Atomic structure and the Periodic Table Flashcards – Cambridge IGCSE (9–1) Chemistry 0971 (Extended) (2026–2028)

How are protons, neutrons and electrons arranged in an atom?

Protons and neutrons are in the central nucleus; electrons surround it in shells.

What are the relative charge and relative mass of a proton?

Charge: $+1$; mass: $1$.

What are the relative charge and relative mass of a neutron?

Charge: $0$; mass: $1$.

What are the relative charge and approximate relative mass of an electron?

Charge: $-1$; mass: approximately $\frac{1}{1836}$.

Where is nearly all of an atom’s mass concentrated?

In its nucleus.

How do the numbers of protons and electrons compare in a neutral atom?

They are equal, so their charges balance.

What is the atomic number (proton number) of an atom?

The number of protons in its nucleus.

What is the mass number (nucleon number) of an atom?

The total number of protons and neutrons in its nucleus.

What does an atom’s atomic number identify?

Its element.

How can you calculate the number of neutrons in an atom from its atomic and mass numbers?

Subtract the atomic number from the mass number.

Why can atoms of the same element have different mass numbers?

They can contain different numbers of neutrons.

What does an electronic configuration show?

How an atom’s or ion’s electrons are distributed among shells around the nucleus, written as a sequence of shell counts.

How does the number of electrons in a neutral atom compare with its number of protons?

They are equal.

How do positive and negative ions form from atoms?

A positive ion forms by losing electrons; a negative ion forms by gaining electrons.

What happens to an atom’s proton number when it forms an ion?

It remains unchanged.

For elements with proton numbers 1–20, how are electrons placed in shells?

The first shell fills to 2, the second to 8 and the third to 8 before the fourth begins to fill.

What must the total of the shell counts in an electronic configuration equal?

The total number of electrons in the atom or ion.

For elements in Groups I to VII, how does the group number relate to the number of outer-shell electrons?

The group number equals the number of outer-shell electrons.

What characterises the outer shells of Group VIII noble gases among the first 20 elements?

They are full: helium has 2 outer-shell electrons; the others have 8.

How does an element’s period number relate to its occupied electron shells?

The period number equals the number of occupied electron shells.