Who created the first widely accepted periodic table, and on what basis did he arrange the elements?

John Dalton (1803), who arranged elements by the number of atoms they contain Dmitri Mendeleev (1869), who arranged elements by increasing atomic mass and noticed repeating patterns in chemical properties Ernest Rutherford (1911), who arranged elements by the number of protons in their nuclei Antoine Lavoisier (1789), who arranged elements by their states at room temperature

In the modern periodic table, what determines an element's position (its atomic number)?

The mass of the atom in atomic mass units The number of protons in the nucleus The number of electrons in the outermost shell The element's reactivity with water

What does a 'period' (row) in the periodic table represent?

A column of elements with the same number of valence electrons A row of elements in which electrons are filling the same outermost shell — each period corresponds to a new shell being filled A group of elements with the same chemical reactivity A set of elements discovered in the same historical period

What does a 'group' (column) in the periodic table represent?

A column of elements in the same electron shell A column of elements discovered at the same time A column of elements that all have the same number of valence electrons and therefore similar chemical properties A column of elements with the same mass number

What general properties distinguish metals from non-metals on the periodic table?

Metals are found in period 2; non-metals in period 3 Metals (left/centre) conduct electricity, are malleable, shiny, and tend to lose electrons; non-metals (upper right) do not conduct, are brittle (if solid), and tend to gain electrons Metals are non-reactive; non-metals are highly reactive Metals have more protons than non-metals of the same period

The alkali metals (group 1: Li, Na, K…) become more reactive as you go down the group. Why?

Lower alkali metals have more valence electrons, making them more reactive Going down the group, the valence electron is in a higher, more distant shell — it is more easily lost because it is further from the nucleus and more shielded by inner electrons Lower alkali metals are physically larger and can hold more reactants Lower alkali metals have more neutrons, making their nuclei less stable

How many naturally occurring elements exist?

Exactly 20 — one for each of the first 20 elements 118 — all elements on the periodic table occur naturally About 90 — elements 1–94 occur in nature (though some in trace amounts); elements above 94 are synthetic Only 4 — hydrogen, oxygen, carbon, and nitrogen are the only natural elements

What is a metalloid (semi-metal), and give one example?

A metal that is also a liquid at room temperature — mercury (Hg) for example An element that reacts with both acids and bases — sodium is an example An element with properties intermediate between metals and non-metals — silicon (Si) is the most important example, used in computer chips A radioactive metal found at the bottom of the periodic table

Mendeleev left gaps in his periodic table for undiscovered elements. How was this predictive power possible?

Mendeleev could see the future — he was a remarkable psychic By observing clear trends in atomic mass and properties across periods and groups, he could predict the properties of missing elements from the surrounding ones He had found the elements but kept them secret to test other chemists He used quantum mechanics to calculate the properties from first principles

How does the periodic table demonstrate that the enormous diversity of matter arises from a small set of building blocks?

The periodic table shows that all matter is made of only 4 elements (earth, water, air, fire) The periodic table shows that each element is unique and cannot combine with any other About 90 natural elements combine in countless ways (millions of compounds) due to different valences, bonding, and quantum rules — producing all the matter in the universe The diversity of matter is an illusion — all matter is ultimately the same single element