Grade 10 Chemistry Lesson

1. Atom Structure
Atomic Number, Mass Number and
Ion Charge

2. 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.

3. 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

4. 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.

5. 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).

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

7. 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.

8. Calculating the number of Neutrons in
an Atom.
• Number of Neutrons = Mass Number -
Number of Protons (atomic number).

9. Calculating the Number of Neutrons
• Can you calculate the amount of neutrons in
the following mercury atom?

10. 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.

11. 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.

12. Notation on the Periodic Table
• Elements are portrayed by the nuclear
notation:
(mass number is always larger than the atomic
number)

13. • 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.

14. 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.

15. 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?

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