Instruction 1-7 Connection Among the Location in the Table, the Atomic Number, and Mass | How to Identify Metals, Semimetals, Nonmetals, and Halogens | How to Identify Alkaline Metals, Alkaline Earth Metals, and Transition Metals | Lanthanide, Actinide, Transactinide, and Transuranium Elements | Ionization Energy, Electronegativity, Relative Sizes | How Many Electrons Can Bond? | Size and Mass | Location and Quantum Electron Configuration | Summary |
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| SIZE AND MASS |
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| We’ve told you a little about the size and mass of atoms. But now it’s
time to go into more detail. Size Since we’re still talking size here, another important atomic measurement is atomic radius. A radius, as you know, is a line drawn from the center of a circle to its circumference (or from the center of a sphere to its surface). Unlike a ball, an atom doesn’t have a fixed radius (because of its constantly shifting and orbiting electrons). So the radius of an atom can only be found by measuring the distance between the nuclei of two touching atoms and then dividing that distance between the 2 atoms. There are certain trends in atomic radii (“radii” is the plural of
radius) in The Periodic Table as shown in Instruction 1-5. As you go The size of an atom changes when an atom becomes an ion. When chlorine
picks up an electron to become an anion, it fills its outer shell. Also,
there is greater electron-electron repulsion, which pushes the electrons
away from each other. This causes an increase in the size of the atom. When
sodium loses an electron to become a cation, it empties its outer shell.
Also, the remaining electrons are pulled towards the nucleus due to the
decrease in the electron-electron repulsion. This causes a decrease in the
size of the atom. Refer to the following web page to see the change in size
when elements change to ions. First, the total mass of an atom is the sum of the mass of its protons, neutrons and electrons. There is some controversy about the terms “atomic weight” and “atomic mass.” Some chemists use the words “weight” and “mass” interchangeably, even though that’s not actually correct. We’ll explain why at the end of this Instruction, but in the meantime we’ll use the word “mass.”
So the nucleus makes up most of the mass of an atom but only a small part of its volume. Electrons make up most of the volume of an atom but only a small part of its mass. To explain, this we’d better define the words volume and mass. Mass is the quantity of matter an object has. Volume is the size of a three-dimensional region in space. They are two very different ideas. Electrons occupy a lot of the volume in an atom because of the way they move around the nucleus (the more electrons, the greater the volume.) The Periodic Table shows the atomic mass of each element. But, as with many things in science, it isn’t that simple. As you know, the atomic number on The Table represents the number of
protons in the element. That never changes. So the number of protons for an
element always remains the same. But there can be different numbers of
neutrons. An element with a different numbers of neutrons is known as an
isotope. Isotopes are designated by their total number of protons and neutrons. As shown in the figure, there are 3 isotopes of hydrogen and the only thing that changes is the number of neutrons. There is only 1 proton and 1 electron in the neutral isotopes of hydrogen. Take carbon, the most common form of carbon has 6 protons (its atomic number is 6) and 6 neutrons. The mass of this atom is 12 daltons and given the label carbon-12. So carbon-12 is a very common, non-radioactive isotope of carbon. Carbon14 is a less common radioactive isotope of carbon with 6 protons and 8 neutrons. It slowly changes into nitrogen-14 and emits radiation when it changes. Have you ever heard of carbon dating (a process that can tell the age of organic items that are thousands of years old)? That’s where carbon-14 comes in handy. When a living organism dies, there is a certain ratio of carbon-14 to carbon-12. Since the organism cannot take in any new carbon-14, it starts to disappear. By taking the current ratio of carbon-14 to carbon-12, we can determine how much time has past since the organism died. After 20,000 years, there is so little carbon-14 left, that it is almost impossible to measure the amount present and determine the length of time that has past. Can an atom have any number of neutrons? No. There are preferred combinations of neutrons and protons, where the forces holding the nucleus together seem to balance best. Light elements tend to have about as many neutrons as protons, while heavy elements tend to have more neutrons. Atoms with too few or two many neutrons can sometimes exist for a while, but they’re unstable. Their nuclei change or “decay” by spitting out radiation in the form of particles or electromagnetic waves (kind of like radio waves, which is where the word “radioactive” comes from). We will learn more about this in the last set of Instructions. Since isotopes of an element have different number of neutrons, their masses are different. Carbon-12 has an atomic mass of 12 daltons; carbon-13 has an atomic mass of 13 daltons; carbon-14 has an atomic mass of 14 daltons. All three isotopes exist in the environment, but in different amounts. 98.9% of carbon is carbon-12; 1.1% of carbon is carbon-13; and carbon-14 is only a very small amount. The atomic mass shown for an element on The Periodic Table is the weighted average of the atomic mass of all its naturally occurring isotopes. What does weighted average mean? The weighted average is obtained when a specific amount of each of the parts is used to find an average. Looking at carbon, we have 98.9% with an atomic mass of 12 daltons and 1.1% with an atomic mass of 13 daltons (we’ll ignore carbon-14 since there is so little of it). We multiply the amount percent by the atomic mass and add all the parts together.
Looking at the Periodic Table, we see that the atomic mass of carbon is
listed as 12.011 daltons. Strictly speaking, weight measures the pull of gravity on an object’s mass – while mass (as we said earlier) measures the quantity of matter in an object. An atom is far too small for its weight to be measured on any kind of
scale – but its mass can be. So the correct term is “mass,” although a lot
of chemists, as we said, still use the words interchangeably.
Now let's do Practice Exercise 1-7 (top).
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down a Group (column) the atoms (and their radii) get bigger. That’s because
you’ve added extra shells to the atom. As you go across the Periods (rows)
from left to right, the atomic radii get smaller. That’s because the
increasing number of protons in the nucleus of each atom pull the electrons
in more tightly and decrease the atom’s overall size. In the transition
elements (transition metals) there’s not much change in size as you move
across the rows. Although there is a contraction at the beginning of the
series, all have pretty much the same radii.
Most
of the mass of an atom comes from its protons and neutrons (the nucleus).
Protons and neutrons have pretty much the same mass. Electrons are very tiny
– only 1/1836 the mass of a proton.
