Atoms and elements
All substances are made of atoms (very small particles). Each atom has a small central nucleus made up of protons and neutrons. The nucleus is surrounded by orbiting electrons.
A substance that contains only one sort of atom is called an element. There are about 100 different elements.
The atoms are of each element represented by a different chemical symbol.
- Sodium = Na
- Carbon = C
- Iron = Fe
The periodic table
Elements are arranged in the periodic table. The groups in the periodic table contains elements that have similar properties.
For example, all Group 1 elements (the alkali metals) that vigourously with water to produce an alkaline solution and hydrogen gas.
Group 1 elements react rapidly with oxygen to form metal oxides. The elements in group 0 are called Noble gases. They are all unreactive elements because the atoms have full outer shells of electrons, meaning they are stable. Atoms of noble gases have eight electrons in the outer shell, except the helium, which only has 2.
Protons, neutrons and electrons have relatively electrical charges.
Atoms, as a whole, have no overall charge because they contain an equal number of protons and electrons.
All atoms in particular element has the same number of protons. Atoms of different elements have different numbers of protons. The number of protons in an atom is called its atomic number. The sum of the protons and neutrons in the atom is its mass number.
Elements are arranged in the modern periodic table in order of atomic number
The number of neutrons in an atom is calculated by:
Mass number - Atomic number
Electron configuration and structure
Electrons in an atom occupy the lowest available energy level (i.e. the innermost available shop). For the first 20 elements:
- The first level can only contain a maximum of two electrons
- The energy levels after this can each hold a maximum of eight electrons
The electron configuration tells us how the electrons are arranged around the nucleus in energy levels or shells . It is written as a series of numbers for example…
- Oxygen is 2,6
- Aluminium is 2,8,3
The periodic table groups elements in terms of electronic structure.
Elements in the same group have the same number of electrons in the highest energy level (outer shell) and this gives them similar chemical properties. A particular energy level is gradually filled with electrons from left to right, arcoss the period.
When elements react that atoms joined with other atoms to form compounds. This involves…
- The giving and taking of electrons to form ions
- The sharing of electrons to form molecules
Metal atoms lose electrons to form positive ions and non-metal atoms gain electrons to form negative ions. Compounds formed between metals nonmetals consists of ions. Compounds formed between non-metal ashrams for molecules or atoms and molecules are held together by covalent bonds.
Compounds represented by a combination of numbers and chemical symbols called a chemical formula.
Scientists use chemical formulae a show…
- The different elements in a compound
- The number of atoms in each element in the compound.
In chemical formula, the position of the numbers tells you what is multiplied:
- A small number that sits below the line multiplies only the symbol that comes immediately before it
- A number that is the same size as the letters multiplies all the symbols that come after it.
You can show what has happened during a chemical reaction by writing the word or symbol equation.
The reactants (i.e. the substances that react) or on one side of the equation and the products (i.e. the new substances that are formed) are on the other.
The total mass of the products of a chemical reaction is also equal to the total mass of the reactants . This is because no atoms are lost or made. The products of a chemical reaction are made up from exactly the same atoms as the reactants.
Chemical symbol equations must always be balanced. There must be the same number of atoms of each element on the reactants side of the equation as there is in the product side.