SNC1D1 Chemistry
Notes Atomic Structure
Atom Diagram
Elements and Atomic Structure
Atoms are composed of three subatomic particles, protons, neutrons and electrons.
(i) Protons are positively charged particles that are found in the dense positive core nucleus of an atom.
(ii) Neutrons are neutral particles that are also found in the nucleus of an atom.
(iii) Electrons are negatively charged particles found circling the nucleus in the outer orbit or shell of the atom
Chemical Symbol
Master Yoda Much Learning You Have Yes!

Elements are all represented by a chemical symbol. The proper chemical symbol notation includes both the mass number and the atomic number.
See example below. Remember that the mass number indicates the number of protons & neutrons while the atomic number indicates the number of protons.
Atom
An atom is mostly empty space and it is made up of protons, neutron and electrons. Both the protons and neutrons are located in the nucleus and the electrons are located in the outer electrons shells. Protons have a positive charge, electrons have negative charge and neutrons have a neutral charge. Compared to the proton and neutrons, the electron is very light. The proton and neutron masses are approximately the same and they both combine to make up more than 99% of the atom’s mass.
Most of the time when you come in contact with an object you don’t get a shock. This suggests that most objects are not charged. Matter that is not charged is said to be neutral. Therefore we can conclude that the number of protons and electrons must be equal in a neutral object thereby the positive and negative charges cancel each other out.
Atomic Number, Mass Number, Atomic Mass & Atomic Mass Unit
Atomic Number
The atomic number of an element conveys the number of protons found in an atom. Knowing the number of protons will also indicate the number of electrons found in a neutral atom because the number of protons and
Master Yoda Much Learning You Have Yes!

neutrons are always the same in a neutral atom. For example the atomic number for fluorine is 9, this means that fluorine has 9 protons and a neutral atom of fluorine would have 9 protons and electrons.
Atomic Mass
The atomic mass is the mass of an average atom of an element, measured in atomic mass units.
Atomic Mass Units
The atomic mass of an element is stated in atomic mass units. Atomic mass units are not measurements of mass in the same way that grams are units for the measurement of the mass of an object placed on a balance scale. Originally, atomic mass units were ratios based on the comparison with a hydrogen atom. When Dalton first proposed atomic mass in the 19 th century, he imagined the masses of atoms relative to each other. Dalton used the hydrogen atom as the starting unit, with an atomic mass of 1 and other atomic masses were expressed in relation to it. For example, a relative atomic mass of 6 on Dalton’s scale means that the atom is six times as heavy as a hydrogen atom. Modern atomic mass units are also ratios, however all atomic masses are defined relative to the mass of a carbon molecule
Mass Number
The mass number conveys the total number of protons and neutrons present in a particular atom. Therefore one can determine the number of neutrons in an atom by subtracting the atomic number from the mass number.
Master Yoda Much Learning You Have Yes!