Atomic Structure

What is the structure of an atom in terms of space and mass?

Atoms are mostly empty space surrounding a very small, dense nucleus. The nucleus contains protons and neutrons and holds most of the atom's mass. Electrons occupy shells and most of the atom's volume.

Where are electrons located in an atom?

Electrons are found in energy levels (shells) in the empty space around the nucleus.

What are the relative charge and mass of a proton?

Charge = +1, Mass = 1.

What are the relative charge and mass of a neutron?

Charge = 0, Mass = 1.

What are the relative charge and mass of an electron?

Charge = –1, Mass = 1/1836.

What determines the identity of an element?

The number of protons (atomic number).

What determines an atom’s reactivity and chemical properties?

The number and arrangement of electrons.

Define atomic number (Z).

The number of protons in the nucleus of an atom.

Define mass number (A).

The total number of protons and neutrons in the nucleus.

How do you calculate the number of neutrons in an atom?

Neutrons = Mass number – Atomic number.

Describe the distribution of mass and charge in an atom.

Mass and positive charge are concentrated in the nucleus. Electrons have little mass and are spread in shells, forming most of the atom’s volume.

What happens to a beam of protons in an electric field?

It deflects slightly toward the negative plate due to its positive charge and greater mass.

What happens to a beam of electrons in an electric field?

It deflects strongly toward the positive plate because of their negative charge and low mass.

What happens to a beam of neutrons in an electric field?

No deflection – they are uncharged.

How do you calculate the number of electrons in:

a neutral atom
a positive ion
a negative ion

Neutral: electrons = protons
Positive ion: electrons = protons – charge
Negative ion: electrons = protons + charge

What happens to atomic radius across a period?

It decreases due to increased nuclear charge pulling electrons closer.

What happens to atomic radius down a group?

It increases due to added electron shells and increased shielding.

Why are cations smaller than their atoms?

They have fewer electrons, so the nucleus pulls the remaining electrons closer.

Why are anions larger than their atoms?

They have more electrons, causing greater repulsion and increased radius.

Draw/describe the structure of an atom, indicating nucleus, electrons, protons, neutrons — and comment on space.

Atom has small dense nucleus (protons & neutrons) and empty space around, with electrons in shells. Most mass is in the nucleus; most volume is empty space.

Proton, neutron, electron: state relative mass and charge.

Proton +1 charge, mass 1; Neutron 0 charge, mass 1; Electron –1 charge, mass ≈1/1836.

Define atomic (proton) number and mass (nucleon) number, and how to calculate neutrons.

Atomic number (Z) = number of protons; Mass number (A) = protons + neutrons; neutrons = A – Z.

How do protons, electrons, and neutrons behave in an electric field?

Protons deflect slightly toward the negative plate; electrons deflect strongly toward the positive plate; neutrons are unaffected.

Explain trend in atomic radius across a period and down a group.

Across: decreases due to increasing nuclear charge, same shell → stronger pull. Down: increases due to more shells and shielding → weaker pull.

Why is a sodium ion smaller and a chloride ion larger than their atoms?

Na⁺: lost electron → same nuclear charge pulling fewer electrons = smaller radius. Cl⁻: gained electron → more repulsion among electrons = larger radius.

Sketch the mass distribution in an atom and use it to explain why most of the atom’s volume is empty space.

Nucleus contains almost all the mass in a very small volume; electrons occupy shells much further out → most of the atom is empty space.

A beam of protons and electrons moving at the same velocity enters a uniform electric field. Compare their deflections.

Electrons deflect more toward the positive plate (lighter and opposite charge); protons deflect less toward the negative plate (heavier).

Define atomic number and mass number, and calculate the number of neutrons for an atom with Z = 12 and A = 25.

Atomic number = number of protons = 12; mass number = protons + neutrons = 25; neutrons = 25 – 12 = 13.

Explain why atomic radius decreases across a period.

Increasing nuclear charge (more protons) attracts electrons more strongly in the same shell → radius decreases.

Without calculation, determine if Al³⁺ is larger, smaller, or the same size as Al. Explain.

Al³⁺ is smaller than Al because it has lost three electrons but retains the same number of protons, so the nuclear pull on fewer electrons is stronger → reduced radius.