atomic structure and isotopes

What is an atom?

The smallest part of an element that can take part in chemical reactions

What are atoms mostly made up of?

Empty space

Where is the mass of an atom concentrated, and why?

In the nucleus, because it contains the heaviest subatomic particles (neutrons and protons)

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

Charge +1, relative mass 1

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

Charge 0 (neutral), relative mass 1

What is the relative charge and relative mass of an electron?

Charge -1, relative mass 1/1836

Why is the nucleus positively charged?

Because it contains protons

What holds an atom together?

The electrostatic attraction between the positive nucleus and the negatively charged electrons orbiting it

Does an atom have an overall charge?

No — an atom is neutral

How are ions formed?

When atoms gain or lose electrons, causing them to become charged

What determines which element an atom or ion is?

Its atomic (proton) number

Do all atoms and ions of the same element have the same number of protons?

Yes — the atomic number

How do you calculate the number of protons if you know the mass number and number of neutrons?

Number of protons = mass number − number of neutrons

How many protons does a Mg²⁺ ion have?

12 (same as the atomic number of magnesium — ion charge doesn't change proton number)

An atom has a mass number of 63 and 34 neutrons. How many protons does it have, and which element is it?

63 − 34 = 29 protons; element X is copper

How many protons and electrons does a neutral atom have?

The same number of each

How does the number of electrons compare to protons in a cation?

A cation has lost electrons, so it has fewer electrons than protons

How does the number of electrons compare to protons in an anion?

An anion has gained electrons, so it has more electrons than protons

How many electrons does a Mg²⁺ ion have?

10 (12 protons − 2 lost electrons)

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

Number of neutrons = mass number (A) − number of protons (Z)

A carbon atom has atomic number 6 and mass number 12. How many neutrons does it have?

12 − 6 = 6 neutrons

What is an isotope?

An atom of the same element with the same number of protons and electrons but a different number of neutrons

How is the symbol for an isotope written?

The chemical symbol (or word) followed by a dash and then the mass number, e.g. carbon-12

What are the isotopes of hydrogen called, and how many neutrons does each have?

Protium (0 neutrons), deuterium (1 neutron), tritium (2 neutrons)

Do isotopes have the same chemical properties?

Yes — because they have the same number of electrons in their outer shells, and electrons determine chemistry

Why do isotopes have different physical properties?

Because they have different numbers of neutrons, which only adds mass — causing small differences in mass and density

What is used as the international standard for relative mass?

The carbon-12 isotope

What is the accepted mass of one atom of carbon-12?

1.992646538 × 10⁻²⁶ kg (fixed as exactly 12 atomic mass units, 12u)

What is the standard mass for atomic mass, and what does it represent?

1u, which is the mass of 1/12th of a carbon-12 atom

What is relative isotopic mass?

The mass of an atom of an isotope relative to 1/12th the mass of a carbon-12 atom

What is the relative atomic mass (Ar)?

The weighted mean/average mass of an atom's isotopes, relative to 1/12th the mass of a carbon-12 atom

Why do most elements on the Periodic Table need a relative atomic mass rather than a single fixed mass?

Because most elements are a mixture of different isotopes