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By the end of this section, you will be able to:
  • Describe matter and elements
  • Describe the interrelationship between protons, neutrons, and electrons, and the ways in which electrons can be donated or shared between atoms

At its most fundamental level, life is made up of matter. Matter occupies space and has mass. All matter is composed of elements , substances that cannot be broken down or transformed chemically into other substances. Each element is made of atoms, each with a constant number of protons and unique properties. A total of 118 elements have been defined; however, only 92 occur naturally, and fewer than 30 are found in living cells. The remaining 26 elements are unstable and, therefore, do not exist for very long or are theoretical and have yet to be detected.

Each element is designated by its chemical symbol (such as H, N, O, C, and Na), and possesses unique properties. These unique properties allow elements to combine and to bond with each other in specific ways.


An atom is the smallest component of an element that retains all of the chemical properties of that element. For example, one hydrogen atom has all of the properties of the element hydrogen, such as it exists as a gas at room temperature, and it bonds with oxygen to create a water molecule. Hydrogen atoms cannot be broken down into anything smaller while still retaining the properties of hydrogen. If a hydrogen atom were broken down into subatomic particles, it would no longer have the properties of hydrogen.

At the most basic level, all organisms are made of a combination of elements. They contain atoms that combine together to form molecules. In multicellular organisms, such as animals, molecules can interact to form cells that combine to form tissues, which make up organs. These combinations continue until entire multicellular organisms are formed.

All atoms contain protons, electrons, and neutrons ( [link] ). The only exception is hydrogen (H), which is made of one proton and one electron. A proton    is a positively charged particle that resides in the nucleus    (the core of the atom) of an atom and has a mass of 1 and a charge of +1. An electron    is a negatively charged particle that travels in the space around the nucleus. In other words, it resides outside of the nucleus. It has a negligible mass and has a charge of –1.

Illustration of an atom showing two neutrons and two protons in the center, with a circle labeled as the nucleus around them. Another circle shows an orbit with two electrons outside of the nucleus
Atoms are made up of protons and neutrons located within the nucleus, and electrons surrounding the nucleus.

Neutrons , like protons, reside in the nucleus of an atom. They have a mass of 1 and no charge. The positive (protons) and negative (electrons) charges balance each other in a neutral atom, which has a net zero charge.

Because protons and neutrons each have a mass of 1, the mass of an atom is equal to the number of protons and neutrons of that atom. The number of electrons does not factor into the overall mass, because their mass is so small.

As stated earlier, each element has its own unique properties. Each contains a different number of protons and neutrons, giving it its own atomic number and mass number. The atomic number    of an element is equal to the number of protons that element contains. The mass number    , or atomic mass, is the number of protons plus the number of neutrons of that element. Therefore, it is possible to determine the number of neutrons by subtracting the atomic number from the mass number.

Questions & Answers

How many bones are in the human skeleton
Treasure Reply
procce of digestion of proteins a long human alimentarycanal
Carson Reply
what are the properties of lipids?
Isiah Reply
They are: Fatty acids, fats, oils, waxes, phospholipid, glycolipids, steroids and some vitamins
explain why a fresh water fish excrete ammonia
Leonard Reply
plz answer my question
sorry i meant it has a nucleous unlike plant cells lol
Ammonia is the end product of protein catabolism and is stored in the body of the fish in high concentrations relative to basal excretion rates. Ammonia, if allowed to accumulate, is toxic and is converted to less toxic compounds or excreted
What are eukaryotic cells?
Nwosueke Reply
cell with no nucleous so not a plant cell
eukaryotic cells are membrane bound organelles that have a membrane bound nucleus
where does the cell get energy for active transport processes?
A'Kaysion Reply
IDK maybe glucose
what is synapsis
Adepoju Reply
how many turns are required to make a molecule of sucrose in Calvin cycle
Amina Reply
why Calvin cycle occurs in stroma
why do humans enhale oxygen and exhale carbondioxide?
Maryam Reply
why do humans enhale oxygen and exhale carbondioxide? For the purpose of breaking down the food
what is allele
uzoka Reply
process of protein synthesis
what is cell
Zulf Reply
a cell is a smallest basic, structural and functional unit of life that is capable of self replication
why does a fresh water fish excrete ammonia
plz answer my question
Ammonia is a toxic colorless gas and when its inside the fish biological system is converted to a less toxic compound then excreted in the form of urea. However too much ammonia will kill the fish " Ammonia Poisoning " which is a very common disease among fish.
what is cytoplasm
uzoka Reply
cytoplasm is fluid of cell.
how many major types of Cloning
Saeed Reply
comparative anatomy of gymnosperms?
Meenakshi Reply
anatomy of gymnosperms

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Source:  OpenStax, Concepts of biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11487/1.9
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