All About Atoms 2 Atomic Number He 4 Atomic Mass Number Protons Electrons Neutrons A helium He atom has two protons so its atomic number is 2 The atomic mass the weight of helium is 4 and is measured by the sum of the protons and neutrons in the atom The number of electrons in an atom doesn’t affect either measurement Small Wonder Atoms are so tiny that they are too small to see except with extremely powerful microscopes Every atom has a dense center called a nucleus Forming the nucleus are two kinds of particles protons which have a positive electrical charge and neutrons which have no charge Bound to the nucleus are one or more electrons which are particles with a negative charge Scientists used to describe atoms as tiny solar systems with the electrons orbiting around the nucleus like planets around the sun but atoms are a lot hazier than that If you imagined the tiny nucleus as a pea it would be centered in a football stadium size cloud that includes the electrons We can’t be sure where the electrons are because they are impossible to pin down Protons We’ve Got Your Number All atoms have at least one proton in their core And protons determine an atom’s uniqueness The number of protons determines the element For example hydrogen has just one proton No other element is made up of atoms that contain one proton For this reason we list the different elements by their number of protons This is called the atomic number The atomic number determines where the element sits on the periodic table of elements the organized chart of all of the elements that have been discovered Neither Here nor There Neutrons have neither a positive nor a negative charge No matter how many neutrons are in an atom they do not affect the charge However neutrons do affect the mass the weight of an atom The atomic mass is the sum of all the protons and neutrons in the element 10
Electrons in Orbit In addition to protons and neutrons all atoms have electrons Electrons are negatively charged particles that orbit the space surrounding the positively charged nuclear core These orbits are called electron shells Each electron shell has a different energy level with the electron shells farthest from the nucleus having the highest energy levels This outermost shell is known as the valence shell and the electrons found in it are called valence electrons The number of electrons in the valence shell determines the atom’s reactivity or tendency to form chemical bonds with other atoms Atoms are most stable and least likely to be reactive when their valence shell is full of all the electrons it can hold In Balance Neutral atoms have an equal number of protons and electrons and the number of protons and neutrons is usually the same but sometimes not Adding or removing neutrons to an atom makes an isotope a heavier or lighter version of that same element with the same chemical properties Changing the number of electrons in an atom also does not change the type of element it is but it does change its chemical behavior the atom is no longer neutral in charge and is now called an ion But adding a proton Suddenly you’ve got an entirely new element with you guessed it a new atomic number An atom is about one angstrom in diameter which is about 100,000 times smaller than a red blood cell Electrons travel at a speed of 1,367 miles 2,200 kilometres per second and at this speed they could orbit Earth in 18 seconds 99 99999999999 percent of an atom is empty space A single human hair is about as thick as one hundred thou sand atoms Make a fist It contains about 15,000,000,000,000,000,000 000,000 that’s 15 septillion atoms If each atom in your fist were the size of a marble your fist would be the size of Earth Nucleus Valence Electrons Protium Deuerium Tritium Hydrogen Isotopes ALL ABOUT ATOMS 11
Compounds Anytime two atoms join together they make a molecule Everything around us is made up of molecules When atoms from two or more different elements bind together the molecule is a compound The chemical bond is so strong that the atoms from the different elements all behave like a single substance Compounds are not simply mixtures or blends but a bind that happens at the atomic level Sometimes an element will lose traits when it is bound in a compound An element that is reactive on its own meaning it will release energy in a chemical reaction with another element like sodium Na can become nonreactive when it binds with another element like chlorine Cl forming sodium chloride NaCl which is everyday table salt All compounds have a definite composition For example a compound you may have heard about before is H2O or water Water always has two hydrogen atoms and one oxygen atom If you take away one of the hydrogen atoms it is no longer water While we have only 118 elements in the periodic table there are millions and millions of compound combinations that are each unique Alloys Versus Compounds Alloys are not compounds An alloy is a mixture of metals or a metal and another element Brass is an alloy of copper and zinc which are both metals Steel is an alloy of iron a metal carbon a nonmetal and sometimes other elements The alloy components are usually melted at very high temperatures mixed and allowed to cool to a solid alloy In general alloys are stronger and more durable than their individual components Unlike compounds alloys do not have chemical bonds between the elements 12 ILLUSTRATED ENCYCLOPEDIA OF THE ELEMENTS
Liquid bleach NaClO common compouns Some compounds you might know by their common names Marble CaCO3 Baking soda NaHCO3 Rubbing alcohol CH3 2CHOH Milk of magnesia Mg OH 2 Table Salt NaCl Baking soda Bleach 30% isopropyl Alcohol Milk of Magnesia Sugar pure Granulated Sugar C12H22O11
