • The chemical formula of an ionic compound tells us the simplest ratio of ions in the compound. For example, NaCl shows that sodium chloride is made from equal numbers of sodium ions and chloride ions
   • When non-metals combine, their atoms share electrons to form covalent bonds and molecules are formed
   • The chemical formula of a molecule tells us the number of atoms that have bonded together in the molecule. Covalent bonds can be shown as lines between the atoms that are bonded together in a diagram

5. 1.5 Chemical Equations

   • In chemical reactions the atoms in the reactants re-arrange themselves to form new substances, the products
   • 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=the mass of the reactants
   • Symbol equations are balanced by changing the large numbers in front of the formulae of the reactants and products. You should balance equations by changing only the large numbers