Matter & Its Properties
Atoms & the Periodic Table
1st Year · 2nd Year · 3rd Year (Junior Cert)
- ✓By the end of this lesson students will be able to identify the subatomic particles (protons, neutrons, electrons) and their relative charges and masses.
- ✓By the end of this lesson students will be able to define atomic number and mass number and use them to determine the number of protons, neutrons, and electrons in an atom.
- ✓By the end of this lesson students will be able to describe the arrangement of elements in the Periodic Table in terms of groups and periods.
- ✓By the end of this lesson students will be able to distinguish between metals and non-metals based on their properties and position in the Periodic Table.
Key concepts
The basic building block of all matter. Atoms are incredibly small particles that make up everything around us. They consist of a central nucleus and electrons orbiting the nucleus.
The particles that make up an atom: protons, neutrons, and electrons. * Protons: Positively charged particles found in the nucleus. They determine the atomic number. Relative mass = 1. Relative charge = +1. * Neutrons: Neutral (no charge) particles found in the nucleus. They contribute to the mass number. Relative mass = 1. Relative charge = 0. * Electrons: Negatively charged particles that orbit the nucleus in energy shells. They determine the chemical properties of an atom. Relative mass = 1/1836 (negligible). Relative charge = -1.
The number of protons in the nucleus of an atom. It is unique for each element and defines the element. In a neutral atom, the number of electrons equals the number of protons.
The total number of protons and neutrons in the nucleus of an atom. It represents the approximate relative atomic mass of an atom.
A chart that organises all known elements based on their atomic number and recurring chemical properties. It is a fundamental tool in chemistry.
The vertical columns in the Periodic Table. Elements in the same group have similar chemical properties because they have the same number of valence (outermost shell) electrons.
The horizontal rows in the Periodic Table. The period number indicates the number of electron shells an atom has.
Elements typically found on the left and centre of the Periodic Table. They are generally shiny (lustrous), good conductors of heat and electricity, malleable (can be hammered into shapes), and ductile (can be drawn into wires). They tend to lose electrons in chemical reactions.
Elements typically found on the right side of the Periodic Table. They are generally dull, poor conductors of heat and electricity (insulators), brittle (break easily), and have low melting points. They tend to gain or share electrons in chemical reactions.
Key facts to remember
- 1Atoms are the smallest particles of an element and are made of protons, neutrons, and electrons.
- 2Protons have a positive charge (+1), neutrons have no charge (0), and electrons have a negative charge (-1).
- 3The atomic number (Z) is the number of protons and uniquely identifies an element.
- 4The mass number (A) is the total number of protons and neutrons in the nucleus.
- 5In a neutral atom, the number of protons is equal to the number of electrons.
- 6The Periodic Table organises elements by increasing atomic number.
- 7Groups are vertical columns (elements have similar chemical properties), and periods are horizontal rows (elements have the same number of electron shells).
- 8Metals are generally found on the left and centre of the Periodic Table, while non-metals are on the right.
Worked examples
Example 1
An atom of Sodium (Na) has an atomic number of 11 and a mass number of 23. Determine the number of protons, neutrons, and electrons in a neutral Sodium atom.
Answer
Number of Protons = 11 Number of Neutrons = 12 Number of Electrons = 11
Example 2
An element is in Group 2 and Period 3 of the Periodic Table. Is it a metal or a non-metal? Describe two properties you would expect it to have.
Answer
The element is a metal. Two expected properties are: it is a good conductor of electricity and it is shiny (lustrous).
Example 3
An atom has 8 protons, 8 neutrons, and 8 electrons. What is its atomic number and mass number? What element is it?
Answer
Atomic Number = 8 Mass Number = 16 The element is Oxygen (O).
Common mistakes
- ✗Confusing atomic number (protons) with mass number (protons + neutrons).
- ✗Forgetting that in a neutral atom, the number of electrons is equal to the number of protons.
- ✗Mixing up the definitions of groups (vertical columns) and periods (horizontal rows) on the Periodic Table.
- ✗Incorrectly calculating the number of neutrons by adding the atomic number and mass number instead of subtracting.
- ✗Assuming all elements are either clearly metals or non-metals, overlooking the existence of metalloids (though less focus at Junior Cycle).
Exam tips
- ★Always clearly state the formula used when calculating the number of subatomic particles (e.g., Neutrons = Mass Number - Protons).
- ★Practise locating elements on the Periodic Table quickly using their group and period numbers.
- ★Memorise the relative charges and masses of protons, neutrons, and electrons.
- ★Be able to list at least two distinguishing physical properties for metals and non-metals.
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