1.1 Atomic structure

Describe the structure of an atom.

An atom consists of a small, dense nucleus containing protons and neutrons, with electrons arranged in shells in the empty space 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 atomic (proton) number.

The atomic number is the number of protons in the nucleus of an atom.

Define the mass (nucleon) number.

The mass number is the total number of protons and neutrons in the nucleus of an atom.

Explain why atoms are electrically neutral.

Atoms are electrically neutral because they contain equal numbers of protons and electrons, so the total positive and negative charges cancel out.

Describe how charged particles behave in an electric field.

Protons are deflected toward the negative plate with moderate deflection. Electrons are deflected toward the positive plate with greater deflection due to lower mass. Neutrons are not deflected because they have no charge.

Calculate the number of subatomic particles in an ion of atomic number 9 and mass number 19 with a 1– charge.

Protons = 9, Neutrons = 10 (19 – 9), Electrons = 10 (9 + 1).

Explain why atomic radius decreases across a period.

Atomic radius decreases across a period because the number of protons increases, increasing nuclear charge. Electrons are added to the same shell, so there is greater attraction between the nucleus and electrons, pulling them closer.

Explain why atomic radius increases down a group.

Atomic radius increases down a group because additional electron shells are added, increasing the distance between the nucleus and outer electrons. Increased shielding reduces the attraction between the nucleus and outermost electrons.

Describe how the ionic radius changes with increasing negative charge.

Ionic radius increases with increasing negative charge because extra electrons increase electron–electron repulsion. The nuclear charge remains the same, so the outer electrons are held less tightly and move further from the nucleus.

Describe how the ionic radius changes with increasing positive charge.

Ionic radius decreases with increasing positive charge because electrons are lost while the nuclear charge remains the same. The remaining electrons experience greater electrostatic attraction to the nucleus and are pulled close.