Atoms are the smallest particle of a substance. There are protons, neutrons, and electrons. It would take about 100 million atoms to stretch across your fingernail. Protons are positively charged and electrons are negatively charged. If an atom has 2 protons and 1 electron it is positive. If an atom has 2 electrons and 1 proton it is negative.
Matter is everything. The main four states are solid, liquid, gas, and plasma. In a gas the atoms move freely and fast. In a liquid the atoms move slower and they are more packed together. In a solid the atoms don't move at all and they are packed together tightly. The three properties of matter are mass, density, and volume.