Chemistry: Atoms

Atoms consist of electrons surrounding a nucleus that contains protons and neutrons. Neutrons are neutral, but protons and electrons are electrically charged; protons have a relative charge of plus one and electrons have a relative charge of minus one.

All substances are made up of tiny atoms. An element is a substance that is made of only one sort of atom. There are about 100 different elements and these are shown in the periodic table. The periodic table is organised in a specific way. The horizontal rows on the table are called periods and the vertical columns are called groups.  

The metals are shown on the left of the periodic table and the non-metals are shown on the left of the table. These are split by a red line. Therfore most of the elements are metals.

The atoms of each element are represented by a chemical symbol. These usually consist of one or two different letters but three are sometimes used for newly discovered elements. For example, O represents oxygen and Na represents a sodium atom.

REMEMBER: The first letter of a chemical symbol is always uppercase and the other letters are always lowercase. For example, a magnesium atom is Mg, not mG, MG or mg.

An atom has a small central nucleus made up of even smaller sub-atomic particles called neutrons and protons. The nucleus is then surrounded with even smaller sub-atomic particles called electrons. The number of electrons in the atom is equal to the number of protons in the atom. Therefore, an atom has no overall electrical charge.

The atomic number of an atom is the same as the number of protons it contains. All the atoms of a particular element will have the same number of protons and therefore the same atomic number. For example, all oxygen atoms have 8 protons and all sodium atoms have 11 protons.

The mass number of an element is the number of protons and the number of neutrons it contains combined. The mass number of an atom is never smaller than the atomic number. They can be the same but the mass number is usually bigger.

You will need to know how to work out the number of each type of particle in an atom when you are given its atomic and mass number. The full chemical symbol of an atom shows its mass number on the top and its atomic number on the bottom, with the chemical symbol in the middle.

Example: A chlorine element

• 35
• CL
• 17

Number of protons: The atomic number, 17

Number of electrons: The same as the amount of protons, 17

Number of neutrons: Difference between the mass and atomic numbers. Therefore, 35-17= 18 neutrons

Electrons orbit the nucleus in certains shells. There is a maximum number of electrons that can fit in each shell. The Periodic Table gives you specific information to write the electronic structure of the element based on its position. the group number gives the number of electrons in the outer shell and the period number gives the number of shells. The number of elements in each period gives the maximum number of electrons in that shell. For example, the first energy level can hold a maximum of two electrons, the second level can hold a maximum of 8, the third can hold a maximum of 8 and so on.

Example: The electronic structure of Phosphorus

Number of electron in its outer shell: Group Number, 5

Number of shells, period number, 3

Therefore, the electronic structure of Phosphorus is 2,8,5

You need to able to draw the electronic structure of any of the first twenty elements.E.g.

Lithium  Li  2,1

How to draw the electronic structure:

• Work out which period the element is in. Draw that number of circles around the nucleus.
• Work out which group it is in and draw that amount of electrons on the outer circle
• Fill the other circles with as many electrons as needed, 2 for the first and 8 for the second.
• Finally, check the amount of electrons is the same as the atomic number