If you keep cutting a solid block of iron into smaller and smaller pieces, eventually you reach a point where it can no longer be divided. All substances in the universe are made of tiny particles called .
An is defined as the smallest part of an that can exist. Matter is classified into , compounds, and mixtures. An is a substance that is made of only one type of , and there are approximately 100 different displayed on the periodic table.
Examples include Iron (Fe), which is a metallic arranged in a regular, giant lattice structure. Oxygen (O) is a non-metal that naturally exists as (), meaning two oxygen are chemically joined. It is strictly classified as an , not a compound, because it contains only one type of .
are incredibly small, with a radius of about nanometres ( m). They consist of a central containing protons and neutrons, surrounded by negatively charged in energy levels (shells).
The radius of the is about of that of the (approximately m). Despite its tiny size, virtually all the mass of an is concentrated in the central .
The number of protons, known as the , is unique to each and dictates its identity. The is the sum of the protons and neutrons in the .
Before the discovery of protons, scientists like Dmitri Mendeleev ordered by . However, this caused problems because were sometimes placed in with completely different chemical properties.
Mendeleev overcame this by leaving gaps for undiscovered and swapping the order of some to match their chemical properties. The later discovery of explained why ordering by was incorrect, as have different masses but identical chemical properties.
The modern periodic table arranges in order of increasing atomic (proton) number. This sequential arrangement ensures that with similar properties fall naturally into the same vertical columns without needing to be manually swapped.
The arrangement of in shells is called the . For the first 20 , the shells follow a strict sequence of maximum capacities: 2 in the first shell, 8 in the second, 8 in the third, and 2 in the fourth ().
The periodic table is directly linked to this atomic structure. (horizontal rows) represent the number of occupied electron shells an has.
(vertical columns) represent the number of in the (outer shell). in the same group have similar chemical properties exactly because they have the same number of in their outer shell.
You can determine an 's position on the periodic table purely from its .
Deduce the group and period of Sodium ( 11).
Step 1: Write the using the rule.
Step 2: Use the number of shells to find the period.
Step 3: Use the final number to find the group.
Because in the same group share the same number of outer , they react in similar ways. However, their reactivity changes as you move down the group due to , where inner electron shells reduce the electrostatic attraction between the positive and the negative outer .
Group 1 metals all have one outer electron and react by losing it to form ions. Reactivity increases down the group because the outer electron is further from the and more shielded, meaning the electrostatic attraction is weaker and the electron is lost more easily.
Group 7 halogens have seven outer and react by gaining one to form ions. Reactivity decreases down the group because a larger atomic radius and more mean the has a weaker attraction for an incoming electron.
Group 0 (noble gases) do NOT react easily. They are inert because they already have a stable, full outer shell of 8 (except Helium, which has a full shell of 2).
Students often define an element just as 'a substance that cannot be broken down'. For AQA, you must explicitly state it is 'a substance made of only one type of atom'.
Do not confuse atomic number with mass number. The atomic number is always the smaller number on the periodic table and strictly tells you the number of protons.
When explaining why elements are in the same group, examiners look for the exact phrase: 'they have the same number of electrons in their outer shell'.
In 6-mark questions explaining reactivity trends, you must link the physical atomic structure (distance from nucleus and shielding) to how easily the atom loses or gains electrons.
Remember that and are still elements, not compounds, because even though they are molecules, they contain only one type of atom.
Atom
The smallest part of an element that can exist.
Element
A substance that is made of only one type of atom.
Diatomic molecules
Molecules composed of two atoms chemically joined together, such as oxygen gas ().
Nucleus
The central part of an atom containing protons and neutrons, where virtually all the mass is concentrated.
Electrons
Negatively charged subatomic particles that orbit the nucleus in energy levels (shells).
Atomic number
The number of protons in the nucleus of an atom.
Mass number
The sum of the protons and neutrons in the nucleus of an atom.
Atomic weight
The historical term for relative atomic mass, used by early scientists to order elements.
Isotopes
Atoms of the same element with the same number of protons but a different number of neutrons.
Electronic configuration
The arrangement of electrons in shells (energy levels) around an atom's nucleus.
Periods
Horizontal rows on the periodic table representing the number of occupied electron shells.
Groups
Vertical columns on the periodic table containing elements with the same number of outer shell electrons.
Valence electrons
The electrons found in the highest occupied energy level (outer shell) of an atom.
Highest occupied energy level
The formal term for the outermost electron shell of an atom.
Shielding
The effect where inner electron shells reduce the electrostatic attraction between the positive nucleus and the negative outer electrons.
Put your knowledge into practice — try past paper questions for Chemistry
Atom
The smallest part of an element that can exist.
Element
A substance that is made of only one type of atom.
Diatomic molecules
Molecules composed of two atoms chemically joined together, such as oxygen gas ().
Nucleus
The central part of an atom containing protons and neutrons, where virtually all the mass is concentrated.
Electrons
Negatively charged subatomic particles that orbit the nucleus in energy levels (shells).
Atomic number
The number of protons in the nucleus of an atom.
Mass number
The sum of the protons and neutrons in the nucleus of an atom.
Atomic weight
The historical term for relative atomic mass, used by early scientists to order elements.
Isotopes
Atoms of the same element with the same number of protons but a different number of neutrons.
Electronic configuration
The arrangement of electrons in shells (energy levels) around an atom's nucleus.
Periods
Horizontal rows on the periodic table representing the number of occupied electron shells.
Groups
Vertical columns on the periodic table containing elements with the same number of outer shell electrons.
Valence electrons
The electrons found in the highest occupied energy level (outer shell) of an atom.
Highest occupied energy level
The formal term for the outermost electron shell of an atom.
Shielding
The effect where inner electron shells reduce the electrostatic attraction between the positive nucleus and the negative outer electrons.