Atoms: The Building Blocks of Matter

The particle theory of matter has been supported from as early as 400 BCE by some Greek thinkers like Democritus.
Nearly almost all chemists in the late 1700s accepted the modern definition of an element as a substance that cannot be broken down any further by any sort of chemical means.
They also assumed that these elements form compounds that have different physical and chemical properties compared to the elements that they are composed of.
The chemists however struggled to understand how chemical reactions truly worked.

Chemical reactions- How different substances could combine with one another to form new ones.
In the 1790s technology started to become even more advanced, and measurement tools became more advanced.
This led to the creation of many laws.
Law of conservation of mass- Mass cannot be created or destroyed during chemical reactions or physical changes.
Law of definite proportions- The fact that a chemical compound has the same number of elements in exactly the same proportions by mass. The size or the source of the compound doesn’t matter.
Two elements can also combine to form more than one compound.
Law of multiple proportions- If 2 or more different compounds have the same 2 elements, then the ratio of the masses of the 2nd element combined with a certain mass of the 1st element is always the ratio of small whole numbers.

Dalton’s Atomic Theory:
- All matter is composed of extremely small particles called atoms.
- Atoms of an element are identical in size, mass, etc. Vice versa for atoms of different elements.
- Atoms cannot be divided, created, or destroyed.
- Atoms of different elements come together to form simple whole-number ratios when forming chemical compounds.
- Atoms are combined, separated, or rearranged in chemical reactions.
Dalton turned Democritus’s idea into a scientific theory that could be tested by experimentation.
However, Dalton’s theory wasn’t entirely accurate according to modern atomic theory.

Changes were made to Dalton’s theory to alter it to what we know today.
Changes made:
- Atoms can be divided into subatomic particles(protons, neutrons, and electrons)
- Atoms of an element can have different masses(due to isotopes.)

In the late 1800s, scientists discovered that atoms are composed of several types of smaller particles.
The number and arrangement of these smaller particles found in the atom determine the atom’s chemical properties.
Atom- The smallest particle of an element that retains the chemical properties of that element.
Parts of an atom:
- Nucleus: a very small area, but located in the center of the atom.
- Protons: They can be found in the nucleus, and they are positively charged particles.
- Neutrons: They are also found in the nucleus, but have no charge(neutral.)
- Electrons: They are negatively charged particles, and are found in areas that are surrounding the nucleus.

The first discovery of subatomic particles was in the late 1800s. Cathode ray tubes were used often to discover these subatomic particles.
Scientists noticed that when current was passed through the tube, the surface of the tube that was opposite the cathode glowed.
The glow was caused by a stream of particles called cathode rays. These rays would travel when a current was passed through the tube.
The cathode ray tubes proved that atoms are divisible into other subatomic particles.
It ended up revealing many observations after the experiment..
- Cathode rays were deflected by a magnetic field in the same manner as a wire carrying an electric current, which was known as a negative charge.
- The rays were deflected away from a negatively charged object.
It leads to the hypothesis that particles found in cathode rays are negatively charged.

Cathode rays have identical properties regardless of the element that’s used to produce them.
It was concluded that electrons are present in atoms of all elements.
The Cathode-Ray experiments provided evidence that atoms are divisible and that one of the atom’s basic constituents is a negatively charged electron.
The discovery of the electron led to more discoveries.
Because atoms are electrically neutral, they must have a positive charge to balance negative electrons.
Because electrons have less mass than atoms, atoms must contain other particles to account for their mass.
Thomson proposed the “plum pudding model”. Where the negative electrons were spread evenly through the positive charge that was found in the rest of the atom.
New experiments that followed after this model disproved it.
It did however start to give us a better understanding of the atom.
Atoms are generally considered electrically neutral because they have the same number of protons and electrons. Note: A neutron is just neutral in general.
Each atom of different elements is unique because they have different numbers of subatomic particles.
Nuclear forces- These are short-range forces(proton-neutron, proton-proton, and neutron-neutron) that hold the nuclear particles together.
Remember that the radius of an atom is the distance from the center of the nucleus to the outer position of the electron cloud. The picometer is used because of the small size of atoms.

All atoms are composed of the same basic particles, but all atoms aren't the same.
Atoms of different elements have different numbers of protons.
Atomic number- Is used to represent the number of protons and electrons in the atom of an element.
The elements on the periodic table are placed in increasing order of their atomic numbers.
The atomic number is used to identify an element.

Isotopes- Atoms of the same element that have different masses.
The detection of these isotopes and determination of their relative proportions has become precise. Scientists can determine where elements come from by measuring the percentages of different isotopes that are found in a sample.
When you try to identify an isotope it requires knowing both the name or atomic number of the element and the mass of an isotope.

Mass number- The total number of protons and neutrons that make up the nucleus of an isotope.

AMU(Atomic Mass Unit)- It’s defined as 1/12th of a Carbon-12 atom.
Note: Isotopes may have different masses, but they don’t differ in their chemical behavior.
Average Atomic Mass- The weighted average of the atomic masses of the naturally occurring isotopes of an element.
Ex: (View image)
Mole(mol)- The amount of a substance that contains as many particles as there are atoms in 12 g of carbon-12.
A mole is just like a counting unit, similar to the concept of a dozen.
The number of particles that are in a mole that has been experimentally determined is called Avogadro’s number.
It’s named after the 19th-century Italian scientist Amedeo Avogadro that made discoveries to help explain the relationship between mass and the number of atoms.
Avogadro’s number: 6.02 * 10^23
Just to get an understanding of how huge Avogadro’s number is consider this; If every person living on Earth worked to count the atoms in one mole of an element, and if each person counted continuously at a rate of 1 atom per second, it would take about 3 million years for all the atoms to be counted.
A mole can also be thought of as the amount of substance that has Avogadro’s number of particles.