1.1 Particles in the atom and atomic radius

Explain why atoms are electrically neutral.

Atoms have equal numbers of protons and electrons. The positive charges of the protons cancel out the negative charges of the electrons, making the atom electrically neutral.

Describe the structure of an atom.

An atom is mostly empty space with a small, dense nucleus containing protons and neutrons. Electrons are found in shells around the nucleus.

State the relative charge and relative mass of a proton.

Charge = +1, Mass = 1.

State the relative charge and relative mass of a neutron.

Charge = 0, Mass = 1.

State the relative charge and relative mass of an electron.

Charge = –1, Mass = 1/1836.

Define the term atomic (proton) number.

The number of protons in the nucleus of an atom.

Define the term mass (nucleon) number.

The total number of protons and neutrons in an atom.

State where mass and positive charge are concentrated in an atom.

In the nucleus (protons and neutrons).

State what determines the volume of an atom.

The spread of electrons in shells (energy levels).

Describe how charged particles behave in an electric field.

• Protons: deflect toward the negative plate (moderate deflection).
• Electrons: deflect toward the positive plate (greater deflection due to smaller mass).
• Neutrons: no deflection (neutral).

Calculate the number of protons in an atom.

Equal to the atomic number.

Calculate the number of neutrons in an atom.

Neutrons = Mass number – Atomic number.

State how to find the number of electrons in a neutral atom.

Electrons = Protons.

Calculate the number of electrons in an ion.

• Positive ion: Electrons = Protons – charge.
• Negative ion: Electrons = Protons + charge.

Explain why atomic radius decreases across a period.

It decreases due to increasing nuclear charge and electrons being added to the same shell, pulling them closer.

Explain why atomic radius increases down a group.

It increases due to more energy levels, increased shielding, and weaker attraction to the nucleus.

Explain why ionic radius increases with increasing negative charge.

• More electrons added.
• Same nuclear charge.
• More repulsion → electrons held further from nucleus.
• Radius increases.

Explain why ionic radius decreases with increasing positive charge.

• Electrons are lost.
• Same nuclear charge.
• Fewer electrons = stronger attraction.
• Radius decrease