atomic number and mass number

features of John Dalton’s atomic theory

• all matter is made up of tiny particles called atoms

• atoms are tiny, hard spheres that cannot be broken down into smaller pieces

• atoms cannot be created or destroyed

• the atoms in an element are all identical

how were the elements in the periodic table originally arranged?

the elements were originally arranged in the order of the masses of their atoms

what problem arose with arranging the elements by atomic mass?

some elements were grouped together with others that had very different properties

what did henry moseley’s experiments confirm about the arrangement of elements

moseley’s experiments confirmed that the elements were correctly ordered by the amount of positive charge in the nucleus, not just by atomic mass

how is the modern periodic table arranged?

the modern periodic table arranges elements by the number of protons in their atoms, known as the atomic number

how can you identify an element based on the structure of its atoms?

each element has a unique number of protons in its nucleus, which distinguishes it from all other elements

where is most of the mass in an atom?

most of the atom’s mass is concentrated in the nucleus

why do atoms contain equal numbers of protons and electrons

atoms are neutral. the charges of a proton and an electron cancel each other out

describe the size of a nucleus in an atom

the size of the nucleus is very small in comparison to the overall atom