![]() Nitrogen (N): often the limiting factor for agriculture or population growth, even though N 2 is 78% of air, because it only reacts under special circumstances.\) we see that there are two types of covalent compounds, what we are calling "classical molecules", and acids.Oxygen (O): we get most of our energy from reactions with oxygen, when we breathe or when we burn fuel O 2 is 21% of air.Helium (He): named after the sun, because it was discovered in the sun before being discovered on Earth (we'll explain how later) it doesn't react with anything.Hydrogen (H): exists as H 2 or in combination with other elements, such as in water.Light elements: the elements with smallest mass.Here are a few that you should learn right now, though, because they are common or important, so that you won't be confused when they are mentioned later. There are lots of elements and you don't need to memorize them all. Or you might just see K +(aq), meaning potassium ions dissolved in water. You might also see something written with (aq), which means "dissolved in water." For instance, NaCl(aq) means salt dissolved in water so there is no solid left. You'll probably never hear about He(s), since it would be very hard to make it a solid. If it's a liquid (4.2K or below, less than -269° C) that is written He(l). For instance, He is almost always a gas, written He(g). The phase of a substance is often indicated by a letter in () after the symbol.This is also called hydroxyl radical, written OH from water, you are left with OH molecule, which is neutral.For instance, water is H 2O, and if you remove hydrogen ion, you are left with hydroxide ion, OH –. Because this is unusual, it might be indicated with a dot, like this: H For instance, H is a radical, because it has one electron. If they have an odd number of electrons, this is called a radical. Most molecules or ions that are stable have an even number of electrons.A few compounds of sodium, however, contain the Na ion, allowing comparison of its size with that of the far more familiar Na + ion, which is found in many compounds. ![]() Then, since the lithium ion has one less electron, remove. Because most elements form either a cation or an anion but not both, there are few opportunities to compare the sizes of a cation and an anion derived from the same neutral atom. With a standard atomic weight of circa 1.008, hydrogen is the lightest element on the periodic table. For example, to find the configuration for the lithium ion (Li), start with neutral lithium (1s☢s¹). Hydrogen is a chemical element with atomic number 1 which means there are 1 protons and 1 electrons in the atomic structure.The chemical symbol for Hydrogen is H. Then, add or remove electrons depending on the ions charge. You can tell that this is different from the charge, because the charge will always include + or – and come after the symbol. To find the electron configuration for an ion, first identify the configuration for the neutral atom. This means carbon with (protons + neutrons) = 13. If we want to show how many protons and neutrons are present in an atom, we can use the mass number, as a superscript before the element symbol, such as 13C.2 O means 2 atoms of oxygen that aren't connected to anything. If the number comes first, though, it has a different meaning. ![]() Sometimes people might write O2 to mean the same thing. If we write O 2 that means the oxygen molecule, which is two atoms of oxygen connected together. This means one carbon atom and 2 oxygen atoms. We indicate the number of atoms of a particular type using a subscript, like this: CO 2.S 2– is sulfide, or sulfur + 2 electrons. For instance, Ca 2 + is a calcium atom – 2 electrons, or calcium ion. If there's an number in the superscript, that says how many electrons are added or removed. If we write H – this means one hydrogen atom + one electron, so a proton and 2 electrons, also called hydride ion. ![]() H + means 1 H atom – 1 electron, so it means just one proton, also called hydrogen ion. If we write just H, that means an H atom, which is one proton and one electron.
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