Living things are comprised of atoms. Think of atoms as being the basic unit of chemistry. The concept of the atom has been pondered since philophosers in ancient Greece in roughly 460 BC. Atoms come from a Greek word “a” and “tomos”. Tomos means cut, and with the prefix of a it means it can’t be cut, or dividied. It was believed to breated by the Greek philosopher Democritus. We however learned in WWII that atoms can be fragmented.

Some people including Isaac Newton credited Mochus the Pheonician as being the first with the atomical hypothesis. Some tired to connect him with the biblical Moses.

It was only until John Dalton.

When the atomic theory was first proposed it was a revelation. The current theory of the atom is an atomic nucleus, with a space called an electron cloud. Electrons are negatively charged particles.

The nucleus which contains protons, and neutrons is dense. Protons and neutrons are in the nucleus so are referred to as nucleons. These nucleons are 1836 times heavier than that os the electron.

The radius of an atom is about 10,000 times of a nucleus.

Einstein mathematically proved an exernal the existence of aotms and molecules in 1905.

It was wondered why a grain of pollen suspended in water would move even seemingly without outside influence. It was later proved by Einstein mathematically that there was invisible to the naked eye there was particles moving in the liquid colliding with the pollen making it move. Atomic theory states that any liquid is made up of molecules, which are always in random, ceaseless motion.

The more we’ve evolved our knolwedge the more we’ve found matter is made up of smaller particles.

Chemistry is fundamentally about the movement of electrons, not so much the nucleus of the atom. Anything that is sub atomic normally falls in the realm of nuclear physics. Electrons need to be attached to an atom, they are not found on their own.

Atoms consist of electrons, protons, and neutrons. Protons have a positive charge, electrons have a negative charge, and neutrons have a neutral charge. Like many things in the sciences many of the names of the atoms come from Latin and Greek. As an example the name of Silver of the periodic table is “ag” which comes from the Latin “Argentum”.

Other names of elements come from English, such as “hydrogen” which was named by Antoine Lavoisier whom some people refer to as the “father” of chemistry, as it was “generated” by water, or “hydro”generated. Hydrogen is almost the most abundant chemical element in the uniervse, making up 75% of normal matter based on mass, and more than 90% by number of atoms.

Electrons may move about in an atom, where as protons are much more stable. The discovery of the sub atomic particle known as electrons was credited to Sir Joseph John Thomson, which had built on the work of people like William Prout whom had “Prout’s hyposthesis” who believed the hydrogen atom is the fundamental particle, and Sir Norman Lockyer whom discovered Helium coming from the greek word ‘Helios’ meaning sun. both whom had suggested atoms were made of a more fundamental unit.

The original name for electrons was “corpuscles”. While historically we use the word corpuscle as an outdated word for a sub particle of matter or light, in modern times in biology we use it to mean a small body or cell in an organisms, parictuarly in the concept of red and white blood cells in vertebrates.

The discovery of the electron came from JJ Thomson studying cathode rays. Out of this came the original Thomson model called the “plum pudding” model, or the “raisin bread” model that was speculated incorrectly at the time that each atom was like a spehere with a liquid known as the pudding, iwith negatively charged electrons called the plums. This out dated style of looking at the atom has been described as a chocolate chip cookie with a positively charged soup with electrons in between.

It was Ernest Rutherford that discovered the Nucleas of an atom. Rutherford was friends with Marie and Pierre Curie and had been studying radioactivity. He aimed particles at a piece of gold foils. He coompared it to bullets being shot at a piece of tissue paper.

He thought the particles would go straight through, instead some particles bounced back. He deduced there was something in the atom that was small, heavy and postively charged. He called it the Nuclear model based on the idea that something was bouncing off the positively charged nucleus of the atoms in the gold foil.

The word proton comes from the Greek word meaning “first” which was named by Ernest Rutherford.


James Chadwick came up with the idea of the neutron. Many people there must be another particle in the nucleus that was creating extra mass.

One atomic mass unit = 1.6605 x 10-24 g

Protons weigh 1.0073 Mass / amu
Neutrons weigh 1.0087 Mass / amu
Electrons weigh 5.4859 x 10 -4

Neutrons help keep the nucleus of the atom in place. The atomic number does not change as the number of neutrons change because the number of protons in the atom has not changed, but the relative atomic mass does. When an atom has the same number of protons, but different number of neutrons to each other it’s called an “isotype”.

Protons and neutrons both can be referred to as nucleons as they are both in the nucleus. You might say the protons in addition to the neutrons equal the total number of necleons.

Most of the atom is empty space. The electrons are around the nucleus. The plural of nucleus is nulcei. They are held together of the stron nuclear force, which is one of the four fundamnetal forcesof physics. As the electrons engage it forms matter, whether ionic or covalent. Together they form elements which form the periodic table.

In atoms there are an equal number of protons and electrons giving a total charge of 0.

The composition of the nucleus is defined by three numbers:

Z – The Atomic number (also called Element number)

This is the number of protons in the nucleus which defines which element the atom is. The number of protons in the nucleus always determines what element it is despite the number of neutrons, or electrons. Think of the atomic number as the number that the element is in the periodic table

N – The Neutron number

The number of neutrons

A – The Mass number

This is the number of protons in addition to the number of neutrons (Z + N = A)