Atom Structure

Atomic Number, Mass Number and Ion Charge

Lesson Outcomes

• By the end of this lesson you should be able to:

– Illustrate the structure of an atom.– Accurately explain the terms “atomic number” and “mass

number” and identify each value on the periodic table.– Calculate the number of neutrons in an atom/ the mass of an

atom when given specific variables. – Calculate the charge on an atom.

Atomic Structure • An atom is composed of:

– A nucleus with: - Protons that are positively charged - Neutrons that are neutral – A space around the nucleus with: - Energy levels surrounding the nucleus - Electrons that are negatively charged

Atomic Number

• The atomic number of an element is the number of protons (+) in the nucleus. It also indicates the position of the element on the periodic table.

Neutral Atoms

• Atoms are capable of having positive and negative charges.

• In a neutral atom the amount of protons (+ charge) is equal to the amount of electrons (- charge).

Can you determine whether this atom is positively/negatively charged?

Neutrons

• Neutrons are particles in atoms that contribute to approximately half of an atom’s mass. This is because protons and neutrons are roughly the same size.

• Neutrons do not possess any charge, unlike positive protons and negative electrons.

Calculating the number of Neutrons in an Atom.

• Number of Neutrons = Mass Number - Number of Protons (atomic number).

Calculating the Number of Neutrons

• Can you calculate the amount of neutrons in the following mercury atom?

Relative Atomic Mass

• Carbon has an atomic mass of exactly 12 amu. The masses of other elements are calculated by comparison to this.

• Thus, the hydrogen atom’s mass is one twelfth of this and so its relative atomic mass is one.

• The relative atomic mass can be read straight off the periodic table.

• Most elements occur in nature as a mixture of isotopes, so the atomic mass of the element will be a weighted average of all of these masses.

Mass Number

• The mass number is the total number of nucleons, that is the number of protons and neutrons in the nucleus, in an element.

• The mass number gives only an indication of the mass of an atom.

• The mass of an atom is determined by its nucleons (protons and neutrons).

• Electrons have a relatively small mass and simply impart volume to the atom instead of mass.

Notation on the Periodic Table

• Elements are portrayed by the nuclear notation:

(mass number is always larger than the atomic number)

• In the example above it means that: – The element is Helium. – There are 4 protons and neutrons in the nucleus. – There are 2 protons. – There are 3 electrons.– Helium is element number 2 on the periodic table.

Ions • An ion is a charged particle • When a neutral atom looses an electron it becomes a positive

ion known as a cation. • When a neutral atom gains an electron it becomes a

negatively charged ion known as an anion. • The number of protons and neutrons DOES NOT CHANGE.

Determining Ion Charge

• What is the charge on a Li atom if it loses an electron?

- is it +/-? - what is the charge value?• What is the charge on an F atom if it gains 2

electrons? - is it +/-? - what is the charge value?

Activity

• Complete the activity sheets in pairs or groups of three.